Comparison of multilocus variable-number tandem repeat analysis and pulsed-field gel electrophoresis in molecular subtyping of Salmonella enterica serovars Paratyphi A

Antimicrobial Resistance and Genomic Characterization of Salmonella Infantis from Different Sources

The epidemiology of Salmonella Infantis is complex in terms of its distribution and transmission. The continuous collection and analysis of updated data on the prevalence and antimicrobic resistance are essential. The present work aimed to investigate the antimicrobial resistance and the correlation among S. Infantis isolates from different sources through the multiple-locus variable-number of tandem repeat (VNTR) analysis (MLVA). A total of 562 Salmonella strains isolated from 2018 to 2020 from poultry, humans, swine, water buffalo, mussels, cattle, and wild boar were serotyped, and 185 S. Infantis strains (32.92%) were identified. S. Infantis was commonly isolated in poultry and, to a lesser extent, in other sources. The isolates were tested against 12 antimicrobials, and a high prevalence of resistant strains was recorded. S. Infantis showed high resistance against fluoroquinolones, ampicillin, and tetracycline, which are commonly used in human and veterinary medicine. From all S. Infantis isolates, five VNTR loci were amplified. The use of MLVA was not sufficient to understand the complexity of the epidemiological relationships between S. Infantis strains. In conclusion, an alternative methodology to investigate genetic similarities and differences among S. Infantis strains is needed.

A comparative study on antibiotic resistance and virulence properties of Staphylococcus aureus isolated from hospitalized patients and hospital environment

BACKGROUND

We compared the characteristics of clinical Staphylococcus aureus and S. aureus isolated from environmental surfaces in 3 hospitals.

METHODS

Clinical S. aureus isolates were collected from hospitalized patients. Environmental surfaces were sampled from the rooms of patients infected with S. aureus. After identifying rooms with the target organism, 3-5 high-touch surfaces in patient care areas were sampled using swabs before room cleaning by environmental services. S. aureus isolates were subjected to genotyping, antimicrobial susceptibility testing, and virulence determinant screening. The isolates were analyzed for integron content and sequences of variable region amplification products.

RESULTS

There were epidemiologically unrelated 79 clinical and 62 environmental S. aureus isolates. Overall, 11.4% of clinical and 59.7% of environmental isolates were methicillin-resistant. The environmental and clinical S. aureus exhibited very different virulence profiles: 79% of the environmental isolates were negative for virulence genes compared to 2.5% of clinical isolates (P < .001). Environmental isolates were more resistant to antibiotics compared to clinical isolates. Class 1 integrons were only detected in 7 of 62 environmental isolates, of which 3 isolates had integrons with cysteine synthase cassette, 1 had aadA1, and 1 had an unknown cassette.

CONCLUSION

These data indicate the different characteristics between environmental and clinical S. aureus, which may reflect different reservoirs from which the 2 groups acquired the strains.

Changes in antimicrobial resistance and molecular attributes of Salmonellae causing enteric fever in Kolkata, India, 2014-2018

Globally, enteric fever caused by Salmonella Typhi (S. Typhi, ST) and S. Paratyphi A (SPA) remain one of the major diseases of public health importance. In this study, a total of 457 (380 ST, 77 SPA) blood isolates were collected from three tertiary care hospitals in Kolkata during 2014-18. Additionally, 66 (3.4%) ST and 5 (0.25%) SPA were recovered from blood culture of 1962 patients attending OPD of one pediatric hospital during 2016-18. The study isolates were tested for antimicrobial resistance (AMR) profiles; AMR genes; molecular sub-types by PFGE, MLVA and CRISPR. Among the total 446 ST and 82 SPA isolates, fluoroquinolone (FQ) resistance was very common in both serovars. Ciprofloxacin resistance of 24.9% and 9.8% & ofloxacin resistance of 20.9% and 87.8% were found in ST and SPA respectively. Majority (>70%) of the isolates showed decreased susceptibility to ciprofloxacin (DCS). A single point mutation in gyrA gene (S83F) was responsible for causing DCS in 37.5% (n = 42/112) ST and 63% (n = 46/73) SPA isolates. Multidrug resistance (MDR) was found only in 3.4% ST isolates and encoded the genes bla_TEM-1, catA, sul, strA-strB, class 1 integron with dfrA7. All MDR ST (n = 15) possessed non-conjugative non-IncHI1 (180 kb) plasmid except one having conjugative IncHI1 (230 kb) plasmid and one without plasmid. The MDR genes were integrated near chromosomal cyaA gene site in ST with/without the presence of plasmid (nonIncH1). Almost 65.7% resistant ST belonged to H58 haplotype. PFGE showed clonally related isolates with 81% similarity in ST and 87% in SPA. Similarly, CRISPR typing showed less diversity among the isolates. However, the isolates (ST and SPA) were found to be more diverse by MLVA typing (D value 0.987 and 0.938). The study reports decrease in MDR and increase in FQ resistance among typhoidal Salmonella isolates over the years giving interesting information for enteric fever treatment.

