Allele distribution and genetic diversity of VNTR loci in Salmonella enterica serotype Enteritidis isolates from different sources

Retracted: Antibiotic Resistance in Salmonella Enteritidis Isolates Recovered from Chicken, Chicken Breast, and Humans Through National Antimicrobial Resistance Monitoring System Between 1996 and 2014

The online e-pub version of the article entitled, Antibiotic Resistance in Salmonella Enteritidis Isolates Recovered from Chicken, Chicken Breast, and Humans Through National Antimicrobial Resistance Monitoring System Between 1996 and 2014" by Paudyal N, Pan H, Li X. Fang W. Yue M., Foodborne Pathog Dis [Epub ahead of print]; DOI: 10.1089/fpd.2017.2402 is being officially retracted from Foodborne Pathogens and Disease (FPD) due to a significant number of errors in reporting and miscalculations of the National Antimicrobial Resistance Monitoring System (NARMS) datasets reported in the paper. Authors evaluated NARMS data, which are in the public domain, and analyzed a subset of NARMS data to address questions about a specific serotype of Salmonella in humans and chicken meat. Authors analyzed Salmonella Enteritidis isolates from humans, chicken and retail chicken meat, with their minimum inhibitory concentrations (MICs) to a range of commonly used antibiotics in the US collected over a period of 1996-2014 by NARMS, to segregate isolates based on their MIC value for a certain antimicrobial and evaluate their relationship along the foodborne transmission pathway. NARMS has data on more than 185,000 isolates that can be downloaded in an accessible format. To help make these large data sets more accessible, the NARMS teams continue to develop new tools to enable users to explore them according to their own interests. As a public health surveillance system, the goal is continuous improvement and open, transparent data sharing. The NARMS partners believe this is the best way to foster a collaborative effort to combat antibiotic resistance. After the online-ahead-of-print version of the paper, which used a subset of NARMS data, was published, a significant concern was brought to the attention of the Editor-in-Chief of FPD indicating that the article contained several significant errors which could potentially lead to a misunderstanding of the resistance situation in the United States. Of particular import is with regard to the authors of the paper reversing the poultry and human resistance data found in Figure 2, which displays the overall antimicrobial resistance data and is a central element of the article. The authors correctly state it in the text, but the figure is incorrect. Additionally, there appeared to be an accidental omission of a reference to a published article which shows a strong association between quinolone-resistant Salmonella Enteritidis infections in humans and international travel (O'Donnell et al., 2014) This appears to be a critical oversight given that the intention of the study was to analyze the NARMS data to help understand the dynamics of Salmonella transmission. The authors of the published article were notified of this communication by the Editor of FPD and were provided an opportunity to respond, which they quickly did. The corresponding author, Dr. Min Yue, agreed that after he and his team reanalyzed the data, there were indeed errors in the published paper and supplied revised versions of Figure 2 and supplemental Figure 3, as well as providing a significantly revised version of the manuscript, based upon the criticisms levied against the published paper. After giving the revised manuscript and figures very careful consideration, and after significant probing of his own, the Editor of FPD determined that the significantly revised manuscript, coupled with the multiple errors presented in the figures, is simply far too weighty for a correction statement to be issued, and determined a full retraction of the published article was warranted. It is important to note that there is no indication whatsoever that the errors or miscalculations were intentional, and that Dr. Yue and his team quickly and honestly replied to the concerns raised about their work. However, in the interest of upholding the proper protocols of peer review, and in accurate and truthful reporting in the scientific literature, the Editor of Foodborne Pathogens and Diseases is issuing this full and formal retraction of the article.

