To analyze the etiology and traceability of foodborne pathogens in two suspected mixed infection food poisoning events in Shaoxing City in 2024.

The re-identified isolates were serotyped by slide agglutination, virulence genes (tlh+/tdh+/trh-) of Vibrio parahaemolyticus were identified by real-time fluorescence polymerase chain reaction, drug resistance was detected by broth microdilution method, and pulsed field gel electrophoresis (PFGE) was used to detect the isolates, and were performed for molecular typing. Acquired drug resistance genes were predicted by ResFinder and multilocus sequence typing (MLST) analysis by PubMLST database. core genome multilocus sequence typing (cgMLST) was used for molecular tracing.

Of the 2 incidents, both 2 isolates of Vibrio parahaemolyticus from incident 1 were serotype O10:K4, with virulence gene profile tlh+/tdh+/trh-, resistant to cefazolin, and exhibited identical PFGE banding patterns, indicating homologous strains. The 2 Salmonella agona isolates were susceptible to all 27 kinds of antibiotics, showed identical PFGE banding patterns, and were identified as homologous strains. The 4 Vibrio parahaemolyticus isolates from incident 2 were all serotype O3:K6, with virulence gene profile tlh+/tdh+/trh- and identical PFGE banding patterns, but exhibited two distinct antimicrobial resistance phenotypes: Cefazolin and ampicillin-cefazolin. The 2 Salmonella ruzizi II isolates showed different antimicrobial resistance phenotypes: Nalidixic‑tetrecycline and ampicillin‑streptomycin‑azithromycin‑trimethoprim/sulfamethoxazole‑gentamicin‑tetrecycline‑chloramphenicol‑florfenicol, with a PFGE band similarity of 51.90%. Whole-genome sequencing results showed that the 2 Vibrio parahaemolyticus isolates from incident 1 belonged to ST3 by MLST, and the β-lactam resistance gene bla_CARB-22 was predicted in each isolate. The Salmonella isolates were identified as ST13, and 4 kinds of antimicrobial resistance genes were predicted in total.The cgMLST clustering results showed high similarity between the isolates, respectively. The 4 Vibrio parahaemolyticus isolates from incident 2 belonged to ST3, and the bla_CARB-22 resistance gene was predicted in each isolate. The 2 Salmonella isolates were identified as ST516 and ST398, with 19 kinds of antimicrobial resistance genes predicted. cgMLST clustering revealed that the Vibrio parahaemolyticus isolates were highly homologous, whereas the Salmonella isolates were highly divergent.

PFGE typing, whole-genome sequencing-based MLST, and cgMLST results demonstrated that incident 1 is a food poisoning case caused by mixed infection with Vibrio parahaemolyticus and Salmonella, whereas incident 2 is attributed to Vibrio parahaemolyticus alone. The etiological characteristics of mixed-infection food poisoning incidents in Shaoxing City provide a theoretical basis for the prevention and control of such mixed infections. It is recommended that relevant departments strengthen food risk supervision capabilities and enhance public food safety awareness.