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

Abstract

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.

Antimicrobial resistance and genomic investigation of Salmonella isolated from retail foods in Guizhou, China

“…The antimicrobial resistance of all Salmonella isolates was examined using a previously reported micro broth dilution method (Xingbai) ( Kong et al, 2022 ). Salmonella isolates was tested against 15 antibiotics across 11 categories: ampicillin (AMP), ampicillin/sulbactam (AMS), tetracycline (TET), cefazolin (CFZ), cefotaxime (CTX), cephalosporin cefoxitin (CFX), ceftazidime (CAZ), imipenem (IMI), Gentamicin (GEN), Polymyxin E (Colistin, CT), Azithromycin (AZM), nalidixic acid (NAL), ciprofloxacin (CIP), chloramphenicol (CHL), and trimethoprim/sulfamethoxazole (SXT).…”

“…Blank and negative controls were also used. Escherichia coli ATCC 25922 and Pseudomonas aeruginosa ATCC 27853 were used as quality control strains to validate the results of the antimicrobial susceptibility testing ( Kong et al, 2022 ). Antimicrobial breakpoints were determined in line with the interpretive standards provided by the Clinical Laboratory Standards Institute (CLSI) guidelines ( CLSI, 2016 ).…”

“…The sequencing results for Salmonella strains were uploaded to the National Foodborne Disease Molecular Tracing Network (TraNet). Multi-locus sequence typing (MLST) and core genome MLST (cgMLST) analysis were performed using BioNumerics version 7.6 software (Applied Maths, Sint-Martens-Latem, Belgium) ( Kong et al, 2022 ). Complete linkage was used to construct a cluster analysis dendrogram.…”

“…This genomic information is crucial for developing effective therapies. Furthermore, comparative genomics among different Salmonella strains helps in tracing the source and transmission of outbreaks, thus playing a vital role in public health surveillance and response strategies ( Kong et al, 2022 ; Fatima et al, 2023 ).…”