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

<b>Background/Objectives:</b><i>Salmonella</i> 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 <i>Salmonella</i>, focusing on extended-spectrum β-lactamase (ESBL)-producing <i>Salmonella</i> in retail poultry meat in Korea. <b>Methods</b>: 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 <i>S</i>. Infantis isolates to identify the genetic characteristics and phylogenetic relationship. <b>Results</b>: <i>Salmonella</i> was detected in 81.3% of raw poultry meat samples, with <i>S</i>. Infantis ST32 being the dominant serotype in chicken (53.0%) and <i>S</i>. 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 (<i>p</i> < 0.001). Notably, 75.3% of chicken MDR isolates were ESBL-producing <i>S</i>. Infantis carrying <i>bla_CTX-M-65</i>. WGS of 28 geographically and phenotypically representative ESBL-producing <i>S</i>. Infantis revealed five clonal clusters, suggesting the widespread dissemination of ESBL-producing <i>S</i>. Infantis across Korea's poultry supply chain. All 28 ESBL-producing <i>S</i>. 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. <b>Conclusions</b>: 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.