Whole-genome sequencing of soil- and foodborne Bacillus cereus sensu lato indicates no clear association between their virulence repertoire, genomic diversity and food matrix

Bacillus cereus sensu lato is frequently involved in foodborne toxico-infections and is found in various foodstuff. It is unclear whether certain strains have a higher affinity for specific food matrices, which can be of interest for risk assessment. This study reports the characterization by whole-genome sequencing of 169 B. cereus isolates, isolated from 12 food types and soil over two decades. Any potential links between the food matrix of isolation, the isolate's genetic lineage and/or their (putative) virulence gene reservoir were investigated. More than 20 % of the strains contained the genes for the main potential enterotoxins (nheABC, hblCDA and cytK_2). Cereulide biosynthesis genes and genes encoding hemolysins and phospholipases, were detected in multiple isolates. Strain typing revealed a high diversity, as illustrated by 84 distinct sequence types, including 26 not previously described. This diversity was also reflected in the detection of all seven panC types and 71 unique virulence gene profiles. Core-genome MLST was used for phylogenomic investigation of the entire collection and SNP-based clustering was performed on the four most abundant sequence types, which did not reveal a clear affinity for specific B. cereus lineages or (putative) virulence genes for certain food matrices. Additionally, minimal genetic overlap was observed between soil and foodborne isolates. Clusters of closely-related isolates with common epidemiological metadata were detected. However, some isolates from different food matrices or collected several years apart were found to be genetically identical. This study provides elements that can be used for risk assessment of B. cereus in food.

Multi-country and intersectoral assessment of cluster congruence between pipelines for genomics surveillance of foodborne pathogens

Different laboratories employ different Whole-Genome Sequencing (WGS) pipelines for Food and Waterborne disease (FWD) surveillance, casting doubt on the comparability of their results and hindering optimal communication at intersectoral and international levels. Through a collaborative effort involving eleven European institutes spanning the food, animal, and human health sectors, we aimed to assess the inter-pipeline clustering congruence across all resolution levels and perform an in-depth comparative analysis of cluster composition at outbreak level for four important foodborne pathogens: Listeria monocytogenes, Salmonella enterica, Escherichia coli, and Campylobacter jejuni. We found a general concordance between allele-based pipelines for all species, except for C. jejuni, where the different resolution power of allele-based schemas led to marked discrepancies. Still, we identified non-negligible differences in outbreak detection and demonstrated how a threshold flexibilization favors the detection of similar outbreak signals by different laboratories. These results, together with the observation that different traditional typing groups (e.g., serotypes) exhibit a remarkably different genetic diversity, represent valuable information for future outbreak case-definitions and WGS-based nomenclature design. This study reinforces the need, while demonstrating the feasibility, of conducting continuous pipeline comparability assessments, and opens good perspectives for a smoother international and intersectoral cooperation towards an efficient One Health FWD surveillance.

Genomic epidemiology of Salmonella Enteritidis human infections in the Netherlands, 2019 to 2023

Salmonella enterica serotype Enteritidis (SE) is a common foodborne pathogen that can cause human salmonellosis. Identifying closely related cases is essential to control the pathogen through, e.g. outbreak investigation, but it is often challenging due to the low genetic diversity of SE, particularly with traditional typing methods. This study aimed to investigate the population structure of SE genomes collected during routine surveillance in the Netherlands using whole-genome sequencing (WGS), their clustering, temporal distribution and the association between epidemiological and phenotypic antimicrobial resistance (AMR) factors and the persistence of SE clusters. We also investigated the distribution of genotypic AMR markers among these isolates. The study collection comprised 1,669 unique SE isolates from human infections collected from Dutch surveillance between 2019 and 2023, and their relatedness was derived using core-genome multi-locus sequence typing and Hamming distances. Based on the results, the 216 clusters comprised 1,085 sequences, in addition to 584 sequences depicted as singletons. These clusters predominantly fell within three major lineages, of which two were the previously described Global and Atlantic lineages. Of these clusters, approximately a third persisted for more than 1 year during the 5-year study period. However, no statistically significant associations were found between epidemiological factors, such as age, gender and travel history, or phenotypic AMR and the persistence of SE clusters. The most common AMR genetic markers observed were related to antimicrobial classes of (fluor)quinolones, β-lactamases and aminoglycosides. This study provides a better understanding of the genomic epidemiology of SE in the Netherlands based on WGS. Further analysis that includes samples from the food-chain supply, along with higher resolution methods during a post-Coronavirus Disease of 2019 (COVID-19) period, may provide more insights into the possible causes of the persistence of SE clusters.

An outbreak of Shiga Toxin-producing Escherichia coli Serotype O145:H28 Associated with Domestic Travel and Consumption of Unpasteurized Cheese, UK, 2023

Unpasteurized dairy products carry an inherent risk of being contaminated with STEC and/or other zoonotic gastrointestinal pathogens. In November 2023, a genetically linked and geographically dispersed outbreak of 36 cases of Shiga toxin-producing Escherichia coli (STEC) O145:H28 was detected by the foodborne gastrointestinal pathogens surveillance systems at the UK Health Security Agency, using whole genome sequencing. Reported symptoms included diarrhoea (81%), bloody diarrhoea (65%), vomiting (84%), and 47% of cases were admitted to hospital. A review of the completed enhanced surveillance questionnaires (n = 29) revealed 18 cases reporting travelling first class on trains operated by the same company prior to onset of symptoms, of which 16/18 consumed the same meal which included an unpasteurized cheese. Microbiological testing of the cheese products did not detect the outbreak strain; however, STEC O145:H28 was detected in two bovine fecal samples collected at the dairy farm where the unpasteurized cheese was produced. Analysis of the genome sequencing data confirmed that the 36 human STEC O145 isolates and the two bovine STEC O145 isolates fell within the same 5 SNP single linkage cluster. These findings indicated that the cattle were the likely source of the human infections, via the consumption of contaminated unpasteurized cheese. The food business operator voluntarily recalled the implicated product from sale. Vulnerable groups, such as those who are very young, elderly, pregnant, or immunocompromised, should avoid consuming raw drinking milk and cheeses. Due to advances in clinical molecular diagnostics and enhanced epidemiological surveillance, notifications of foodborne outbreaks of STEC other than serogroups O157 are increasing in the UK. Further improvements in microbiological methods for detecting STEC on the farm and in food are essential for the presale identification of contaminated food items and to reduce the risks to public health.

Comprehensive epidemiological profiling of poultry-derived Salmonella spp. in Shandong, China, 2019-2022: a longitudinal study of prevalence, antibiotic resistances, virulence factors and molecular characteristics

Salmonella spp., as a major foodborne pathogen, pose significant threats to public health globally and has been an important zoonotic contamination for poultry industry that should receive increasing attentions. This study aimed to comprehensively investigate the prevalence, antimicrobial resistances, virulence factors, and plasmid types of Salmonella isolates collected from chickens, ducks, and geese across eight cities in Shandong between 2019 and 2022. Out of 300 samples, 53 Salmonella strains (17.67%) were isolated, with varied prevalence from 8.33% to 25.00% in different cities of Shandong. A total of seven serotypes were identified among the 53 Salmonella isolates, wherein the S. Enteritidis (45.28%), S. Pullorum (22.64%) and S. Typhimurium (16.98%) were identified as the most prevalent. Whole-genome sequencing analysis revealed that ST11, ST92, and ST19 were the predominant sequence types for S. Enteritidis, S. Pullorum, and S. Typhimurium, respectively. Phylogenetic analysis indicated that potential clonal spread of S. Enteritidis, S. Pullorum, and S. Typhimurium occurred across different regions, particularly the evidences supported that the S. Typhimurium isolates were dispersed in a cross-species manner. Finally, the phenotypic and genotypic profiling of antibiotic resistance among the isolates revealed that these isolates were multidrug resistant with corresponding antibiotic resistance genes (ARGs) including bla TEM, aac, aph, tet(A), and tet(B) to confer them with resistances to commonly-used veterinary drugs such as β-lactams, quinolones, macrolides. To sum, this study provides valuable insights into the current epidemiology of Salmonella in poultry industry in one of the biggest provinces in China, and shedding the light on the urgent necessity for further approaches to prevent and decontaminate such MDR Salmonella in livestock under One Health concept.

Salmonella Infantis outbreak on six broiler units in Great Britain: investigation, epidemiology, and control

AIMS

To describe the analysis, epidemiology, and control of six contemporaneous and linked outbreaks of Salmonella enterica subsp. enterica serovar Infantis on British broiler farms. Salmonella Infantis is a potentially multidrug-resistant foodborne zoonosis and can persistently colonize poultry flocks and farms.

METHODS AND RESULTS

Routine monitoring initially identified the organism, which was tracked to six farms associated with a single company. Extensive, repeat sampling identified widespread and, in some cases, persistent contamination. Salmonella Infantis was also isolated from three associated processing factories and catching crew equipment, but not from associated hatcheries and feed mills. Whole genome sequencing and resistance phenotyping revealed one strain was present in the processing plants and on five farms. However, on one of those farms, several highly genetically distinct strains were also detected, including one also found in one of the processing plants. The sixth farm had a strain that was genetically unrelated to strains collected from the other premises and which exhibited an extended spectrum beta-lactamase phenotype. Cleaning and disinfection were enhanced, and the organism was eventually cleared from all farms.

CONCLUSIONS

There were multiple incursions of varied strains, with a possible link to processing factories. Elimination of S. Infantis from premises can be challenging but achievable.

An outbreak of Shiga toxin-producing Escherichia coli serotype O103:H2 associated with unpasteurized soft cheese, England and Wales, 2022

In July 2022, a genetically linked and geographically dispersed cluster of 12 cases of Shiga toxin-producing Escherichia coli (STEC) O103:H2 was detected by the UK Health Security Agency using whole genome sequencing. Review of food history questionnaires identified cheese (particularly an unpasteurized brie-style cheese) and mixed salad leaves as potential vehicles. A case-control study was conducted to investigate exposure to these products. Case food history information was collected by telephone. Controls were recruited using a market research panel and self-completed an online questionnaire. Univariable and multivariable analyses were undertaken using Firth Logistic Regression. Eleven cases and 24 controls were included in the analysis. Consumption of the brie-style cheese of interest was associated with illness (OR 57.5, 95% confidence interval: 3.10-1,060). Concurrently, the production of the brie-style cheese was investigated. Microbiological sample results for the cheese products and implicated dairy herd did not identify the outbreak strain, but did identify the presence of stx genes and STEC, respectively. Together, epidemiological, microbiological, and environmental investigations provided evidence that the brie-style cheese was the vehicle for this outbreak. Production of unpasteurized dairy products was suspended by the business operator, and a review of practices was performed.