Retrospective genome-wide comparisons of Salmonella enterica serovar Enteritidis from suspected outbreaks in Singapore

The number of salmonellosis cases in Singapore has increased over the years. Salmonella enterica serovar Enteritidis has always been the most predominant serovar in the last five years. The National Public Health Laboratory assisted outbreak investigations by performing multilocus variable number tandem repeat analysis (MLVA) on isolates that were collected at the time of the investigations. Isolates were defined as belonging to a particular cluster if they had identical MLVA patterns. Whilst MLVA has been instrumental in outbreak investigations, it may not be useful when outbreaks are caused by an endemic MLVA type. In this study, we analysed 67 isolates from 12 suspected outbreaks with known epidemiological links to explore the use of next-generation sequencing (NGS) for defining outbreaks. We found that NGS can confidently group isolates into their respective outbreaks. The isolates from each suspected outbreak were closely related and differed by a maximum of 3 single nucleotide polymorphisms (SNPs). They were also clearly separated from isolates that belonged to different suspected outbreaks. This study provides an important insight and further evidence on the value of NGS for routine surveillance and outbreak detection of S. Enteritidis.

Multi-laboratory validation study of multilocus variable-number tandem repeat analysis (MLVA) for Salmonella enterica serovar Enteritidis, 2015

Multilocus variable-number tandem repeat analysis (MLVA) is a rapid and reproducible typing method that is an important tool for investigation, as well as detection, of national and multinational outbreaks of a range of food-borne pathogens. Salmonella enterica serovar Enteritidis is the most common Salmonella serovar associated with human salmonellosis in the European Union/European Economic Area and North America. Fourteen laboratories from 13 countries in Europe and North America participated in a validation study for MLVA of S. Enteritidis targeting five loci. Following normalisation of fragment sizes using a set of reference strains, a blinded set of 24 strains with known allele sizes was analysed by each participant. The S. Enteritidis 5-loci MLVA protocol was shown to produce internationally comparable results as more than 90% of the participants reported less than 5% discrepant MLVA profiles. All 14 participating laboratories performed well, even those where experience with this typing method was limited. The raw fragment length data were consistent throughout, and the inter-laboratory validation helped to standardise the conversion of raw data to repeat numbers with at least two countries updating their internal procedures. However, differences in assigned MLVA profiles remain between well-established protocols and should be taken into account when exchanging data.

Multiple-locus variable-number tandem repeat analysis (MLVA) for genotyping of Salmonella enterica subspecies enterica serotype Infantis isolated from human sources

Salmonella is an important cause of food-borne infection worldwide. Detection of outbreaks caused by Salmonella spp. relies on suitable and robust methods for genotyping. Little is known about the genetic diversity of the Salmonella enterica subspecies enterica serotype Infantis strains isolated from human sources in Iran. In this study, 40 isolates of S. Infantis, which were previously recovered from patients with gastroenteritis or diarrhea in Tehran between years 2007 and 2009, were subjected to multiple-locus variable-number of tandem repeat (VNTR) analysis (MLVA), pulsed-field gel electrophoresis (PFGE), and ERIC-PCR. Using MLVA method, 31 types were identified. The MLVA clustering of the isolates by the unweighted pair group method with arithmetic mean (UPGMA) revealed the presence of two major clusters. The discriminatory power of MLVA was superior to that of PFGE and ERIC-PCR. Overall, our data showed that MLVA assay could effectively differentiate closely related strains. It is technically simple and inexpensive to perform. Furthermore, MLVA can be used as a helpful method for epidemiological investigations.

Genetic diversity and antimicrobial resistance pattern of Salmonella enterica serovar Enteritidis clinical isolates in Thailand

OBJECTIVE

To trace the history of antimicrobial resistance in Salmonella enterica serovar Enteritidis (S. Enteritidis, SE) circulating in Thailand, we characterised clinical isolates obtained during 2004-2007.

METHODS

Antimicrobial resistance profiles, multi-locus variable number tandem repeat analysis (MLVA) types and 3 representative virulence determinants (spvA, sodCI and sopE) were established from SE isolates (n = 192) collected from stool and blood of patients throughout Thailand during the period 2004-2007.

RESULTS

Resistance was found in SE against 10 out of 11 antimicrobials studied. The highest resistance ratios were observed for nalidixic acid (83.2%), ciprofloxacin (51.1%) and ampicillin (50.5%), and 25.5% were multidrug resistant. Based on five polymorphic tandem repeat loci analysis, MLVA identified 20 distinct types with three closely related predominant types. A significant increase of AMP resistance from 2004 to 2006 was strongly correlated with that of a MLVA type, 5-5-11-7-3.