Genomic analysis of Salmonella enterica serotype Paratyphi A during an outbreak in Cambodia, 2013-2015

In 2013, an unusual increase in the number of Salmonella enterica serotype Paratyphi A (Salmonella Paratyphi A) infections was reported in patients in Phnom Penh, Cambodia, and in European, American and Japanese travellers returning from Cambodia. Epidemiological investigations did not identify a common source of exposure. To analyse the population structure and genetic diversity of these Salmonella Paratyphi A isolates, we used whole-genome sequencing on 65 isolates collected from 1999 to 2014: 55 from infections acquired in Cambodia and 10 from infections acquired in other countries in Asia, Africa and Europe. Short-read sequences from 80 published genomes from around the world and from 13 published genomes associated with an outbreak in China were also included. Pulsed-field gel electrophoresis (PFGE) was performed on a subset of isolates. Genomic analyses were found to provide much more accurate information for tracking the individual strains than PFGE. All but 2 of the 36 isolates acquired in Cambodia during 2013–2014 belonged to the same clade, C5, of lineage C. This clade has been isolated in Cambodia since at least 1999. The Chinese outbreak isolates belonged to a different clade (C4) and were resistant to nalidixic acid, whereas the Cambodian outbreak isolates displayed pan-susceptibility to antibiotics. Since 2014, the total number of cases has decreased, but there has been an increase in the frequency with which nalidixic acid-resistant C5 isolates are isolated. The frequency of these isolates should be monitored over time, because they display decreased susceptibility to ciprofloxacin, the first-choice antibiotic for treating paratyphoid fever.

Development and comparison of a generic multiple-locus variable-number tandem repeat analysis with pulsed-field gel electrophoresis for typing of Salmonella enterica subsp. enterica

AIMS

Salmonella enterica subsp. enterica causes salmonellosis in humans and animals. Serovar-specific multiple-locus variable-number tandem repeat analysis (MLVA) is widely used for Salmonella surveillance; however, isolates have to be serotyped prior to MLVA typing and only the most common serovars can be typed. We developed a MLVA scheme for high-discriminatory typing of Salmonella.

METHODS AND RESULTS

Sixty-six unique VNTRs were investigated and the polymorphisms of seven promising VNTRs were evaluated with a panel 163 diverse isolates of 14 serotypes of significance for human health. Five VNTRs were selected for MLVA analysis. The discriminatory power was evaluated within serovars by 163 isolates and MLVA yielded 79 genotypes (DI of 0·9790) and pulsed-field gel electrophoresis (PFGE) revealed 87 genotypes (DI of 0·9989). MLVA divided each serotype into 2-8 different profiles and identified six pairs of outbreak-related strains.

CONCLUSIONS

The technique showed a high-discriminatory power within most serotypes comparable with or better than that of PFGE.

SIGNIFICANCE AND IMPACT OF THE STUDY

This MLVA assay makes it possible to use a single typing method for Salmonella surveillance and outbreak investigations. This allows inexpensive and fast surveillance for laboratories without resources for both serotyping and molecular typing, e.g. PFGE or sequence-based methods, and thereby improve the effectiveness of epidemiological investigations of Salmonella infections globally.

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.

