Research – Phenotypic and genotypic characterization of salmonella Enteritidis isolated from two consecutive Food-Poisoning outbreaks in Sichuan, China

Salmonella enterica serotype Enteritidis (SE) is a primary pathogen that causes foodborne diseases in humans. Although whole-genome sequencing (WGS) -based typing analyses have been increasingly used to investigate food-poisoning outbreaks, they are rarely applied to the epidemiology of multiple Salmonella outbreaks in Sichuan, China. This study therefore isolated SE from patients and food of two consecutive food-poisoning outbreaks during 2020 in Sichuan, China. We tracked outbreak origin using epidemiological investigation, serotyping, antimicrobial susceptibility testing (AST), pulsed-field gel electrophoresis (PFGE), and WGS. We also determined phylogenetic relationships using PFGE, whole and core genome multilocus sequence typing (wg/cgMLST), and whole-genome single nucleotide polymorphism (wgSNP) analyses. Epidemiological investigations identified a correlation between cake consumption and food poisoning. Thirteen strains isolated from patients and one strain isolated from the cake were confirmed as SE. Among the 14 strains, only six shared the same AST pattern (AMP-AMS-Sul-STR). Isolates from patients and cakes were indistinguishable in PFGE results. All four methods, namely PFGE, wgMLST, cgMLST, and wgSNP were appropriate for bacterial typing in SE-related outbreak investigation. However, wgSNP can assign 12 SE strains from the first outbreak to one cluster and assign two SE strains from the second outbreak to another cluster, while PFGE, wgMLST, cgMLST did not successfully distinguish the SE strains from different outbreaks. Thus, we conclude that SNP-based phylogenetic analysis might be a viable method for differentiating SE strains at the outbreak level.