Potential links between human bloodstream infection by Salmonella enterica serovar Typhimurium and international transmission to Colombia

Salmonella enterica serovar Typhimurium is a prevalent food-borne pathogen that is usually associated with gastroenteritis infection. S. Typhimurium is also a major cause of bloodstream infections in sub-Saharan Africa, and is responsible for invasive non-typhoidal Salmonella (iNTS) disease. The pathogen also causes bloodstream infection in Colombia, but there has been a lack of information about the S. Typhimurium isolates that were responsible. Here, we investigated the genomic characteristics of 270 S. Typhimurium isolates from bloodstream infection patients in Colombia, collected between 1997 and 2017. We used whole-genome sequencing to analyse multidrug-resistant (MDR) profiles, plasmid distribution, and to define phylogenetic relationships. The study identified the distinct sequence types and phylogenetic clusters of S. Typhimurium prevalent in Colombia. The majority of isolates (90.8%) were ST19, which is distinct from the iNTS-associated S. Typhimurium in sub-Saharan Africa (ST313). The two prominent clusters of MDR S. Typhimurium were either DT104 or closely related to the LT2 reference strain. We used a phylogenetic approach to associate the Colombian clusters with global S. Typhimurium isolates from public databases. By putting the Colombian S. Typhimurium isolates in the context of the global spread of DT104, ST313 and LT2-related variants, we found that the Colombian clusters were introduced to the country via multiple independent events that were consistent with international transmission. We suggest that the acquisition of quinolone and chloramphenicol resistance by the Colombian S. Typhimurium isolates was driven by horizontal gene transfer. Three ST313 isolates that caused bloodstream infection in Colombia were identified. These ST313 isolates were related to the Malawian ST313 lineage 3 & UK ST313, and shared a similarly high invasiveness index. To our knowledge, this is the first report of ST313 in Colombia.

Hybrid strains of enterotoxigenic/Shiga toxin-producing Escherichia coli, United Kingdom, 2014-2023

Introduction. Diarrhoeagenic Escherichia coli (DEC) pathotypes are defined by genes located on mobile genetic elements, and more than one definitive pathogenicity gene may be present in the same strain. In August 2022, UK Health Security Agency (UKHSA) surveillance systems detected an outbreak of hybrid Shiga toxin-producing E. coli/enterotoxigenic E. coli (STEC-ETEC) serotype O101:H33 harbouring both Shiga toxin (stx) and heat-stable toxin (st).Gap statement. These hybrid strains of DEC are a public health concern, as they are often associated with enhanced pathogenicity. However, little is known about their epidemiology, clinical significance and associated public health burden.Aim. The aim of this study was to describe the microbiology, epidemiology and genomic analysis of this novel hybrid serotype in the context of the STEC-ETEC strains in the UKHSA archive.Methodology. From 2014 to 2023, STEC isolated from faecal specimens testing positive for STEC by PCR were sequenced on the NextSeq 1000 short read platform and a subset were selected for long read nanopore sequencing. Genomes were analysed to determine serotype, stx subtype, DEC pathogenicity genes and antimicrobial resistance determinants.Results. There were 162 STEC-ETEC strains isolated between 2014 and 2023, of which 117/162 were human clinical isolates and 45 were of food or animal origin. An average of 16 STEC-ETEC strains were identified each year, exhibiting a range of different stx subtypes, the most common profiles being stx2g,st (n=65, 40%) and stx2a,st (n=48, 30%). The most common sequence types were ST329 and ST200 (n=24 each), and the most frequently detected serotype was O187:H28 (n=25). Nine cases of genetically linked STEC-ETEC O101:H33, stx1a,st were detected between 8 August and 21 September 2022. Although the temporal and geographical distribution of the cases was characteristic of a foodborne outbreak, the contaminated vehicle was not identified.Conclusions. Phylogenetic analysis and long-read sequencing of the outbreak strain provided insight into the stepwise acquisition of st and stx and the evolutionary history of STEC-ETEC pathotypes. The integration of epidemiological data and whole-genome sequencing for routine surveillance of gastrointestinal pathogens is key to understanding the emergence of zoonotic hybrid DEC pathotypes and monitoring foodborne threats to public health.

Comparison of three source attribution methods applied to whole genome sequencing data of monophasic and biphasic Salmonella Typhimurium isolates from the British Isles and Denmark

Methodologies for source attribution (SA) of foodborne illnesses comprise a rapidly expanding suite of techniques for estimating the most important source or sources of human infection. Recently, the increasing availability of whole genome sequencing (WGS) data for a wide range of bacterial strains has led to the development of novel SA methods. These techniques utilize the unique features of bacterial genomes adapted to different host types and hence offer increased resolution of the outputs. Comparative studies of different SA techniques reliant on WGS data are currently lacking. Here, we critically assessed and compared the outputs of three SA methods: a supervised classification random forest machine learning algorithm (RandomForest), an Accessory genes-Based Source Attribution method (AB_SA), and a Bayesian frequency matching method (Bayesian). Each technique was applied to the WGS data of a panel of 902 reservoir host and human monophasic and biphasic Salmonella enterica subsp. enterica serovar Typhimurium isolates sampled in the British Isles (BI) and Denmark from 2012 to 2016. Additionally, for RandomForest and Bayesian, we explored whether utilization of accessory genome features as model inputs improved attribution accuracy of these methods over using the core genome derived features only. Results indicated that this was the case for RandomForest, but for Bayesian the overall attribution estimates varied little regardless of the inclusion or not of the accessory genome features. All three methods attributed the vast majority of human isolates to the Pigs primary source class, which was expected given the known high relative prevalence rates in pigs, and hence routes of infection into the human population, of monophasic and biphasic S. Typhimurium in the BI and Denmark. The accuracy of AB_SA was lower than of RandomForest when attributing the primary source classes to the 120 animal test set isolates with known primary sources. A major advantage of both AB_SA and Bayesian was a much faster execution time as compared to RandomForest. Overall, the SA method comparison presented in this study describes the strengths and weaknesses of each of the three methods applied to attributing potential monophasic and biphasic S. Typhimurium animal sources to human infections that could be valuable when deciding which SA methodology would be the most applicable to foodborne disease outbreak scenarios involving monophasic and biphasic S. Typhimurium.

Genotypic analysis of Shiga toxin-producing Escherichia coli clonal complex 17 in England and Wales, 2014-2022

Introduction. Shiga toxin-producing Escherichia coli (STEC) are zoonotic, gastrointestinal pathogens characterized by the presence of the Shiga toxin (stx) gene. Historically, STEC O157:H7 clonal complex (CC) 11 has been the most clinically significant serotype; however, recently there has been an increase in non-O157 STEC serotypes, including STEC O103:H2 belonging to CC17.Gap statement. STEC O103:H2 is an STEC serotype frequently isolated in England, although little is known about the epidemiology, clinical significance, associated public health burden or evolutionary context of this strain.Aim. Surveillance data and whole-genome sequencing data were analysed to determine the microbiological characteristics and public health burden of CC17, including the clinically significant serotype O103:H2, in England and Wales.Methodology. Isolates of E. coli belonging to CC17 (n=425) submitted to the Gastrointestinal Bacteria Reference Unit from 2014 to 2022 were whole genome sequenced, integrated with enhanced surveillance questionnaire data and analysed retrospectively.Results. Overall, diagnoses of CC17 infection increased every year since 2014. Most cases were female (58.5%), with the highest proportion of cases belonging to the 0-4 age group (n=83/424, 19.6%). Clinical presentation data identified diarrhoea (92.1%), abdominal pain (72.4%) and blood in stool (55.3%) as the most frequent symptoms, while 20.4% cases were admitted to hospital and 1.3% developed haemolytic uraemic syndrome. The five most common established serotypes were O103:H2 (64.5%), O123:H2 (11.1%), O151:H2 (6.6%), O71:H2 (3.3%) and O4:H2 (2.6%). The majority of CC17 isolates (78.6%) had the stx1a/eae virulence gene combination. Nine outbreak clusters of STEC infections that were mainly geographically dispersed and temporally related were identified and associated with foodborne transmission.Conclusions. Nationwide implementation of PCR to detect non-O157 STEC and improvements to algorithms for the follow-up of PCR-positive faecal specimens is recommended. Enhanced surveillance is necessary to assess the incidence of CC17 infection and overall burden of this CC within the UK population.

Microbiological quality of vegan alternatives to dairy and meat products in England during 2022-3

AIMS

Plant-based alternatives to meat and dairy products have become increasingly popular in the UK. Despite a public perception that they have a relatively low microbiological risk, outbreaks of illness have been linked with these foods. This study aimed to assess the microbiological safety and quality of vegan alternatives to dairy and meat products available in England.

METHODS AND RESULTS

Samples were collected between September 2022 and March 2023 from retail, production, and catering premises, and tested for a range of bacterial pathogens and hygiene indicators using standard procedures. A total of 937 samples were tested, of which 92% were of a satisfactory microbiological quality, 3% were borderline, and 5% were unsatisfactory. Those interpreted as unsatisfactory were due to elevated counts of Enterobacteriaceae and Escherichia coli (indicators of poor hygiene) rather than pathogenic microorganisms. Listeria monocytogenes was present in five samples of tofu, all from the same producer (all at counts of <100 CFU g-1), while other Listeria species were detected at counts of <20 CFU g-1 in two burgers and two 'vegan chicken' products. The majority of samples did not have pH and water activity values that would significantly contribute to preventing microbial growth: 62.4% had pH > 5.0 and 82.4% had Aw > 0.94.

CONCLUSIONS

The majority of vegan products examined were of a satisfactory quality, but results demonstrate that microbiological control must be maintained using appropriate processing and storage temperatures, and application of a safe length of shelf life.

