Tracing and pathogen-analyzing of a foodborne disease outbreak of Salmonella typhimurium in a university in Chongqing City by whole-genome sequencing

Abstract

ObjectiveLaboratory pathogenic analysis of Salmonella that caused a foodborne outbreak in a university in Chongqing were carried out.MethodsBiochemical characterization and serotyping of 11 strains of Salmonella were carried out based on national standard method （GB 4789.4—2016）；antibiotics sensitivity testingwere tested by micro-broth dilution method； the molecular typing， virulence and drug resistance genes were analyzed by pulsed-Field gel electrophoresis （PFGE）and whole genome sequencing （WGS）.ResultsA total of 11 strains of Salmonella were Salmonella typhimurium； among the 15 antibiotics tested， all strains were only resistant to tetracycline； The PFGE bands of all strains are consistent； WGS identification is consistent with the results of serological phenotype experiments， and all strains carry 2 tetracycline resistance genes and 10 virulence islands； The traceability analysis results showed that all strains had the same ST type， and the wgSNP evolutionary tree showed that all strains could cluster together in same branch.ConclusionThe 11 strains of Salmonella typhimurium have highly homologous sequences， indicating that the source of the patient， environment， and food isolates are the same. Based on epidemiological investigation results， it is determined that the outbreak of foodborne diseases was caused by food contamination caused by food processors not distinguishing between raw and cooked food when using utensils.

Keywords

• salmonella typhimurium
• pfge
• whole genome sequencing
• virulence island
• drug resistance gene