Emergence and characteristics of multidrug-resistant <i>Salmonella enterica</i> subspecies <i>enterica</i> serovar Infantis harboring the pESI plasmid in chicken slaughterhouses in South Korea

<i>Salmonella enterica</i> subsp. <i>enterica</i> serovar Infantis (<i>S</i>. Infantis), a major cause of human salmonellosis, is commonly associated with transmission via contaminated chicken meat. This study, as part of the national <i>Salmonella</i> monitoring program, assessed the prevalence of <i>S. enterica,</i> including <i>S</i>. Infantis, in chicken slaughterhouses across South Korea from 2014-2022. The presence of a megaplasmid, known as plasmid of emerging <i>S</i>. Infantis (pESI), was confirmed. This confirmation was based on multidrug-resistant and third-generation cephalosporin-resistant <i>S</i>. Infantis isolates using whole-genome sequencing. Phenotypic and genotypic characterization involved antimicrobial susceptibility tests, pulsed-field gel electrophoresis, polymerase chain reaction to screen for pESI plasmids, plasmid profiling, and conjugation assays. <i>S</i>. Infantis was identified in 9.3% of <i>Salmonella</i>-positive samples in 2014, undetected from 2015-2020, but re-emerged as the predominant serovar in 2021 (54.7%) and 2022 (75.5%). The isolates in 2014 were antibiotic susceptible, whereas most isolates from 2021 to 2022 exhibited multidrug-resistance, including resistance to third-generation cephalosporins. All isolates were sequence type 32 (ST32), with core genome multilocus sequence typing demonstrating pESI plasmid-based clustering. The pESI⁺ isolates harbored genes, such as <i>aadA1</i>, <i>dfrA14</i>, <i>sul1</i>, and <i>tetA</i>, and three multidrug-resistant pESI⁺ isolates harbored <i>bla_CTX-M-65</i>. The plasmids were genetically similar to those observed in <i>S</i>. Infantis from broilers, chicken meat, and human clinical samples across various countries. This study highlights the spread of multidrug-resistant <i>S</i>. Infantis harboring the pESI plasmid with <i>bla_CTX-M-65</i> during early chicken production in South Korea. Continuous monitoring and control of resistant <i>S</i>. Infantis throughout the food chain is essential to inform public health initiatives.IMPORTANCEThis study highlights the critical emergence of multidrug-resistant (MDR) <i>Salmonella enterica</i> serovar Infantis (<i>S</i>. Infantis) in South Korea's chicken slaughterhouses, driven by the acquisition of the pESI megaplasmid harboring the extended-spectrum beta-lactamase (ESBL) determinant <i>bla_CTX-M-65</i>. Using whole-genome sequencing and comprehensive phenotypic-genotypic analyses, the findings reveal that pESI⁺ isolates in South Korea are genetically similar to strains from broilers, chicken meat, and human clinical cases worldwide. This underscores the transboundary nature of <i>S</i>. Infantis and its potential as a global public health threat.