An outbreak of Shiga toxin-producing Escherichia coli (STEC) O157:H7 associated with contaminated lettuce and the cascading risks from climate change, the United Kingdom, August to September 2022

Shiga-toxin producing Escherichia coli (STEC) O157 is a food-borne pathogen which causes gastrointestinal illness in humans. Ruminants are considered the main reservoir of infection, and STEC exceedance has been associated with heavy rainfall. In September 2022, a large outbreak of STEC O157:H7 was identified in the United Kingdom (UK). A national-level investigation was undertaken to identify the source of the outbreak and inform risk mitigation strategies. Whole genome sequencing (WGS) was used to identify outbreak cases. Overall, 259 cases with illness onset dates between 5 August and 12 October 2022, were confirmed across the UK. Epidemiological investigations supported a UK grown, nationally distributed, short shelf-life food item as the source of the outbreak. Analytical epidemiology and food chain analysis suggested lettuce as the likely vehicle of infection. Food supply chain tracing identified Grower X as the likely implicated producer. Independent of the food chain investigations, a novel geospatial analysis triangulating meteorological, flood risk, animal density and land use data was developed, also identifying Grower X as the likely source. Novel geospatial analysis and One Health approaches are potential tools for upstream data analysis to predict and prevent contamination events before they occur and to support evidence generation in outbreak investigations.

refMLST: reference-based multilocus sequence typing enables universal bacterial typing

BACKGROUND

Commonly used approaches for genomic investigation of bacterial outbreaks, including SNP and gene-by-gene approaches, are limited by the requirement for background genomes and curated allele schemes, respectively. As a result, they only work on a select subset of known organisms, and fail on novel or less studied pathogens. We introduce refMLST, a gene-by-gene approach using the reference genome of a bacterium to form a scalable, reproducible and robust method to perform outbreak investigation.

RESULTS

When applied to multiple outbreak causing bacteria including 1263 Salmonella enterica, 331 Yersinia enterocolitica and 6526 Campylobacter jejuni genomes, refMLST enabled consistent clustering, improved resolution, and faster processing in comparison to commonly used tools like chewieSnake.

CONCLUSIONS

refMLST is a novel multilocus sequence typing approach that is applicable to any bacterial species with a public reference genome, does not require a curated scheme, and automatically accounts for genetic recombination.

AVAILABILITY AND IMPLEMENTATION

refMLST is freely available for academic use at https://bugseq.com/academic .

Can genomics and meteorology predict outbreaks of legionellosis in urban settings?

Legionella pneumophila is ubiquitous and sporadically infects humans causing Legionnaire's disease (LD). Globally, reported cases of LD have risen fourfold from 2000 to 2014. In 2016, Sydney, Australia was the epicenter of an outbreak caused by L. pneumophila serogroup 1 (Lpsg1). Whole-genome sequencing was instrumental in identifying the causal clone which was found in multiple locations across the city. This study examined the epidemiology of Lpsg1 in an urban environment, assessed typing schemes to classify resident clones, and investigated the association between local climate variables and LD outbreaks. Of 223 local Lpsg1 isolates, we identified dominant clones with one clone isolated from patients in high frequency during outbreak investigations. The core genome multi-locus sequence typing scheme was the most reliable in identifying this Lpsg1 clone. While an increase in humidity and rainfall was found to coincide with a rise in LD cases, the incidence of the major L. pneumophila outbreak clone did not link to weather phenomena. These findings demonstrated the role of high-resolution typing and weather context assessment in determining source attribution for LD outbreaks in urban settings, particularly when clinical isolates remain scarce.IMPORTANCEWe investigated the genomic and meteorological influences of infections caused by Legionella pneumophila in Sydney, Australia. Our study contributes to a knowledge gap of factors that drive outbreaks of legionellosis compared to sporadic infections in urban settings. In such cases, clinical isolates can be rare, and thus, other data are needed to inform decision-making around control measures. The study revealed that core genome multi-locus sequence typing is a reliable and adaptable technique when investigating Lpsg1 outbreaks. In Sydney, the genomic profile of Lpsg1 was dominated by a single clone, which was linked to numerous community cases over a period of 40 years. Interestingly, the peak in legionellosis cases during Autumn was not associated with this prevalent outbreak clone. Incorporating meteorological data with Lpsg1 genomics can support risk assessment strategies for legionellosis in urban environments, and this approach may be relevant for other densely populated regions globally.

mtDNA "nomenclutter" and its consequences on the interpretation of genetic data

Population-based studies of human mitochondrial genetic diversity often require the classification of mitochondrial DNA (mtDNA) haplotypes into more than 5400 described haplogroups, and further grouping those into hierarchically higher haplogroups. Such secondary haplogroup groupings (e.g., "macro-haplogroups") vary across studies, as they depend on the sample quality, technical factors of haplogroup calling, the aims of the study, and the researchers' understanding of the mtDNA haplogroup nomenclature. Retention of historical nomenclature coupled with a growing number of newly described mtDNA lineages results in increasingly complex and inconsistent nomenclature that does not reflect phylogeny well. This "clutter" leaves room for grouping errors and inconsistencies across scientific publications, especially when the haplogroup names are used as a proxy for secondary groupings, and represents a source for scientific misinterpretation. Here we explore the effects of phylogenetically insensitive secondary mtDNA haplogroup groupings, and the lack of standardized secondary haplogroup groupings on downstream analyses and interpretation of genetic data. We demonstrate that frequency-based analyses produce inconsistent results when different secondary mtDNA groupings are applied, and thus allow for vastly different interpretations of the same genetic data. The lack of guidelines and recommendations on how to choose appropriate secondary haplogroup groupings presents an issue for the interpretation of results, as well as their comparison and reproducibility across studies. To reduce biases originating from arbitrarily defined secondary nomenclature-based groupings, we suggest that future updates of mtDNA phylogenies aimed for the use in mtDNA haplogroup nomenclature should also provide well-defined and standardized sets of phylogenetically meaningful algorithm-based secondary haplogroup groupings such as "macro-haplogroups", "meso-haplogroups", and "micro-haplogroups". Ideally, each of the secondary haplogroup grouping levels should be informative about different human population history events. Those phylogenetically informative levels of haplogroup groupings can be easily defined using TreeCluster, and then implemented into haplogroup callers such as HaploGrep3. This would foster reproducibility across studies, provide a grouping standard for population-based studies, and reduce errors associated with haplogroup nomenclatures in future studies.

Genetic Diversity in Salmonella enterica in Outbreaks of Foodborne and Zoonotic Origin in the USA in 2006-2017

Whole genome sequencing is replacing traditional laboratory surveillance methods as the primary tool to track and characterize clusters and outbreaks of the foodborne and zoonotic pathogen Salmonella enterica (S. enterica). In this study, 438 S. enterica isolates representing 35 serovars and 13 broad vehicle categories from one hundred epidemiologically confirmed outbreaks were evaluated for genetic variation to develop epidemiologically relevant interpretation guidelines for Salmonella disease cluster detection. The Illumina sequences were analyzed by core genome multi-locus sequence typing (cgMLST) and screened for antimicrobial resistance (AR) determinants and plasmids. Ninety-three of the one hundred outbreaks exhibited a close allele range (less than 10 allele differences with a subset closer than 5). The remaining seven outbreaks showed increased variation, of which three were considered polyclonal. A total of 16 and 28 outbreaks, respectively, showed variations in the AR and plasmid profiles. The serovars Newport and I 4,[5],12:i:-, as well as the zoonotic and poultry product vehicles, were overrepresented among the outbreaks, showing increased variation. A close allele range in cgMLST profiles can be considered a reliable proxy for epidemiological relatedness for the vast majority of S. enterica outbreak investigations. Variations associated with mobile elements happen relatively frequently during outbreaks and could be reflective of changing selective pressures.

The microbiological quality of flour products in the UK with respect to Salmonella and Shiga-toxin-producing Escherichia coli

AIM

To investigate the possible contamination of raw flour and raw flour-based products, such as pancake/batter mixes, with Salmonella, generic Escherichia coli, and Shiga-toxin-producing E. coli (STEC). Samples included flours available for sale in the UK over a period of four months (January to April 2020). The Bread and Flour regulations, 1998 state the permitted ingredients in flour and bread but it does not specify the regular monitoring of the microbiological quality of flour and flour-based products.

METHODS AND RESULTS

Samples of raw flour were collected by local authority sampling officers in accordance with current guidance on microbiological food sampling then transported to the laboratory for examination. Microbiological testing was performed to detect Salmonella spp., generic E. coli, and STEC characterized for the presence of STEC virulence genes: stx1, stx2, and subtypes, eae, ipah, aggR, lt, sth, and stp, using molecular methods Polymerase Chain Reaction (PCR). Of the 882 flours sampled, the incidence of Salmonella was 0.1% (a single positive sample that contained multiple ingredients such as flour, dried egg, and dried milk, milled in the UK), and 68 samples (7.7%) contained generic E. coli at a level of >20 CFU/g. Molecular characterization of flour samples revealed the presence of the Shiga-toxin (stx) gene in 10 samples (5 imported and 5 from the UK) (1.1%), from which STEC was isolated from 7 samples (0.8%). Salmonella and STEC isolates were sequenced to provide further characterization of genotypes and to compare to sequences of human clinical isolates held in the UKHSA archive. Using our interpretive criteria based on genetic similarity, none of the STEC flour isolates correlated with previously observed human cases, while the singular Salmonella serotype Newport isolate from the mixed ingredient product was similar to a human case in 2019, from the UK, of S. Newport. Although there have been no reported human cases of STEC matching the isolates from these flour samples, some of the same serotypes and stx subtypes detected are known to have caused illness in other contexts.

CONCLUSION

Results indicate that while the incidence was low, there is a potential for the presence of Salmonella and STEC in flour, and a genetic link was demonstrated between a Salmonella isolate from a flour-based product and a human case of salmonellosis.

DODGE: automated point source bacterial outbreak detection using cumulative long term genomic surveillance

SUMMARY

The reliable and timely recognition of outbreaks is a key component of public health surveillance for foodborne diseases. Whole genome sequencing (WGS) offers high resolution typing of foodborne bacterial pathogens and facilitates the accurate detection of outbreaks. This detection relies on grouping WGS data into clusters at an appropriate genetic threshold. However, methods and tools for selecting and adjusting such thresholds according to the required resolution of surveillance and epidemiological context are lacking. Here we present DODGE (Dynamic Outbreak Detection for Genomic Epidemiology), an algorithm to dynamically select and compare these genetic thresholds. DODGE can analyse expanding datasets over time and clusters that are predicted to correspond to outbreaks (or "investigation clusters") can be named with established genomic nomenclature systems to facilitate integrated analysis across jurisdictions. DODGE was tested in two real-world Salmonella genomic surveillance datasets of different duration, 2 months from Australia and 9 years from the United Kingdom. In both cases only a minority of isolates were identified as investigation clusters. Two known outbreaks in the United Kingdom dataset were detected by DODGE and were recognized at an earlier timepoint than the outbreaks were reported. These findings demonstrated the potential of the DODGE approach to improve the effectiveness and timeliness of genomic surveillance for foodborne diseases and the effectiveness of the algorithm developed.

