Emergence and Clonal Spread of Extended-Spectrum β-Lactamase-Producing Salmonella Infantis Carrying pESI Megaplasmids in Korean Retail Poultry Meat.

Background/Objectives:Salmonella is a major cause of foodborne illnesses, with multidrug-resistant (MDR) strains posing significant threats to public health worldwide. This study investigated the prevalence and antimicrobial resistance (AMR) of Salmonella, focusing on extended-spectrum β-lactamase (ESBL)-producing Salmonella in retail poultry meat in Korea. Methods: A total of 300 poultry meat samples were collected nationwide from retail markets. Multi-locus sequence typing, serotyping, and antimicrobial susceptibility testing were performed. Whole-genome sequencing (WGS) analysis was conducted against 28 representative ESBL-producing S. Infantis isolates to identify the genetic characteristics and phylogenetic relationship. Results: Salmonella was detected in 81.3% of raw poultry meat samples, with S. Infantis ST32 being the dominant serotype in chicken (53.0%) and S. Typhimurium ST19 predominant in duck (39.0%). MDR was identified in 58.2% of samples, with a significantly higher rate in chicken isolates than in duck isolates (p < 0.001). Notably, 75.3% of chicken MDR isolates were ESBL-producing S. Infantis carrying blaCTX-M-65. WGS of 28 geographically and phenotypically representative ESBL-producing S. Infantis revealed five clonal clusters, suggesting the widespread dissemination of ESBL-producing S. Infantis across Korea's poultry supply chain. All 28 ESBL-producing S. Infantis isolates contained a pESI-like megaplasmid, carrying multiple resistance and virulence genes, with sequences highly identical to plasmids reported in the United States, indicating potential international transmission. Conclusions: This study emphasizes the urgent need for continuous surveillance and responsible antibiotic use in livestock under a One Health framework. WGS can provide an effective tool for tracking AMR evolution and clonal spread within and across regions.