Salmonella antisera

All Salmonella isolates were serotyped by slide agglutination with Salmonella antisera (Statens Serum Institut, Copenhagen, Denmark) and serovar names assigned according to the WKL; distinction between the biotypes of S. Choleraesuis was performed by biochemical tests (H₂S production, mucate and dulcitol fermentation) (1 ).

A total of 3,200 RTE pork samples were collected from 32 retail outlets and 32 commercial hypermarkets, in 32 provincial capitals of China in 2014 (Figure 1). Fifty RTE pork samples were collected at each sampling site and all were stored inside tightly sealed aseptic bags, surrounded by a biological ice bag, and then placed in a box maintained at a temperature lower than 4°C. Samples were immediately transported to the laboratory and subjected to microbiological analysis within 2 h. All samples were subjected to qualitative analysis for Salmonella using an enrichment method described by the National Food Safety Standard of China-Food microbiological examination, Salmonella (GB 4789.4-2010). Finally, presumptive Salmonella were selected for biochemical confirmation using API 20E test identification test strips (bioMérieux, Marcy l′ Etoile, France), as well as for molecular identification using PCR assay targeting the invA gene (Malorny et al., 2003 (link)). For all of the confirmed Salmonella isolates, serotypes were determined by the slide agglutination test, using Salmonella antisera (Statens Serum Institute, Denmark) according to the Kauffmann–White scheme. All confirmed Salmonella isolates were stored in brain heart infusion broth with 40% [v/v] glycerol (Land Bridge, Beijing, China) at -80°C. Each sample retained was represented by at least one bacterial isolate.

Human stool sample was inoculated onto Salmonella-Shigella and xylose lysine deoxycholate agar mediums, and incubated at 37 °C overnight. Bacterial culture was identified by using standard biochemical tests (the use of glucose, citrate, and indole by bacteria, the production of gas and H₂S upon lactose fermentation of bacteria, the determination of the mobility and the ability to split urea). The cultured colonies used glucose, citrate and indole confirmed the presence of Salmonella bacteria. The isolate was serogrouped and serotyped using polyvalent and monovalent Salmonella antisera (Statens Serum Institut, Copenhagen, Denmark) according to the Kauffmann-White scheme [26 ]. The colonies that produced H₂S were considered as a pure colony based on their morphology. After the confirmation and serotyping, the strain was immediately frozen in tryptic soy broth with 16% glycerol at −80 °C.