Metal nanoparticle assisted polymerase chain reaction for strain typing of Salmonella Typhi

Salmonella enterica serotype Typhi (S. Typhi) is the causative agent of typhoid fever and remains a major health threat in most of the developing countries. The prompt diagnosis of typhoid directly from the patient's blood requires high level of sensitivity and specificity. Some of us were the first to report PCR based diagnosis of typhoid. This approach has since then been reported by many scientists using different genomic targets. Since the number of bacteria circulating in the blood of a patient can be as low as 0.3 cfu ml(-1), there is always a room for improvement in diagnostic PCR. In the present study, the role of different types of nanoparticles was investigated to improve the existing PCR based methods for diagnosis and strain typing of S. Typhi (targeting Variable Number of Tandem Repeats [VNTR]) by using optimized PCR systems. Three different types of nanoparticles were used i.e., citrate stabilized gold nanoparticles, rhamnolipid stabilized gold and silver nanoparticles, and magnetic iron oxide nanoparticles. The non-specific amplification was significantly reduced in VNTR typing when gold and silver nanoparticles were used in an appropriate concentration. More importantly, the addition of nanoparticles decreased the non-specificity to a significant level in the case of multiplex PCR thus further validating the reliability of PCR for the diagnosis of typhoid.

Phylogenetic and Variable-Number Tandem-Repeat Analyses Identify Nonpathogenic Xanthomonas arboricola Lineages Lacking the Canonical Type III Secretion System

Xanthomonas arboricola is conventionally known as a taxon of plant-pathogenic bacteria that includes seven pathovars. This study showed that X. arboricola also encompasses nonpathogenic bacteria that cause no apparent disease symptoms on their hosts. The aim of this study was to assess the X. arboricola population structure associated with walnut, including nonpathogenic strains, in order to gain a better understanding of the role of nonpathogenic xanthomonads in walnut microbiota. A multilocus sequence analysis (MLSA) was performed on a collection of 100 X. arboricola strains, including 27 nonpathogenic strains isolated from walnut. Nonpathogenic strains grouped outside clusters defined by pathovars and formed separate genetic lineages. A multilocus variable-number tandem-repeat analysis (MLVA) conducted on a collection of X. arboricola strains isolated from walnut showed that nonpathogenic strains clustered separately from clonal complexes containing Xanthomonas arboricola pv. juglandis strains. Some nonpathogenic strains of X. arboricola did not contain the canonical type III secretion system (T3SS) and harbored only one to three type III effector (T3E) genes. In the nonpathogenic strains CFBP 7640 and CFBP 7653, neither T3SS genes nor any of the analyzed T3E genes were detected. This finding raises a question about the origin of nonpathogenic strains and the evolution of plant pathogenicity in X. arboricola. T3E genes that were not detected in any nonpathogenic isolates studied represent excellent candidates to be those responsible for pathogenicity in X. arboricola.

Characterization of Salmonella Typhimurium isolates from domestically acquired infections in Finland by phage typing, antimicrobial susceptibility testing, PFGE and MLVA

BACKGROUND

Salmonella enterica spp. enterica serotype Typhimurium (STM) is the most common agent of domestically acquired salmonellosis in Finland. Subtyping methods which allow the characterization of STM are essential for effective laboratory-based STM surveillance and for recognition of outbreaks. This study describes the diversity of Finnish STM isolates using phage typing, antimicrobial susceptible testing, pulsed-field gel electrophoresis (PFGE) and multilocus variable-number tandem repeat analysis (MLVA), and compares the discriminatory power and the concordance of these methods.

RESULTS

A total of 375 sporadic STM isolates were analysed. The isolates were divided into 31 definite phage (DT) types, dominated by DT1 (47 % of the isolates), U277 (9 % of the isolates) and DT104 (8 % of the isolates). Of all the isolates, 62 % were susceptible to all the 12 antimicrobials tested and 11 % were multidrug resistant. Subtyping resulted in 83 different XbaI-PFGE profiles and 111 MLVA types. The three most common XbaI-PFGE profiles (STYM1, STYM7 and STYM8) and one MLVA profile with three single locus variants accounted for 56 % and 49 % of the STM isolates, respectively. The studied isolates showed a genetic similarity of more than 70 % by XbaI-PFGE. In MLVA, 71 % of the isolates lacked STTR6 and 77 % missed STTR10p loci. Nevertheless, the calculated Simpson's diversity index for XbaI-PFGE was 0.829 (95 % CI 0.792-0.865) and for MLVA 0.867 (95 % CI 0.835-0.898). However, the discriminatory power of the 5-loci MLVA varied among the phage types. The highest concordance of the results was found between XbaI-PFGE and phage typing (adjusted Wallace coefficient was 0.833 and adjusted Rand coefficient was 0.627).