AVAILABILITY AND IMPLEMENTATION

DODGE is freely available at https://github.com/LanLab/dodge and can easily be installed using Conda.

Emergence and control of an outbreak of PVL-positive MRSA in a UK-based maternity setting

This paper aims to describe the investigation and control of an outbreak of USA300 ST8 Panton-Valentine leucocidin (PVL)-positive meticillin-resistant Staphylococcus aureus (MRSA), confirmed by whole genome sequencing (WGS), within a maternity and neonatal setting in the UK. The identification of two linked PVL-MRSA cases led to an outbreak investigation. A lookback exercise conducted using the infection control surveillance database, typing of saved MRSA isolates, enhanced patient screening, and staff screening were used to identify further cases. Environmental screening was also performed. Genetic relatedness between isolates was assessed by WGS. During the outbreak, 18 cases were identified between 11th July 2021 and 22nd December 2022: 10 cases were infections and eight cases were colonizations. A healthcare worker (HCW) tested positive for colonization with the same strain, and environmental swabbing identified contaminated information technology equipment in the hospital. The outbreak was brought to an end by exclusion of the colonized HCW from work, and infection prevention and control measures. Since the end of the outbreak, cases of PVL-MRSA with similar molecular profiles have been found in the community. It is likely that the HCW played a role in the transmission of PVL-MRSA. Their exclusion from work and decolonization were key to preventing further cases. WGS was valuable in identifying and linking cases. The identification of community cases of PVL-MRSA with similar molecular profiles confirms transmission of the organism outside of healthcare settings.

Characterization and resistance mechanism of phage-resistant strains of Salmonella enteritidis

In the face of the increasingly severe problem of antibiotic resistance, phage therapy is regarded as a highly potential alternative. Compared with traditional antimicrobial agents, a key research area of phage therapy is the study of phage-resistant mutant bacteria. To effectively monitor and prevent this resistance, it is crucial to conduct in-depth exploration of the mechanism behind phage resistance. In this study, a strain of Salmonella enteritidis (sm140) and the corresponding phage (Psm140) were isolated from chicken liver and sewage, respectively. Using the double-layer plate method, successfully screened out phage-resistant mutant strains. Whole-genome resequencing of 3 resistant strains found that the wbaP gene of all 3 strains had mutations at a specific position (1,118), with the base changing from G to A. This mutation causes the gene-encoded glycine to be replaced by aspartic acid. Subsequent studies found that the frequency of this gene mutation is extremely high, reaching 84%, and all mutations occur at the same position. To further explore the relationship between the wbaP gene and phage resistance, knockout strains and complement strains of the wbaP gene were constructed. The experimental results confirmed the association between the wbaP gene and phage resistance. At the same time, biological characteristics and virulence were evaluated for wild strains, resistant strains, knockout strains, and complement strains. It was found that mutations or deletions of the wbaP gene lead to a decrease in bacterial environmental adaptability and virulence. Through systematic research on the mechanism and biological characteristics of phage resistance, this study provides important references and guidance for the development of new phage therapies, promoting progress in the field of antimicrobial treatment. At the same time, the emergence of phage resistance due to wbaP gene mutations is reported for the first time in salmonella, providing a new perspective and ideas for further studying phage resistance mechanisms.

Salmonella Saintpaul outbreak associated with cantaloupe consumption, the United Kingdom and Portugal, September to November 2023

In September 2023, the UK Health Security Agency identified cases of Salmonella Saintpaul distributed across England, Scotland, and Wales, all with very low genetic diversity. Additional cases were identified in Portugal following an alert raised by the United Kingdom. Ninety-eight cases with a similar genetic sequence were identified, 93 in the United Kingdom and 5 in Portugal, of which 46% were aged under 10 years. Cases formed a phylogenetic cluster with a maximum distance of six single nucleotide polymorphisms (SNPs) and average of less than one SNP between isolates. An outbreak investigation was undertaken, including a case-control study. Among the 25 UK cases included in this study, 13 reported blood in stool and 5 were hospitalized. One hundred controls were recruited via a market research panel using frequency matching for age. Multivariable logistic regression analysis of food exposures in cases and controls identified a strong association with cantaloupe consumption (adjusted odds ratio: 14.22; 95% confidence interval: 2.83-71.43; p-value: 0.001). This outbreak, together with other recent national and international incidents, points to an increase in identifications of large outbreaks of Salmonella linked to melon consumption. We recommend detailed questioning and triangulation of information sources to delineate consumption of specific fruit varieties during Salmonella outbreaks.

SaLTy: a novel Staphylococcus aureus Lineage Typer

Staphylococcus aureus asymptomatically colonises 30 % of humans but can also cause a range of diseases, which can be fatal. In 2017 S. aureus was associated with 20 000 deaths in the USA alone. Dividing S. aureus isolates into smaller sub-groups can reveal the emergence of distinct sub-populations with varying potential to cause infections. Despite multiple molecular typing methods categorising such sub-groups, they do not take full advantage of S. aureus genome sequences when describing the fundamental population structure of the species. In this study, we developed Staphylococcus aureus Lineage Typing (SaLTy), which rapidly divides the species into 61 phylogenetically congruent lineages. Alleles of three core genes were identified that uniquely define the 61 lineages and were used for SaLTy typing. SaLTy was validated on 5000 genomes and 99.12 % (4956/5000) of isolates were assigned the correct lineage. We compared SaLTy lineages to previously calculated clonal complexes (CCs) from BIGSdb (n=21 173). SALTy improves on CCs by grouping isolates congruently with phylogenetic structure. SaLTy lineages were further used to describe the carriage of Staphylococcal chromosomal cassette containing mecA (SCCmec) which is carried by methicillin-resistant S. aureus (MRSA). Most lineages had isolates lacking SCCmec and the four largest lineages varied in SCCmec over time. Classifying isolates into SaLTy lineages, which were further SCCmec typed, allowed SaLTy to describe high-level MRSA epidemiology. We provide SaLTy as a simple typing method that defines phylogenetic lineages (https://github.com/LanLab/SaLTy). SaLTy is highly accurate and can quickly analyse large amounts of S. aureus genome data. SaLTy will aid the characterisation of S. aureus populations and ongoing surveillance of sub-groups that threaten human health.

Genomic analysis of an outbreak of Shiga toxin-producing Escherichia coli O183:H18 in the United Kingdom, 2023

In June 2023, UKHSA surveillance systems detected an outbreak of severe gastrointestinal symptoms caused by a rare serotype of Shiga toxin-producing Escherichia coli, STEC O183:H18. There were 26 cases aged 6 months to 74 years (42 % cases were aged 0-9 years), distributed across the UK with onset dates range between 22 May 2023 and 4 July 2023. The epidemiological and food chain investigations were inconclusive, although meat products made from beef mince were implicated as a potential vehicle. The outbreak strain belonged to sequence type (ST) 657 and harboured a Shiga toxin (stx) subtype stx2a located on a prophage that was unique in the UKHSA stx-encoding bacteriophage database. Plasmid encoded, putative virulence genes subA, ehxA, saa, iha, lpfA and iss were detected, however, the established STEC virulence genes involved in attachment to the gut mucosa (eae and aggR) were absent. The acquisition of stx across the global population structure of ST657 appeared to correspond with the presence of subA, ehxA, saa, iha, lpfA and iss. During the outbreak investigation, we used long read sequencing to characterise the plasmid and prophage content of this atypical STEC, to look for evidence to explain its recent emergence. Although we were unable to determine source and transmission route of the outbreak strain, the genomic analysis revealed potential clues as to how novel strains for STEC evolve. With the implementation of PCR capable of detecting all STEC, and genome sequencing for typing and virulence profiling, we have the tools to enable us to monitor the changing landscape of STEC. Improvements in the standardised collection of epidemiological data and trace-back strategies within the food industry, will ensure we have a surveillance system capable of alerting us to emerging threats to public health.

Presence of phage-plasmids in multiple serovars of Salmonella enterica

Evidence is accumulating in the literature that the horizontal spread of antimicrobial resistance (AMR) genes mediated by bacteriophages and bacteriophage-like plasmid (phage-plasmid) elements is much more common than previously envisioned. For instance, we recently identified and characterized a circular P1-like phage-plasmid harbouring a bla CTX-M-15 gene conferring extended-spectrum beta-lactamase (ESBL) resistance in Salmonella enterica serovar Typhi. As the prevalence and epidemiological relevance of such mechanisms has never been systematically assessed in Enterobacterales, in this study we carried out a follow-up retrospective analysis of UK Salmonella isolates previously sequenced as part of routine surveillance protocols between 2016 and 2021. Using a high-throughput bioinformatics pipeline we screened 47 784 isolates for the presence of the P1 lytic replication gene repL, identifying 226 positive isolates from 25 serovars and demonstrating that phage-plasmid elements are more frequent than previously thought. The affinity for phage-plasmids appears highly serovar-dependent, with several serovars being more likely hosts than others; most of the positive isolates (170/226) belonged to S. Typhimurium ST34 and ST19. The phage-plasmids ranged between 85.8 and 98.2 kb in size, with an average length of 92.1 kb; detailed analysis indicated a high amount of diversity in gene content and genomic architecture. In total, 132 phage-plasmids had the p0111 plasmid replication type, and 94 the IncY type; phylogenetic analysis indicated that both horizontal and vertical gene transmission mechanisms are likely to be involved in phage-plasmid propagation. Finally, phage-plasmids were present in isolates that were resistant and non-resistant to antimicrobials. In addition to providing a first comprehensive view of the presence of phage-plasmids in Salmonella, our work highlights the need for a better surveillance and understanding of phage-plasmids as AMR carriers, especially through their characterization with long-read sequencing.

