Api 20e test identification test strips

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.

During our routine surveillance of foodborne pathogens on various food products, a Salmonella isolate (named GSJ/2017-Sal.-014, hereafter 17Sal014) was recovered from a roasted duck product in Guangzhou, southern China, in 2017. The isolate was first identified by biochemical confirmation using API 20E test identification test strips (bioMérieux, France) and further by 16S ribosomal RNA (rRNA) gene sequencing using the universal primers 27F (5′-AGAGTTTGATCCTG GCTCAG-3′) and 1492R (5′-GGCTACCTTGTTACGACTT-3′). The serotype was determined by the slide agglutination test using Salmonella antisera (SSI Diagnostica, Denmark) according to the Kauffmann–White scheme.
The strain was routinely grown in Luria–Bertani (LB; Guangdong Huankai Microbial Sci. & Tech., Guangzhou, China) broth or agar plates at 37°C for 12–24 h. Escherichia coli J53 was cultured in LB broth or agar plates with 150 μg/ml sodium azide (Sigma–Aldrich, St. Louis, MO, United States) and incubated at 37°C for 12–24 h.

During our routine surveillance of foodborne pathogens from various food products during 2016–2017 in Guangdong, China, 11 S. typhimurium isolates resistant to colistin and carrying the mcr-1 gene were recovered. One of the isolates (named GSJ/2017-Sal.-008, hereafter 17Sal008) was isolated from a retail RTE dumpling with pork and cabbage stuffing in Guangzhou in 2017, while the remaining 10 isolates were collected from raw pork products from retail markets in Guangzhou and Heyuan city in 2016 (Supplementary Table 1). The isolates were identified by biochemical confirmation using API 20E test identification test strips (bioMérieux, France), as well as amplification of the invA gene by PCR (Bai et al., 2016 (link)). The serotype was determined by the slide agglutination test, using Salmonella antisera (SSI Diagnostica, Denmark) according to the White-Kauffmann-Le Minor scheme.
The isolates were routinely grown in Luria-Bertani (LB; Guangdong Huankai Microbial Sci & Tech, Guangzhou, China) broth or on LB agar plates at 37°C for 12–24 h.