Molecular tracing and pathogenic analysis of two suspected homologous Staphylococcus aureus food poisoning incidents

ObjectiveTraceability and pathogenetic analysis of two suspected homologous （S. aureus） food poisoning cases.MethodsWe conducted initial screening of pathogens in collected samples of suspected contaminated food， anal swabs from cases， and environmental swabs using a multiplex PCR kit for foodborne pathogens. The isolation， culture， and identification of pathogenic bacteria were performed according to GB 4789.10—2016 in combination with MALDI-TOF MS. Whole genome sequencing and bioinformatics analysis were performed on the isolates. Broth microdilution method for determining resistance to 15 antimicrobial agents， and ELISA for detecting enterotoxins.ResultsBoth incidents shared a common food exposure history. Incident 1 yielded 12 ST2315-t11687 S. aureus isolates， while Incident 2 yielded 2 ST2315-t11687 and 1 ST398-t2346 isolates. cgSNP analysis showed high homology among ST2315-t11687 isolates from both incidents （SNP differences≤8）， while ST398-t2346 was genetically distant. The ST2315-t11687 strain consistently carries enterotoxin genes sea， sec， and sell. ELISA results showed that some strains were positive for SEA/SEC/SEE （n=9）， a few were positive only for SEA （n=2）， or negative （n=3）. The ST398-t2346 S. aureus strains were negative for both enterotoxin genes and enterotoxin production phenotypically. The ST2315-t11687 strains displayed consistent antimicrobial resistance profiles， being resistant to both penicillin and levofloxacin. Importantly， the ST398-t2346 strains were confirmed as methicillin-resistant S. aureus through detection of the mecA gene and demonstrated resistance to penicillin， oxacillin， and cefoxitin. Virulence gene analysis showed ST2315-t11687 carried the immune evasion cluster （sea-sak-chp-scn） and hemolysin genes （hly/hla， hlb， hld， hlgA， hlgB， hlgC）. ST398-t2346 lacked sea but possessed hemolysins and other immune-modulating virulence factors.ConclusionBoth food poisoning incidents were caused by enterotoxigenic S. aureus， with transmission linkage of ST2315-t11687 clone between incidents. The prevalent strain exhibits strong pathogenicity， highlighting the need to enhance surveillance of highly pathogenic S. aureus in foodborne disease monitoring.