Development and validation of a random forest algorithm for source attribution of animal and human Salmonella Typhimurium and monophasic variants of S. Typhimurium isolates in England and Wales utilising whole genome sequencing data

Source attribution has traditionally involved combining epidemiological data with different pathogen characterisation methods, including 7-gene multi locus sequence typing (MLST) or serotyping, however, these approaches have limited resolution. In contrast, whole genome sequencing data provide an overview of the whole genome that can be used by attribution algorithms. Here, we applied a random forest (RF) algorithm to predict the primary sources of human clinical Salmonella Typhimurium (S. Typhimurium) and monophasic variants (monophasic S. Typhimurium) isolates. To this end, we utilised single nucleotide polymorphism diversity in the core genome MLST alleles obtained from 1,061 laboratory-confirmed human and animal S. Typhimurium and monophasic S. Typhimurium isolates as inputs into a RF model. The algorithm was used for supervised learning to classify 399 animal S. Typhimurium and monophasic S. Typhimurium isolates into one of eight distinct primary source classes comprising common livestock and pet animal species: cattle, pigs, sheep, other mammals (pets: mostly dogs and horses), broilers, layers, turkeys, and game birds (pheasants, quail, and pigeons). When applied to the training set animal isolates, model accuracy was 0.929 and kappa 0.905, whereas for the test set animal isolates, for which the primary source class information was withheld from the model, the accuracy was 0.779 and kappa 0.700. Subsequently, the model was applied to assign 662 human clinical cases to the eight primary source classes. In the dataset, 60/399 (15.0%) of the animal and 141/662 (21.3%) of the human isolates were associated with a known outbreak of S. Typhimurium definitive type (DT) 104. All but two of the 141 DT104 outbreak linked human isolates were correctly attributed by the model to the primary source classes identified as the origin of the DT104 outbreak. A model that was run without the clonal DT104 animal isolates produced largely congruent outputs (training set accuracy 0.989 and kappa 0.985; test set accuracy 0.781 and kappa 0.663). Overall, our results show that RF offers considerable promise as a suitable methodology for epidemiological tracking and source attribution for foodborne pathogens.

Re-analysis of an outbreak of Shiga toxin-producing Escherichia coli O157:H7 associated with raw drinking milk using Nanopore sequencing

The aim of this study was to compare Illumina and Oxford Nanopore Technology (ONT) sequencing data to quantify genetic variation to assess within-outbreak strain relatedness and characterise microevolutionary events in the accessory genomes of a cluster of 23 genetically and epidemiologically linked isolates related to an outbreak of Shiga toxin-producing Escherichia coli O157:H7 caused by the consumption of raw drinking milk. There were seven discrepant variants called between the two technologies, five were false-negative or false-positive variants in the Illumina data and two were false-negative calls in ONT data. After masking horizontally acquired sequences such as prophages, analysis of both short and long-read sequences revealed the 20 isolates linked to the outbreak in 2017 had a maximum SNP distance of one SNP between each other, and a maximum of five SNPs when including three additional strains identified in 2019. Analysis of the ONT data revealed a 47 kbp deletion event in a terminal compound prophage within one sample relative to the remaining samples, and a 0.65 Mbp large chromosomal rearrangement (inversion), within one sample relative to the remaining samples. Furthermore, we detected two bacteriophages encoding the highly pathogenic Shiga toxin (Stx) subtype, Stx2a. One was typical of Stx2a-phage in this sub-lineage (Ic), the other was atypical and inserted into a site usually occupied by Stx2c-encoding phage. Finally, we observed an increase in the size of the pO157 IncFIB plasmid (1.6 kbp) in isolates from 2019 compared to those from 2017, due to the duplication of insertion elements within the plasmids from the more recently isolated strains. The ability to characterize the accessory genome in this way is the first step to understanding the significance of these microevolutionary events and their impact on the genome plasticity and virulence between strains of this zoonotic, foodborne pathogen.

Geographical and temporal distribution of multidrug-resistant Salmonella Infantis in Europe and the Americas

Recently emerged S. Infantis strains carrying resistance to several commonly used antimicrobials have been reported from different parts of the globe, causing human cases of salmonellosis and with occurrence reported predominantly in broiler chickens. Here, we performed phylogenetic and genetic clustering analyses to describe the population structure of 417 S. Infantis originating from multiple European countries and the Americas collected between 1985 and 2019. Of these, 171 were collected from 56 distinct premises located in England and Wales (E/W) between 2009 and 2019, including isolates linked to incursions of multidrug-resistant (MDR) strains from Europe associated with imported poultry meat. The analysis facilitated the comparison of isolates from different E/W sources with isolates originating from other countries. There was a high degree of congruency between the outputs of different types of population structure analyses revealing that the E/W and central European (Germany, Hungary, and Poland) isolates formed several disparate groups, which were distinct from the cluster relating to the United States (USA) and Ecuador/Peru, but that isolates from Brazil were closely related to the E/W and the central European isolates. Nearly half of the analysed strains/genomes (194/417) harboured the IncFIB(pN55391) replicon typical of the "parasitic" pESI-like megaplasmid found in diverse strains of S. Infantis. The isolates that contained the IncFIB(pN55391) replicon clustered together, despite originating from different parts of the globe. This outcome was corroborated by the time-measured phylogeny, which indicated that the initial acquisition of IncFIB(pN55391) likely occurred in Europe in the late 1980s, with a single introduction of IncFIB(pN55391)-carrying S. Infantis to the Americas several years later. Most of the antimicrobial resistance (AMR) genes were identified in isolates that harboured one or more different plasmids, but based on the short-read assemblies, only a minority of the resistance genes found in these isolates were identified as being associated with the detected plasmids, whereas the hybrid assemblies comprising the short and long reads demonstrated that the majority of the identified AMR genes were associated with IncFIB(pN55391) and other detected plasmid replicon types. This finding underlies the importance of applying appropriate methodologies to investigate associations of AMR genes with bacterial plasmids.

Clinical and public health implications of increasing notifications of LEE-negative Shiga toxin-producing Escherichia coli in England, 2014-2022

Introduction. Shiga toxin-producing Escherichia coli (STEC) belong to a diverse group of gastrointestinal pathogens. The pathogenic potential of STEC is enhanced by the presence of the pathogenicity island called the Locus of Enterocyte Effacement (LEE), including the intimin encoding gene eae.Gap statement. STEC serotypes O128:H2 (Clonal Complex [CC]25), O91:H14 (CC33), and O146:H21 (CC442) are consistently in the top five STEC serotypes isolated from patients reporting gastrointestinal symptoms in England. However, they are eae/LEE-negative and perceived to be a low risk to public health, and we know little about their microbiology and epidemiology.Aim. We analysed clinical outcomes and genome sequencing data linked to patients infected with LEE-negative STEC belonging to CC25 (O128:H2, O21:H2), CC33 (O91:H14) and, and CC442 (O146:H21, O174:H21) in England to assess the risk to public health.Results. There was an almost ten-fold increase between 2014 and 2022 in the detection of all STEC belonging to CC25, CC33 and CC442 (2014 n=38, 2022 n=336), and a total of 1417 cases. There was a higher proportion of female cases (55-70 %) and more adults than children, with patients aged between 20-40 and >70 most at risk across the different serotypes. Symptoms were consistent across the three dominant serotypes O91:H14 (CC33), O146:H21 (CC442) and O128:H2 (CC25) (diarrhoea >75 %; bloody diarrhoea 25-32 %; abdominal pain 64-72 %; nausea 37-45 %; vomiting 10-24 %; and fever 27-30 %). Phylogenetic analyses revealed multiple events of acquisition and loss of different stx-encoding prophage. Additional putative virulence genes were identified including iha, agn43 and subA.Conclusions. Continued monitoring and surveillance of LEE-negative STEC infections is essential due to the increasing burden of infectious intestinal disease, and the risk that highly pathogenic strains may emerge following acquisition of the Shiga toxin subtypes associated with the most severe clinical outcomes.

WGS of a cluster of MDR Shigella sonnei utilizing Oxford Nanopore R10.4.1 long-read sequencing

OBJECTIVES

To utilize long-read nanopore sequencing (R10.4.1 flowcells) for WGS of a cluster of MDR Shigella sonnei, specifically characterizing genetic predictors of antimicrobial resistance (AMR).

METHODS

WGS was performed on S. sonnei isolates identified from stool and blood between September 2021 and October 2022. Bacterial DNA from clinical isolates was extracted on the MagNA Pure 24 and sequenced on the GridION utilizing R10.4.1 flowcells. Phenotypic antimicrobial susceptibility testing was interpreted based on CLSI breakpoints. Sequencing data were processed with BugSeq, and AMR was assessed with BugSplit and ResFinder.

RESULTS

Fifty-six isolates were sequenced, including 53 related to the cluster of cases. All cluster isolates were identified as S. sonnei by sequencing, with global genotype 3.6.1.1.2 (CipR.MSM5), MLST 152 and PopPUNK cluster 3. Core genome MLST (cgMLST, examining 2513 loci) and reference-based MLST (refMLST, examining 4091 loci) both confirmed the clonality of the isolates. Cluster isolates were resistant to ampicillin (blaTEM-1), trimethoprim/sulfamethoxazole (dfA1, dfrA17; sul1, sul2), azithromycin (ermB, mphA) and ciprofloxacin (gyrA S83L, gyrA D87G, parC S80I). No genomic predictors of resistance to carbapenems were identified.

CONCLUSIONS

WGS with R10.4.1 enabled rapid sequencing and identification of an MDR S. sonnei community cluster. Genetic predictors of AMR were concordant with phenotypic antimicrobial susceptibility testing.

Multinational collaboration in solving a European Salmonella Braenderup outbreak linked to imported melons, 2021

A genomic cluster of Salmonella Braenderup ST22, a serovar of Salmonella enterica subsp. enterica which causes symptoms of gastrointestinal illness, was notified by Danish authorities to the European Centre for Disease Prevention and Control (ECDC) on 3 May 2021. By 6 July 2021, S. Braenderup outbreak cases (n = 348) had been reported from 12 countries in the European Union/European Economic Area (EU/EEA) and the United Kingdom (UK), including 68 hospitalised cases. With support from affected EU/EEA countries, and in partnership with the European Food Safety Authority (EFSA), ECDC established an international outbreak investigation team to rapidly identify the source and prevent outbreak spread. Consumption information was shared with affected countries through a standard line list, revealing that 124 of 197 cases (63%) reported having eaten (any) melons within 7 days prior to disease onset. The speed and completeness of the investigation, which identified the outbreak vehicle as galia melons imported from Honduras in June 2021, was a direct result of extensive collaboration and information sharing between countries' national food safety and public health authorities. This article describes the outbreak and the benefits, successes, and challenges of multi-country collaboration for consideration in future large foodborne outbreaks across Europe.