CONCLUSION

The usage of antimicrobials in human medicine or farm settings might act as selective pressures and cause the spread of resistant strains. Hence, a strict policy on antimicrobial usage needs to be implemented to achieve the control of resistant SE in Thailand.

Antimicrobial drug resistance patterns among cattle- and human-associated Salmonella strains

During the year 2004, 178 human and 158 bovine clinical Salmonella isolates were collected across New York State to better understand the transmission dynamics and genetic determinants of antimicrobial resistance among human and bovine hosts. Serotyping, sequence typing, and pulsed-field gel electrophoresis typing results have been reported previously. Here we tested all isolates for phenotypic susceptibility to 15 antimicrobial drugs that are part of the National Antimicrobial Monitoring System bovine susceptibility panel. PCR was performed on a representative subset of unique isolates (n = 53) to screen for the presence of 21 known antimicrobial resistance genes (i.e., ampC, blaTEM-1, blaCMY-2, blaPSE-1, cat1, cat2, cmlA, flo, aadA1, aadA2, aacC2, strA, strB, aphA1-IAB, dhrfI, dhrfXII, sulI, sulII, tetA, tetB, and tetG); selected fluoroquinolone- and nalidixic acid-resistant (n = 3) and -sensitive (n = 6) isolates were also tested for known resistance-conferring mutations in gyrA and parC. Genes responsible for antimicrobial resistance were shared among isolates of human and bovine origin. However, bovine isolates were significantly more likely than human isolates to be multidrug resistant (P < 0.0001; Fisher's exact test). Our analyses showed perfect categorical agreement between phenotypic and genotypic resistance for beta-lactam and chloramphenicol. Our data confirm that resistance profiles of amoxicillin-clavulanic acid, chloramphenicol, kanamycin, and tetracycline were strongly associated with the presence of blaCMY or ampC, flo, aphA1-IAB, and tetA, respectively. Our findings provide evidence for the clinical value of genotypic resistance typing if incorporating multiple known genes that can confer a phenotypic resistance profile.

Salmonella source attribution based on microbial subtyping

Source attribution of cases of food-borne disease represents a valuable tool for identifying and prioritizing effective food-safety interventions. Microbial subtyping is one of the most common methods to infer potential sources of human food-borne infections. So far, Salmonella microbial subtyping source attribution models have been implemented by using serotyping and phage-typing data. Molecular-based methods may prove to be similarly valuable in the future, as already demonstrated for other food-borne pathogens like Campylobacter. This review assesses the state of the art concerning Salmonella source attribution through microbial subtyping approach. It summarizes the available microbial subtyping attribution models and discusses the use of conventional phenotypic typing methods, as well as of the most commonly applied molecular typing methods in the European Union (EU) laboratories in the context of their potential applicability for Salmonella source attribution studies.

Molecular characterization of Salmonella Enteritidis: comparison of an optimized multi-locus variable-number of tandem repeat analysis (MLVA) and pulsed-field gel electrophoresis

Salmonella Enteritidis (SE) is a genetically homogenous serovar, which makes optimal subtype discrimination crucial for epidemiological research. This study describes the development and evaluation of an optimized multiple-locus variable number tandem-repeat assay (MLVA) for characterization of SE. The typeability and discriminatory power of this MLVA was determined on a selected collection of 60 SE isolates and compared with pulsed-field gel electrophoresis (PFGE) using restriction enzymes XbaI, NotI, or SfiI. In addition, the estimated Wallace coefficient (W) was calculated to assess the congruence of the typing methods. Selection of epidemiologically unrelated isolates and more related isolates (originating from layer farms) was also based on the given phage type (PT). When targeting six loci, MLVA generated 16 profiles, while PFGE produced 10, 9, and 16 pulsotypes using XbaI, NotI, and SfiI, respectively, for the entire strain collection. For the epidemiologically unrelated isolates, MLVA had the highest discriminatory power and showed good discrimination between isolates from different layer farms and among isolates from the same layer farm. MLVA performed together with PT showed higher discriminatory power compared to PFGE using one restriction enzyme together with PT. Results showed that combining PT with the optimized MLVA presented here provides a rapid typing tool with good discriminatory power for characterizing SE isolates of various origins and isolates originating from the same layer farm.

