Outbreak of Salmonella Typhimurium infection traced to imported cured sausage using MLVA-subtyping

Occurrence and traceability of Salmonella spp. in five Sardinian fermented sausage facilities

The aims of the present study were to evaluate the presence of Salmonella in five fermented sausage processing plants and their products during the production process, and to trace the possible sources of contamination. A total of 270 samples were collected: mixture of ground pork meat and fat, products at the end of acidification, sausages at the end of ripening and, during production stages, surfaces in contact with meat and surfaces not in contact with meat. For samples of ground meat, product at the end of acidification and sausages at the end of ripening, the pH and water activity (aw), were determined. All the samples were tested for the presence of Salmonella. Thirtytwo Salmonella isolates were obtained, subjected to serotyping and PFGE. The sausages at the end of ripening pH and aw mean values were 5.39±0.24 and 0.91±0.03, respectively. Salmonella was detected in three processing plants with an overall prevalence of 16.7% in food samples and 5.8% in environmental samples. Salmonella prevalence was 24% in ground meat and products at the end of acidification and was also detected in a sample of sausage at the end of ripening (2%). In environmental samples, Salmonella was detected in 6.6% of surfaces in contact with meat and 5% of surfaces not in contact with meat. Five serotypes were identified among 32 isolates: S. Derby (37.5%), S. Typhimurium and S. Rissen (both 25%), S. Give and monophasic S. Typhimurium (both 6.25%). Six different pulsotypes were obtained with PFGE. The serotypes and the PFGE pattern of the strains were specific for each facility with no overlapping between different processing plants. The same observation can be pointed out considering different sampling days for the same processing plants, thus presumably indicating the raw material (ground pork meat and fat) as the source of contamination. The detection of Salmonella in a sample of sausage at the end of ripening highlights the ability of the pathogen to survive during manufacturing process.

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.

Evaluation of molecular typing of foodborne pathogens in European reference laboratories from 2012 to 2013

In 2012, the European Centre for Disease Prevention and Control (ECDC) initiated external quality assessment (EQA) schemes for molecular typing including the National Public Health Reference Laboratories in Europe. The overall aim for these EQA schemes was to enhance the European surveillance of food-borne pathogens by evaluating and improving the quality and comparability of molecular typing. The EQAs were organised by Statens Serum Institut (SSI) and included Salmonella enterica subsp. enterica, verocytotoxin-producing Escherichia coli (VTEC) and Listeria monocytogenes. Inter-laboratory comparable pulsed-field gel electrophoresis (PFGE) images were obtained from 10 of 17 of the participating laboratories for Listeria, 15 of 25 for Salmonella, but only nine of 20 for VTEC. Most problems were related to PFGE running conditions and/or incorrect use of image acquisition. Analysis of the gels was done in good accordance with the provided guidelines. Furthermore, we assessed the multilocus variable-number tandem repeat analysis (MLVA) scheme for S. Typhimurium. Of 15 laboratories, nine submitted correct results for all analysed strains, and four had difficulties with one strain only. In conclusion, both PFGE and MLVA are prone to variation in quality, and there is therefore a continuous need for standardisation and validation of laboratory performance for molecular typing methods of food-borne pathogens in the human public health sector.

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.

The prevalence of multidrug resistance is higher among bovine than human Salmonella enterica serotype Newport, Typhimurium, and 4,5,12:i:- isolates in the United States but differs by serotype and geographic region

Salmonella represents an important zoonotic pathogen worldwide, but the transmission dynamics between humans and animals as well as within animal populations are incompletely understood. We characterized Salmonella isolates from cattle and humans in two geographic regions of the United States, the Pacific Northwest and the Northeast, using three common subtyping methods (pulsed-field gel electrophoresis [PFGE], multilocus variable number of tandem repeat analysis [MLVA], and multilocus sequence typing [MLST]). In addition, we analyzed the distribution of antimicrobial resistance among human and cattle Salmonella isolates from the two study areas and characterized Salmonella persistence on individual dairy farms. For both Salmonella enterica subsp. enterica serotypes Newport and Typhimurium, we found multidrug resistance to be significantly associated with bovine origin of isolates, with the odds of multidrug resistance for Newport isolates from cattle approximately 18 times higher than for Newport isolates from humans. Isolates from the Northwest were significantly more likely to be multidrug resistant than those from the Northeast, and susceptible and resistant isolates appeared to represent distinct Salmonella subtypes. We detected evidence for strain diversification during Salmonella persistence on farms, which included changes in antimicrobial resistance as well as genetic changes manifested in PFGE and MLVA pattern shifts. While discriminatory power was serotype dependent, the combination of PFGE data with either MLVA or resistance typing data consistently allowed for improved subtype discrimination. Our results are consistent with the idea that cattle are an important reservoir of multidrug-resistant Salmonella infections in humans. In addition, the study provides evidence for the value of including antimicrobial resistance data in epidemiological investigations and highlights the benefits and potential problems of combining subtyping methods.