Genomic epidemiology of the clinically dominant clonal complex 1 in the Listeria monocytogenes population in the UK

Listeria monocytogenes is a food-borne pathogen, typically affecting the elderly, immunocompromised patients and pregnant women. The aim of this study was to determine the population structure of L. monocytogenes clonal complex 1 (CC1) in the UK and describe the genomic epidemiology of this clinically significant CC. We interrogated a working dataset of 4073 sequences of L. monocytogenes isolated between January 2015 and December 2020 from human clinical specimens, food and/or food-production environments. A minimum spanning tree was reconstructed to determine the population structure of L. monocytogenes in the UK. Subsequent analysis focused on L. monocytogenes CC1, as the cause of the highest proportion of invasive listeriosis in humans. Sequencing data was integrated with metadata on food and environmental isolates, and information from patient questionnaires, including age, sex and clinical outcomes. All isolates either belonged to lineage I (n=1299/4073, 32%) or lineage II (n=2774/4073, 68%), with clinical isolates from human cases more likely to belong to lineage I (n=546/928, 59%) and food isolates more likely to belong to lineage II (n=2352/3067, 77%). Of the four largest CCs, CC1 (n=237) had the highest proportion of isolates from human cases of disease (CC1 n=160/237, 67.5 %; CC121 n=13/843, 2 %; CC9 n=53/360, 15 %; CC2 n=69/339, 20%). Within CC1, most cases were female (n=95/160, 59%, P=0.01771) and the highest proportion of cases were in people >60 years old (39/95, 41%, P=1.314×10-6) with a high number of them aged 20-39 years old (n=35/95, 37%) most linked to pregnancy-related listeriosis (n=29/35, 83%). Most of the male cases were in men aged over 60 years old (40/65, 62%), and most of the fatal cases in both males and females were identified in this age group (42/55, 76%). Phylogenetic analysis revealed 23 5 SNP single linkage clusters comprising 80/237 (34 %) isolates with cluster sizes ranging from 2 to 19. Five 5 SNP clusters comprised isolates from human cases and an implicated food item. Expanding the analysis to 25 SNP single linkage clusters resolved an additional two clusters linking human cases to a potential food vehicle. Analysis of demographic and clinical outcome data identified CC1 as a clinically significant cause of invasive listeriosis in the elderly population and in women of child-bearing age. Phylogenetic analysis revealed the population structure of CC1 in the UK comprised small, sparsely populated genomic clusters. Only clusters containing isolates from an implicated food vehicle, or food processing or farming environments, were resolved, emphasizing the need for clinical, food and animal-health agencies to share sequencing data in real time, and the importance of a One Health approach to public-health surveillance of listeriosis.

Whole genome sequencing-based identification of human tuberculosis caused by animal-lineage Mycobacterium orygis

A recently described member of the Mycobacterium tuberculosis complex (MTBC) is Mycobacterium orygis, which can cause disease primarily in animals but also in humans. Although M. orygis has been reported from different geographic regions around the world, due to a lack of proper identification techniques, the contribution of this emerging pathogen to the global burden of zoonotic tuberculosis is not fully understood. In the present work, we report single nucleotide polymorphism (SNP) analysis using whole genome sequencing (WGS) that can accurately identify M. orygis and differentiate it from other members of the MTBC species. WGS-based SNP analysis was performed for 61 isolates from different provinces in Canada that were identified as M. orygis. A total of 56 M. orygis sequences from the public databases were also included in the analysis. Several unique SNPs in the gyrB, PPE55, Rv2042c, leuS, mmpL6, and mmpS6 genes were used to determine their effectiveness as genetic markers for the identification of M. orygis. To the best of our knowledge, five of these SNPs, viz., gyrB 277 (A→G), gyrB 1478 (T→C), leuS 1064 (A→T), mmpL6 486 (T→C), and mmpS6 334 (C→G), are reported for the first time in this study. Our results also revealed several SNPs specific to other species within MTBC. The phylogenetic analysis shows that the studied genomes were genetically diverse and clustered with M. orygis sequences of human and animal origin reported from different geographic locations. Therefore, the present study provides a new insight into the high-confidence identification of M. orygis from MTBC species based on WGS data, which can be useful for reference and diagnostic laboratories.

Whole-genome sequencing links cases dispersed in time, place, and person while supporting healthcare worker management in an outbreak of Panton-Valentine leucocidin meticillin-resistant Staphylococcus aureus; and a review of literature

This is a report on an outbreak of Panton-Valentine leucocidin-producing meticillin-resistant Staphylococcus aureus (PVL-MRSA) in an intensive care unit (ICU) during the COVID-19 pandemic that affected seven patients and a member of staff. Six patients were infected over a period of ten months on ICU by the same strain of PVL-MRSA, and a historic case identified outside of the ICU. All cases were linked to a healthcare worker (HCW) who was colonized with the organism. Failed topical decolonization therapy, without systemic antibiotic therapy, resulted in ongoing transmission and one preventable acquisition of PVL-MRSA. The outbreak identifies the support that may be needed for HCWs implicated in outbreaks. It also demonstrates the role of whole-genome sequencing in identifying dispersed and historic cases related to the outbreak, which in turn aids decision-making in outbreak management and HCW support. This report also includes a review of literature of PVL-MRSA-associated outbreaks in healthcare and highlights the need for review of current national guidance in the management of HCWs' decolonization regimen and return-to-work recommendations in such outbreaks.

Case of Carbapenem-Resistant Salmonella Typhi Infection, Pakistan, 2022

Salmonella Typhi infection in a patient in Pakistan initially responded to standard treatment but failed to respond to subsequent treatment. The first strain was susceptible to carbapenems and azithromycin; subsequent strains harbored the NDM-5 gene. Treatment with a combination of intravenous meropenem and colistin was successful. Carbapenem-resistant Salmonella Typhi emergence will hinder treatment.

Using SNP addresses for Salmonella Typhimurium DT104 in routine veterinary outbreak detection

SNP addresses are a pathogen typing method based on whole-genome sequences (WGSs), assigning groups at seven different levels of genetic similarity. Public health surveillance uses it for several gastro-intestinal infections; this work trialled its use in veterinary surveillance for salmonella outbreak detection. Comparisons were made between temporal and spatio-temporal cluster detection models that either defined cases by their SNP address or by phage type, using historical data sets. Clusters of SNP incidents were effectively detected by both methods, but spatio-temporal models consistently detected these clusters earlier than the corresponding temporal models. Unlike phage type, SNP addresses appeared spatially and temporally limited, which facilitated the differentiation of novel, stable, or expanding clusters in spatio-temporal models. Furthermore, these models flagged spatio-temporal clusters containing only two to three cases at first detection, compared with a median of seven cases in phage-type models. The large number of SNP addresses will require automated methods to implement these detection models routinely. Further work is required to explore how temporal changes and different host species may impact the sensitivity and specificity of cluster detection. In conclusion, given validation with more sequencing data, SNP addresses are likely to be a valuable addition to early warning systems in veterinary surveillance.

Genomic epidemiology of syphilis in England: a population-based study

BACKGROUND

Syphilis is a sexually transmitted bacterial infection caused by Treponema pallidum subspecies pallidum. Since 2012, syphilis rates have risen dramatically in many high-income countries, including England. Although this increase in syphilis prevalence is known to be associated with high-risk sexual activity in gay, bisexual, and other men who have sex with men (GBMSM), cases are rising in heterosexual men and women. The transmission dynamics within and between sexual networks of GBMSM and heterosexual people are not well understood. We aimed to investigate if whole genome sequencing could be used to supplement or enhance epidemiological insights around syphilis transmission.

METHODS

We linked national patient demographic, geospatial, and behavioural metadata to whole T pallidum genome sequences previously generated from patient samples collected from across England between Jan 1, 2012, and Oct 31, 2018, and performed detailed phylogenomic analyses.

FINDINGS

Of 497 English samples submitted for sequencing, we recovered 240 genomes (198 from the UK Health Security Agency reference laboratory and 42 from other laboratories). Three duplicate samples (same patient and collection date) were included in the main phylogenies, but removed from further analyses of English populations, leaving 237 genomes. 220 (92·8%) of 237 samples were from men, nine (3·8%) were from women, and eight (3·4%) were of unknown gender. Samples were mostly from London (n=118 [49·8%]), followed by southeast England (n=29 [12·2%]), northeast England (n=24 [10·1%]), and southwest England (n=15 [6·3%]). 180 (76·0%) of 237 genomes came from GBMSM, compared with 25 (10·5%) from those identifying as men who have sex with women, 15 (6·3%) from men with unrecorded sexual orientation, nine (3·8%) from those identifying as women who have sex with men, and eight (3·4%) from people of unknown gender and sexual orientation. Phylogenomic analysis and clustering revealed two dominant T pallidum sublineages in England. Sublineage 1 was found throughout England and across all patient groups, whereas sublineage 14 occurred predominantly in GBMSM older than 34 years and was absent from samples sequenced from the north of England. These different spatiotemporal trends, linked to demography or behaviour in the dominant sublineages, suggest they represent different sexual networks. By focusing on different regions of England we were able to distinguish a local heterosexual transmission cluster from a background of transmission in GBMSM.

INTERPRETATION

These findings show that, despite extremely close genetic relationships between T pallidum genomes globally, genomics can still be used to identify putative transmission clusters for epidemiological follow-up. This could be of value for deconvoluting putative outbreaks and for informing public health interventions.

FUNDING

Wellcome funding to the Sanger Institute, UK Research and Innovation, National Institute for Health and Care Research, European and Developing Countries Clinical Trials Partnership, and UK Health Security Agency.

Surveillance of antimicrobial resistant Shiga toxin-producing E. coli O157:H7 in England, 2016-2020

OBJECTIVES

Shiga toxin-producing Escherichia coli (STEC) O157:H7 are zoonotic pathogens and transmission to humans occurs via contaminated food or contact with infected animals. The aim of this study was to describe the frequency, and distribution across the phylogeny, of antimicrobial resistance (AMR) determinants in STEC O157:H7 isolated from human cases in England.