CONCLUSIONS

In general, the calculated discriminatory power was higher for genotyping methods (MLVA and XbaI-PFGE) than for phenotyping methods (phage typing). Overall, comparable diversity indices were calculated for PFGE and MLVA (both DI > 0.8). However, MLVA was phage type dependent providing better discrimination of the most common phage types. Furthermore, 5-loci MLVA was a less laborious method and easier to interpret than XbaI-PFGE. Thus, the laboratory-based surveillance of the Finnish human STM infections has been conducted with a combination of phage typing, antimicrobial susceptibility testing and 5-loci MLVA since January 2014.

Multiple-locus variable number of tandem repeats analysis of Salmonella enterica serotype paratyphi A from Yuxi and comparison with isolates from the Chinese Medical Culture Collection Center

The aim of the present study was to genotype Salmonella enterica serotype paratyphi A (SPA) isolated from Yuxi, China, in a multiple-locus variable number of tandem repeats (VNTRs) analysis (MLVA) and to compare them with isolates from the Chinese Medical Culture Collection Center (CMCC). Potential VNTRs were screened from the genomes of ATCC9150 and AKU_12601 using the Tandem Repeats Finder program. Nine VNTRs were established for MLVA typing of 195 SPA isolates from Yuxi and 20 isolates from CMCC. The dendogram for MLVA profiles and minimum spanning tree (MST) were drawn using the categorical coefficient calculated by BioNumerics software. A total of 23 MLVA types were identified in 215 SPA isolates and were grouped into six distinct cluster groups A, B, C, D, E and F. A total of 195 Yuxi SPA isolates were exclusively grouped into cluster C with nine MLVA genotypes. A total of 20 CMCC isolates were grouped in clusters A B, D, E and F with the other 14 MLVA types. The MLVA with nine VNTR loci, which was exploited in the present study, represents a successful strategy for genotyping SPA. Furthermore, the 195 Yuxi isolates appear to be closely related to each other and distinct from the 20 CMCC strains.

Diversity of Salmonella enterica serovar Typhi strains collected from india using variable number tandem repeat (VNTR)-PCR analysis

BACKGROUND

Typhoid fever is endemic in India, and a seasonal increase of cases is observed annually. In spite of effective therapies and the availability of vaccines, morbidity is widespread owing to the circulation of multiple genetic variants, frequent migration of asymptomatic carriers, unhygienic food practices and the emergence of multidrug resistance and thus continues to be a major public health problem in developing countries, particularly in India. Classical methods of strain typing such as pulsed-field gel electrophoresis, ribotyping, random amplification of polymorphic DNA and amplified fragment length polymorphism are either laborious and technically complicated or less discriminatory.

METHODS

We investigated the molecular diversity of Indian strains of Salmonella enterica serovar Typhi (S. Typhi) isolated from humans from different parts of India to establish the molecular epidemiology of the organism using the variable number tandem repeat (VNTR)-PCR analysis. The electrophoretic band pattern was analysed using the GelCompar II software program.

RESULTS

Of the 94 strains tested for three VNTRs loci, 75 VNTR genotypes were obtained. Of the three VNTRs tested in this study, VNTR1 was amplified in all the strains except one and found to be predominant. VNTR2 was amplified only in 57 strains with a Simpson diversity index of 0.93 indicating the high variability of this region within the strains. VNTR3 was amplified in 90 strains. The discriminatory power of this typing tool has been greatly enhanced by this VNTR2 region as the other two regions could not discriminate strains significantly. In our study, about 55 % of the strains amplified all three VNTR regions and 39 % of the strains lacked the VNTR2 region. Among the three VNTR regions tested, the majority of the strains produced similar banding pattern for any two regions grouped into a cluster. The strains grouped as a genotype were from the same geographical location. Strains collected from each geographical region were also highly heterogeneous.