Pulsed-field gel electrophoresis diversity of human and bovine clinical Salmonella isolates

Pulsed-field gel electrophoresis (PFGE) characterization of 335 temporally and spatially matched clinical, bovine, and human Salmonella enterica subsp. enterica isolates revealed 167 XbaI PFGE patterns. These isolates were previously classified into 51 serotypes and 73 sequence types, as determined by multilocus sequence typing. Discriminatory power of PFGE (Simpson's index, D = 0.991) was considerably higher than that of multilocus sequence typing (D = 0.920) or serotyping (D = 0.913). Although 128 PFGE types each only represented a single isolate, 8 PFGE types represented >4 isolates, including (i) three serotype Enteritidis and Heidelberg patterns that were only identified among human isolates, (ii) two PFGE patterns (each representing serotypes Bardo and Newport) that were significantly more common among bovine isolates as compared with human isolates; (iii) two PFGE types that each includes two serotypes (4,5,12:i:- and Typhimurium; Thompson and 1,7:-:1,5); and (iv) one PFGE type that includes eight Typhimurium isolates from humans and cattle. Characterization of isolates collected over multiple farm visits indicated that given specific PFGE types persisted over time on 11 farms. On an additional seven farms, isolates with a given sequence type represented multiple PFGE type, which typically only differed by <3 bands, suggesting PFGE type diversification during strain persistence. Sixteen PFGE types were isolated from 2 or more farms, including two widely distributed serotype Newport-associated PFGE types each found on 10 farms. In six instances two or three human isolates collected in the same county in the same or consecutive months represented the same subtypes, suggesting small human case clusters. PFGE-based characterization and surveillance of human and animal isolates can provide improved understanding of Salmonella diversity and epidemiology, including identification of possible host-associated and common, widely distributed PFGE types.

Comparison of genotypes of Salmonella enterica serovar Enteritidis phage type 30 and 9c strains isolated during three outbreaks associated with raw almonds

In 2000 to 2001, 2003 to 2004, and 2005 to 2006, three outbreaks of Salmonella enterica serovar Enteritidis were linked with the consumption of raw almonds. The S. Enteritidis strains from these outbreaks had rare phage types (PT), PT30 and PT9c. Clinical and environmental S. Enteritidis strains were subjected to pulsed-field gel electrophoresis (PFGE), multilocus variable-number tandem repeat analysis (MLVA), and DNA microarray-based comparative genomic indexing (CGI) to evaluate their genetic relatedness. All three methods differentiated these S. Enteritidis strains in a manner that correlated with PT. The CGI analysis confirmed that the majority of the differences between the S. Enteritidis PT9c and PT30 strains corresponded to bacteriophage-related genes present in the sequenced genomes of S. Enteritidis PT4 and S. enterica serovar Typhimurium LT2. However, PFGE, MLVA, and CGI failed to discriminate between S. Enteritidis PT30 strains related to outbreaks from unrelated clinical strains or between strains separated by up to 5 years. However, metabolic fingerprinting demonstrated that S. Enteritidis PT4, PT8, PT13a, and clinical PT30 strains metabolized L-aspartic acid, L-glutamic acid, L-proline, L-alanine, and D-alanine amino acids more efficiently than S. Enteritidis PT30 strains isolated from orchards. These data indicate that S. Enteritidis PT9c and 30 strains are highly related genetically and that PT30 orchard strains differ from clinical PT30 strains metabolically, possibly due to fitness adaptations.