METHODS

Short-read whole-genome sequencing data from 1473 isolates of STEC O157:H7 from all seven sub-lineages (Ia-Ic, IIa-IIc and I/II) were mapped to genes known to confer phenotypic resistance to 10 different classes of antibiotic. Long-read sequencing was used to determine the location and genomic architecture of the AMR determinants within phylogenetic clusters exhibiting multidrug resistance.

RESULTS

Overall, 216/1473 (14.7%) isolates had at least one AMR determinant, although the proportion of isolates exhibiting AMR varied by sub-lineage. The highest proportion of AMR determinants were detected in sub-lineages Ib (28/64, 43.7%), I/II (18/51, 35.3%) and IIc (122/440, 27.7%). In all sub-lineages, the most commonly detected AMR determinants conferred resistance to the aminoglycosides, tetracyclines and sulphonamides, while AMR determinants conferring resistance to fluroquinolones, macrolides and third-generation cephalosporins were rarely detected. Long-read sequencing analysis showed that the AMR determinants were co-located on the chromosome in sub-lineages Ib and lineage I/II, whereas those associated with sub-lineage IIc were encoded on the chromosome and/or large plasmids.

CONCLUSIONS

AMR genes were unevenly distributed across the different sub-lineages of STEC O157:H7 and between different clades within the same sub-lineage. Long-read sequencing facilitates tracking the transmission of AMR at the pathogen and mobile genetic element level.

Prevalence of Shiga Toxin-Producing Escherichia coli O157 in Wild Scottish Deer with High Human Pathogenic Potential

Shiga toxin-producing E. coli (STEC) infections associated with wildlife are increasing globally, highlighting many 'spillover' species as important reservoirs for these zoonotic pathogens. A human outbreak of STEC serogroup O157 in 2015 in Scotland, associated with the consumption of venison meat products, highlighted several knowledge gaps, including the prevalence of STEC O157 in Scottish wild deer and the potential risk to humans from wild deer isolates. In this study, we undertook a nationwide survey of wild deer in Scotland and determined that the prevalence of STEC O157 in wild deer is low 0.28% (95% confidence interval = 0.06-0.80). Despite the low prevalence of STEC O157 in Scottish wild deer, identified isolates were present in deer faeces at high levels (>104 colony forming units/g faeces) and had high human pathogenic potential based on whole genome sequencing and virulence gene profiling. A retrospective epidemiological investigation also identified one wild deer isolate from this study as a possible source of a Scottish human outbreak in 2017. These results emphasise the importance of food hygiene practices during the processing of wild deer carcasses for human consumption.

Analysis of Escherichia coli O157 strains in cattle and humans between Scotland and England & Wales: implications for human health

For the last two decades, the human infection frequency of Escherichia coli O157 (O157) in Scotland has been 2.5-fold higher than in England and Wales. Results from national cattle surveys conducted in Scotland and England and Wales in 2014/2015 were combined with data on reported human clinical cases from the same time frame to determine if strain differences in national populations of O157 in cattle could be associated with higher human infection rates in Scotland. Shiga toxin subtype (Stx) and phage type (PT) were examined within and between host (cattle vs human) and nation (Scotland vs England and Wales). For a subset of the strains, whole genome sequencing (WGS) provided further insights into geographical and host association. All three major O157 lineages (I, II, I/II) and most sub-lineages (Ia, Ib, Ic, IIa, IIb, IIc) were represented in cattle and humans in both nations. While the relative contribution of different reservoir hosts to human infection is unknown, WGS analysis indicated that the majority of O157 diversity in human cases was captured by isolates from cattle. Despite comparable cattle O157 prevalence between nations, strain types were localized. PT21/28 (sub-lineage Ic, Stx2a+) was significantly more prevalent in Scottish cattle [odds ratio (OR) 8.7 (2.3-33.7; P<0.001] and humans [OR 2.2 (1.5-3.2); P<0.001]. In England and Wales, cattle had a significantly higher association with sub-lineage IIa strains [PT54, Stx2c; OR 5.6 (1.27-33.3); P=0.011] while humans were significantly more closely associated with sub-lineage IIb [PT8, Stx1 and Stx2c; OR 29 (4.9-1161); P<0.001]. Therefore, cattle farms in Scotland were more likely to harbour Stx2a+O157 strains compared to farms in E and W (P<0.001). There was evidence of limited cattle strain migration between nations and clinical isolates from one nation were more similar to cattle isolates from the same nation, with sub-lineage Ic (mainly PT21/28) exhibiting clear national association and evidence of local transmission in Scotland. While we propose the higher rate of O157 clinical cases in Scotland, compared to England and Wales, is a consequence of the nationally higher level of Stx2a+O157 strains in Scottish cattle, we discuss the multiple additional factors that may also contribute to the different infection rates between these nations.

Whole-Genome Sequencing-Based Screening of MRSA in Patients and Healthcare Workers in Public Hospitals in Benin

Methicillin-resistant Staphylococcus aureus (MRSA) constitutes a serious public health concern, with a considerable impact on patients' health, and substantial healthcare costs. In this study, patients and healthcare workers (HCWs) from six public hospitals in Benin were screened for MRSA. Strains were identified as MRSA using conventional microbiological methods in Benin, and confirmed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry in Belgium. Whole-genome sequencing (WGS) was used on the confirmed MRSA isolates, to characterize their genomic content and study their relatedness. Amongst the 305 isolates (304 wound swabs and 61 nasal swabs) that were collected from patients and HCWs, we detected 32 and 15 cases of MRSA, respectively. From this collection, 27 high-quality WGS datasets were obtained, which carried numerous genes and mutations associated with antimicrobial resistance. The mecA gene was detected in all the sequenced isolates. These isolates were assigned to five sequence types (STs), with ST8 (55.56%, n = 15/27), ST152 (18.52%, n = 5/27), and ST121 (18.52%, n = 5/27) being the most common. These 27 isolates carried multiple virulence genes, including the genes encoding the Panton-Valentine leukocidin toxin (48.15%, n = 13/27), and the tst gene (29.63%, n = 8/27), associated with toxic shock syndrome. This study highlights the need to implement a multimodal strategy for reducing the risk of the cross-transmission of MRSA in hospitals.

Local Salmonella Enteritidis restaurant outbreak investigation in England provides further evidence for eggs as source in widespread international cluster, March to April 2023

We report a 5-single nucleotide polymorphism cluster of Salmonella Enteriditis in England, part of a global cluster of S. Enteritidis ST11. Forty-seven confirmed cases have been investigated of whom 25 were linked to a restaurant. In addition, there were 18 probable cases with restaurant exposure. Epidemiological investigations suggested eggs or chicken as the most likely cause of the outbreak but were unable to distinguish between those two food vehicles. Ongoing food chain investigations indicated links to imported eggs from Poland.

ReporTree: a surveillance-oriented tool to strengthen the linkage between pathogen genetic clusters and epidemiological data

BACKGROUND

Genomics-informed pathogen surveillance strengthens public health decision-making, playing an important role in infectious diseases' prevention and control. A pivotal outcome of genomics surveillance is the identification of pathogen genetic clusters and their characterization in terms of geotemporal spread or linkage to clinical and demographic data. This task often consists of the visual exploration of (large) phylogenetic trees and associated metadata, being time-consuming and difficult to reproduce.

RESULTS

We developed ReporTree, a flexible bioinformatics pipeline that allows diving into the complexity of pathogen diversity to rapidly identify genetic clusters at any (or all) distance threshold(s) or cluster stability regions and to generate surveillance-oriented reports based on the available metadata, such as timespan, geography, or vaccination/clinical status. ReporTree is able to maintain cluster nomenclature in subsequent analyses and to generate a nomenclature code combining cluster information at different hierarchical levels, thus facilitating the active surveillance of clusters of interest. By handling several input formats and clustering methods, ReporTree is applicable to multiple pathogens, constituting a flexible resource that can be smoothly deployed in routine surveillance bioinformatics workflows with negligible computational and time costs. This is demonstrated through a comprehensive benchmarking of (i) the cg/wgMLST workflow with large datasets of four foodborne bacterial pathogens and (ii) the alignment-based SNP workflow with a large dataset of Mycobacterium tuberculosis. To further validate this tool, we reproduced a previous large-scale study on Neisseria gonorrhoeae, demonstrating how ReporTree is able to rapidly identify the main species genogroups and characterize them with key surveillance metadata, such as antibiotic resistance data. By providing examples for SARS-CoV-2 and the foodborne bacterial pathogen Listeria monocytogenes, we show how this tool is currently a useful asset in genomics-informed routine surveillance and outbreak detection of a wide variety of species.

CONCLUSIONS

In summary, ReporTree is a pan-pathogen tool for automated and reproducible identification and characterization of genetic clusters that contributes to a sustainable and efficient public health genomics-informed pathogen surveillance. ReporTree is implemented in python 3.8 and is freely available at https://github.com/insapathogenomics/ReporTree .

Genetic Characterization of Carbapenem-Resistant Klebsiella pneumoniae Clinical Isolates in a Tertiary Hospital in Greece, 2018-2022

BACKGROUND

Carbapenem-resistant Klebsiella pneumoniae (CRKP) is a serious public health issue. The study aimed to identify the antimicrobial resistance and accessory genes, the clonal relatedness, and the evolutionary dynamics of selected CRKP isolates recovered in an adult and pediatric intensive care unit of a tertiary hospital in Greece.

METHODS

Twenty-four CRKP isolates recovered during 2018-2022 were included in the study. Next-generation sequencing was performed using the Ion Torrent PGM Platform. The identification of the plasmid content, MLST, and antimicrobial resistance genes, as well as the comparison of multiple genome alignments and the identification of core genome single-nucleotide polymorphism sites, were performed using various bioinformatics software.

RESULTS

The isolates belonged to eight sequence types: 11, 15, 30, 35, 39, 307, 323, and 512. A variety of carbapenemases (KPC, VIM, NDM, and OXA-48) and resistance genes were detected. CRKP strains shared visually common genomic regions with the reference strain (NTUH-K2044). ST15, ST323, ST39, and ST11 CRKP isolates presented on average 17, 6, 16, and 866 recombined SNPs, respectively. All isolates belonging to ST15, ST323, and ST39 were classified into distinct phylogenetic branches, while ST11 isolates were assigned to a two-subclade branch. For large CRKP sets, the phylogeny seems to change approximately every seven SNPs.

CONCLUSIONS

The current study provides insight into the genetic characterization of CRKP isolates in the ICUs of a tertiary hospital. Our results indicate clonal dispersion of ST15, ST323, and ST39 and highly diverged ST11 isolates.