CONCLUSION

Such analysis is important to identify the genetic clones of the pathogen associated with sporadic infections and disease outbreak to identify the common source and implement public health measures.

Unusual increase in reported cases of paratyphoid A fever among travellers returning from Cambodia, January to September 2013

From January to September 2013, a marked increase in notifications of Salmonella Paratyphi A infections among travellers returning from Cambodia occurred in France. An investigation revealed 35 cases without a common source: 21 in France, five in Germany, three in the Netherlands, one in Norway, one in the United Kingdom, four in New-Zealand. Data suggest an ongoing event that should trigger further investigation. Travellers to Cambodia should observe preventive measures including good personal hygiene and food handling practices.

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.

Housekeeping gene sequencing and multilocus variable-number tandem-repeat analysis to identify subpopulations within Pseudomonas syringae pv. maculicola and Pseudomonas syringae pv. tomato that correlate with host specificity

Pseudomonas syringae pv. maculicola causes bacterial spot on Brassicaceae worldwide, and for the last 10 years severe outbreaks have been reported in the Loire Valley, France. P. syringae pv. maculicola resembles P. syringae pv. tomato in that it is also pathogenic for tomato and causes the same types of symptoms. We used a collection of 106 strains of P. syringae to characterize the relationships between P. syringae pv. maculicola and related pathovars, paying special attention to P. syringae pv. tomato. Phylogenetic analysis of gyrB and rpoD gene sequences showed that P. syringae pv. maculicola, which causes diseases in Brassicaceae, forms six genetic lineages within genomospecies 3 of P. syringae strains as defined by L. Gardan et al. (Int. J. Syst. Bacteriol. 49[Pt 2]:469-478, 1999), whereas P. syringae pv. tomato forms two distinct genetic lineages. A multilocus variable-number tandem-repeat (VNTR) analysis (MLVA) conducted with eight minisatellite loci confirmed the genetic structure obtained with rpoD and gyrB sequence analyses. These results provide promising tools for fine-scale epidemiological studies on diseases caused by P. syringae pv. maculicola and P. syringae pv. tomato. The two pathovars had distinct host ranges; only P. syringae pv. maculicola strains were pathogenic for Brassicaceae. A subpopulation of P. syringae pv. maculicola strains that are pathogenic for Pto-expressing tomato plants were shown to lack avrPto1 and avrPtoB or to contain a disrupted avrPtoB homolog. Taking phylogenetic and pathological features into account, our data suggest that the DC3000 strain belongs to P. syringae pv. maculicola. This study shows that P. syringae pv. maculicola and P. syringae pv. tomato appear multiclonal, as they did not diverge from a single common ancestral group within the ancestral P. syringae genomospecies 3, and suggests that pathovar specificity within P. syringae may be due to independent genetic events.

Methodologies for Salmonella enterica subsp. enterica subtyping: gold standards and alternatives

For more than 80 years, subtyping of Salmonella enterica has been routinely performed by serotyping, a method in which surface antigens are identified based on agglutination reactions with specific antibodies. The serotyping scheme, which is continuously updated as new serovars are discovered, has generated over time a data set of the utmost significance, allowing long-term epidemiological surveillance of Salmonella in the food chain and in public health control. Conceptually, serotyping provides no information regarding the phyletic relationships inside the different Salmonella enterica subspecies. In epidemiological investigations, identification and tracking of salmonellosis outbreaks require the use of methods that can fingerprint the causative strains at a taxonomic level far more specific than the one achieved by serotyping. During the last 2 decades, alternative methods that could successfully identify the serovar of a given strain by probing its DNA have emerged, and molecular biology-based methods have been made available to address phylogeny and fingerprinting issues. At the same time, accredited diagnostics have become increasingly generalized, imposing stringent methodological requirements in terms of traceability and measurability. In these new contexts, the hand-crafted character of classical serotyping is being challenged, although it is widely accepted that classification into serovars should be maintained. This review summarizes and discusses modern typing methods, with a particular focus on those having potential as alternatives for classical serotyping or for subtyping Salmonella strains at a deeper level.