Advances in subtyping methods of foodborne disease pathogens

Current subtyping methods for the detection of foodborne disease outbreaks have limitations that reduce their use by public health laboratories. Recent advances in subtyping of foodborne disease pathogens utilize techniques that identify nucleic acid polymorphisms. Recent methods of nucleic acid characterization such as microarrays and mass spectrometry (MS) may provide improvements such as increasing speed and data portability while decreasing labor compared to current methods. This article discusses multiple-locus variable-number tandem-repeat analysis, single-nucleotide polymorphisms, nucleic acid sequencing, whole genome sequencing, variable absent or present loci, microarrays and MS as potential subtyping methods to enhance our ability to detect foodborne disease outbreaks.

Analysis of an outbreak of Salmonella enteritidis by repetitive-sequence-based PCR and pulsed-field gel electrophoresis

OBJECTIVE

The aim of this study was to investigate a large food-borne outbreak associated with eggs contaminated by Salmonella Enteritidis in a military unit using pulse field gel electrophoresis (PFGE) and the Repetitive-sequence-based PCR (rep-PCR) employing the DiversiLab system.

MATERIALS AND METHODS

In mid-January 2008, a food-borne outbreak associated with S. Enteritidis occurred in a military unit located in the city centre of Isparta. A total of 2,469 patients were registered to six hospitals with gastrointestinal disease symptoms such as diarrhea and abdominal pain. Of those registered, 445 were hospitalized. S. Enteritidis was isolated from 276 stool samples and a blood sample of the hospitalized patients and from a food item. The PFGE patterns after XbaI digestion and rep-PCR profiles produced by the DiversiLab system were determined for eight randomly selected stool isolates, one blood isolate and one food isolate.

RESULTS

The PFGE patterns of all isolates were identical. The Rep-PCR profiles produced by using the DiversiLab system showed that all isolates were indistinguishable. The PFGE and rep-PCR interpretations were concordant for the S. Enteritidis isolates. All stool isolates, one blood isolate and one food isolate were susceptible to all antibiotics tested.

CONCLUSION

This data suggest that the DiversiLab system may be a reasonable alternative to PFGE for investigation and control of S. Enteritidis outbreaks, since it is easy to use, rapid and does not require highly skilled operators.

Multiple-locus variable-number tandem-repeat analysis of Salmonella enterica serovar Typhi

Multilocus variable-number tandem repeats (VNTRs) are widely used as molecular markers to differentiate isolates of homogenous pathogenic clones. We explored the genomes of Salmonella enterica serovar Typhi strains CT18 and Ty2 for potential VNTRs. Among the 43 potential VNTRs screened, 2 were found to be polymorphic. Together with seven polymorphic VNTRs from previous studies, they were used to type 73 global serovar Typhi isolates. A total of 70 multilocus VNTR analysis (MLVA) profiles were found, distinguishing all except three pairs of isolates into individual profiles. The discriminatory power was 0.999. Phylogenetic analysis showed that the MLVA profiles can be divided into seven clusters. However, except for the closely related isolates, the relationships derived were in conflict with those inferred from single nucleotide polymorphism (SNP) typing using 38 SNPs done previously. We concluded that MLVA can resolve the relationships only among closely related isolates. A combination of SNP typing and MLVA typing offers the best approach for local and global epidemiology and the evolutionary analysis of serovar Typhi. We suggest that seven of the nine most polymorphic VNTRs be used as a standardized typing scheme for epidemiological typing.

Novel genetic tools for studying food-borne Salmonella

Nontyphoidal Salmonellae are highly prevalent food-borne pathogens. High-throughput sequencing of Salmonella genomes is expanding our knowledge of the evolution of serovars and epidemic isolates. Genome sequences have also allowed the creation of complete microarrays. Microarrays have improved the throughput of in vivo expression technology (IVET) used to uncover promoters active during infection. In another method, signature tagged mutagenesis (STM), pools of mutants are subjected to selection. Changes in the population are monitored on a microarray, revealing genes under selection. Complete genome sequences permit the construction of pools of targeted in-frame deletions that have improved STM by minimizing the number of clones and the polarity of each mutant. Together, genome sequences and the continuing development of new tools for functional genomics will drive a revolution in the understanding of Salmonellae in many different niches that are critical for food safety.