Rapid geographical source attribution of Salmonella enterica serovar Enteritidis genomes using hierarchical machine learning

Salmonella enterica serovar Enteritidis is one of the most frequent causes of Salmonellosis globally and is commonly transmitted from animals to humans by the consumption of contaminated foodstuffs. In the UK and many other countries in the Global North, a significant proportion of cases are caused by the consumption of imported food products or contracted during foreign travel, therefore, making the rapid identification of the geographical source of new infections a requirement for robust public health outbreak investigations. Herein, we detail the development and application of a hierarchical machine learning model to rapidly identify and trace the geographical source of S. Enteritidis infections from whole genome sequencing data. 2313 S. Enteritidis genomes, collected by the UKHSA between 2014-2019, were used to train a 'local classifier per node' hierarchical classifier to attribute isolates to four continents, 11 sub-regions, and 38 countries (53 classes). The highest classification accuracy was achieved at the continental level followed by the sub-regional and country levels (macro F1: 0.954, 0.718, 0.661, respectively). A number of countries commonly visited by UK travelers were predicted with high accuracy (hF1: >0.9). Longitudinal analysis and validation with publicly accessible international samples indicated that predictions were robust to prospective external datasets. The hierarchical machine learning framework provided granular geographical source prediction directly from sequencing reads in <4 min per sample, facilitating rapid outbreak resolution and real-time genomic epidemiology. The results suggest additional application to a broader range of pathogens and other geographically structured problems, such as antimicrobial resistance prediction, is warranted.

Foodborne outbreak of extended spectrum beta lactamase producing Shigella sonnei associated with contaminated spring onions in the United Kingdom

Globalization of the food supply chain has created conditions favourable for emergence and spread of multidrug resistant (MDR) foodborne pathogens. In November 2021, the UK Health Security Agency detected an outbreak of 17 cases infected with the same strain of MDR extended spectrum beta-lactamase (ESBL)-producing Shigella sonnei. Phylogenetic analysis of whole genome sequencing data revealed the outbreak was closely related to strains of S. sonnei isolated from travellers returning to the UK from Egypt. None of the outbreak cases reported travel and all 17 cases reported eating food from a restaurant/food outlet in the week prior to symptom onset, of which 11/17 (64.7%) ate at branches of the same national restaurant franchise. All 17 cases were adults and 14/17 (82.4%) were female. Ingredient-level analyses of the meals consumed by the cases identified spring onions as the common ingredient. Food chain investigations revealed that the spring onions served at the implicated restaurants could be traced back to a single Egyptian producer. The foodborne transmission of ESBL-producing bacteria is an emerging global health concern, and concerted action from all stakeholders is required to ensure an effective response to mitigate the risks to public health.

Evaluation of Genomic Typing Methods in the Salmonella Reference Laboratory in Public Health, England, 2012-2020

We aim to provide an evidence-based evaluation of whole genome sequence (WGS) methods, employed at the Salmonella reference laboratory in England, in terms of its impact on public health and whether these methods remain a fit for purpose test under UKAS ISO 15189. The evaluation of the genomic methods were mapped against the value of detecting microbiological clusters to support the investigation of food-borne outbreaks of Salmonella in England between 2012-2020. The analysis of WGS with both SNP- and allelic-based methods provided an unprecedented level of strain discrimination and detection of additional clusters when comparing to all of the previous typing methods. The robustness of the routine genomic sequencing at the reference laboratory ensured confidence in the microbiological identifications, even in large outbreaks with complex international food distribution networks. There was evidence that the phylogeny derived from the WGS data can be used to inform the provenance of strains and support discrimination between domestic and non-domestic transmission events. Further insight on the evolutionary context of the emerging pathogenic strains was enabled with a deep dive of the phylogenetic data, including the detection of nested clusters. The public availability of the WGS data linked to the clinical, epidemiological and environmental context of the sequenced strains has improved the trace-back investigations during outbreaks. The global expansion in the use of WGS-based typing in reference laboratories has shown that the WGS methods are a fit for purpose test in public health as it has ensured the rapid implementation of interventions to protect public health, informed risk assessment and has facilitated the management of national and international food-borne outbreaks of Salmonella.

Emergence of extensively drug-resistant and multidrug-resistant Shigella flexneri serotype 2a associated with sexual transmission among gay, bisexual, and other men who have sex with men, in England: a descriptive epidemiological study

BACKGROUND

Shigellosis, also known as bacillary dysentery, is caused by Shigella spp that spread through fecal-oral contact and was traditionally associated with international travel in England. However, sexual transmission of Shigella flexneri and Shigella sonnei among gay, bisexual, and other men who have sex with men (MSM) is now common. In September, 2021, emergence of extensively drug-resistant (XDR) S sonnei harbouring plasmid-encoded bla_CTX-M-27 raised concerns over further spread of this extended-spectrum β-lactamase-producing gene. Using national surveillance in England, we identified and characterised isolates of S flexneri harbouring bla_CTX-M-27.

METHODS

In this epidemiological study, we identified and phylogenetically characterised S flexneri isolates harbouring bla_CTX-M-27 that were referred to the Gastrointestinal Bacterial Reference Unit (GBRU) at the UK Health Security Agency. All isolates referred to the GBRU undergo whole-genome sequencing, enabling antimicrobial resistance determination using genetic markers. Cases were defined as individuals diagnosed with S flexneri harbouring bla_CTX-M-27 in England, with a specimen date between Sept 1, 2015, and June 12, 2022, who were phylogenetically confirmed as part of two t10 (approximately ten single nucleotide polymorphisms) clusters. Long-read sequencing elucidated the genomic location of bla_CTX-M-27. Laboratory data, integrated with available demographic and clinical information from patient questionnaires, were summarised using descriptive statistics.

FINDINGS

A sustained increase in cases of S flexneri harbouring bla_CTX-M-27 (n=26) occurred from September, 2021, having been sporadically reported (n=11) in the preceding 6 years. bla_CTX-M-27 acquisition events within S flexneri 2a established an XDR paraphyletic (n=8) cluster and a multidrug-resistant monophyletic (n=18) cluster. Cases were among adult male individuals (median age 37 years [IQR 31-46]) and, of the 13 individuals who completed a patient questionnaire, ten (77%) identified as MSM. Antimicrobial treatment was received by seven (54%) of 13 individuals, and four (31%) individuals were admitted to hospital. The IncFII plasmids harbouring bla_CTX-M-27 showed high similarity to the XDR S sonnei outbreak plasmid, with 82% and 99% nucleotide similarity between the cluster plasmids and the XDR S sonnei outbreak plasmid.

INTERPRETATION

We report emergence of XDR and multidrug-resistant S flexneri 2a harbouring bla_CTX-M-27 among MSM in England. Epidemiological and plasmid similarities with the XDR S sonnei outbreak support horizontal acquisition events, emphasising the importance of mobilisable antimicrobial resistance and the need for genomic-based surveillance.

FUNDING

National Institute for Health and Care Research Health Protection Research Unit in Gastrointestinal Infections at the University of Liverpool in partnership with the UK Health Security Agency.

Two Outbreaks of Foodborne Gastrointestinal Infection Linked to Consumption of Imported Melons, United Kingdom, March to August 2021

The aim of this study was to describe two foodborne outbreaks caused by contaminated imported melon and make recommendations for future practice. Between March and July 2021, there was an outbreak of 113 cases of Salmonella Braenderup in the UK (62% female, median age 61 years, 33% hospitalized). Analytical epidemiological studies identified Galia melons as the vehicle of infection (OR 671.9, 95% CI 39.0-58,074.0, p < 0.001). Subsequently, the outbreak strain was isolated from two samples of Galia melon imported from Latin America. In July and August 2021, there was an outbreak of 17 cases of Shiga toxin-producing Escherichia coli (STEC) O157:H7 in the UK (53% female, median age 21 years, 35% were hospitalized). Review of the STEC surveillance questionnaire data, followed by the analysis of responses from a modified hypothesis-generating questionnaire, implicated eating precut watermelon from retailer B sourced from Europe as the vehicle of infection. Outbreaks of gastrointestinal pathogens caused by contaminated food of nonanimal origin are a global public health concern. Given the difficulty in removing pathogens from the flesh of ready-to-eat fruit and vegetables, public health interventions should target all steps of the food chain prior to consumption, from cultivation on the farm to processing/packing and distribution.

Application of prophage sequence analysis to investigate a disease outbreak involving Salmonella Adjame, a rare serovar and implications for the population structure

Introduction

Outbreak investigation of foodborne salmonellosis is hindered when the food source is contaminated by multiple strains of Salmonella, creating difficulties matching an incriminated organism recovered from patients with the specific strain in the suspect food. An outbreak of the rare Salmonella Adjame was caused by multiple strains of the organism as revealed by single-nucleotide polymorphism (SNP) variation. The use of highly discriminatory prophage analysis to characterize strains of Salmonella should enable a more precise strain characterization and aid the investigation of foodborne salmonellosis.

Methods

We have carried out genomic analysis of S. Adjame strains recovered during the course of a recent outbreak and compared them with other strains of the organism (n = 38 strains), using SNPs to evaluate strain differences present in the core genome, and prophage sequence typing (PST) to evaluate the accessory genome. Phylogenetic analyses were performed using both total prophage content and conserved prophages.

Results

The PST analysis of the S. Adjame isolates showed a high degree of strain heterogeneity. We observed small clusters made up of 2-6 isolates (n = 27) and singletons (n = 11) in stark contrast with the three clusters observed by SNP analysis. In total, we detected 24 prophages of which only four were highly prevalent, namely: Entero_p88 (36/38 strains), Salmon_SEN34 (35/38 strains), Burkho_phiE255 (33/38 strains) and Edward_GF (28/38 strains). Despite the marked strain diversity seen with prophage analysis, the distribution of the four most common prophages matched the clustering observed using core genome.

Discussion

Mutations in the core and accessory genomes of S. Adjame have shed light on the evolutionary relationships among the Adjame strains and demonstrated a convergence of the variations observed in both fractions of the genome. We conclude that core and accessory genomes analyses should be adopted in foodborne bacteria outbreak investigations to provide a more accurate strain description and facilitate reliable matching of isolates from patients and incriminated food sources. The outcomes should translate to a better understanding of the microbial population structure and an 46 improved source attribution in foodborne illnesses.