Avian pathogenic Escherichia coli (APEC) causes colibacillosis, which is an economically important disease in the poultry industry worldwide. The present study investigated O-serogroups, phylogenetic groups, antimicrobial resistance, and the existence of virulence-associated genes (VAGs) and antimicrobial resistance genes in 125 APEC isolates between 2018 and 2019 in Korea. The phylogenetic group B2 isolates were confirmed for human-related sequence types (STs) through multi-locus sequence typing (MLST). O-serogroups O2 (12.5%) and O78 (10.3%) and phylogenetic group B1 (36.5%) and A (34.5%) were predominant in chicken and duck isolates, respectively. Out of 14 VAGs, iucD, iroN, hlyF, and iss were found significantly more in chicken isolates than duck isolates (p < 0.05). The resistance to ampicillin, ceftiofur, ceftriaxone, and gentamicin was higher in chicken isolates than duck isolates (p < 0.05). The multidrug resistance (MDR) rates of chicken and duck isolates were 77.1% and 65.5%, respectively. One isolate resistant to colistin (MIC 16 μg/mL) carried mcr-1. The B2-ST95 APEC isolates possessed more than 9 VAGs, and most of them were MDR (82.4%). This report is the first to compare the characteristics of APEC isolates from chickens and ducks in Korea and to demonstrate that B2-ST95 isolates circulating in Korea have zoonotic potential and pose a public health risk.

Salmonella enterica subsp. enterica serovar Infantis ( S . Infantis), a major cause of human salmonellosis, is commonly associated with transmission via contaminated chicken meat. This study, as part of the national Salmonella monitoring program, assessed the prevalence of S. enterica, including S . Infantis, in chicken slaughterhouses across South Korea from 2014–2022. The presence of a megaplasmid, known as plasmid of emerging S . Infantis (pESI), was confirmed. This confirmation was based on multidrug-resistant and third-generation cephalosporin-resistant S . 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. S . Infantis was identified in 9.3% of Salmonella -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 aadA1 , dfrA14 , sul1 , and tetA , and three multidrug-resistant pESI ⁺ isolates harbored bla _CTX-M-65 . The plasmids were genetically similar to those observed in S . Infantis from broilers, chicken meat, and human clinical samples across various countries. This study highlights the spread of multidrug-resistant S . Infantis harboring the pESI plasmid with bla _CTX-M-65 during early chicken production in South Korea. Continuous monitoring and control of resistant S . Infantis throughout the food chain is essential to inform public health initiatives. IMPORTANCE: This study highlights the critical emergence of multidrug-resistant (MDR) Salmonella enterica serovar Infantis ( S . Infantis) in South Korea’s chicken slaughterhouses, driven by the acquisition of the pESI megaplasmid harboring the extended-spectrum beta-lactamase (ESBL) determinant bla _CTX-M-65 . 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 S . Infantis and its potential as a global public health threat.

Campylobacter jejuni represents one of the leading causes of bacterial gastroenteritis in humans and is primarily linked to chicken meat contamination. In the present study, we analyzed the virulence and survival genes, antimicrobial resistance, and the clonal distribution of 50 C. jejuni isolates obtained from various sources in 14 chicken slaughterhouses across 8 provinces in South Korea from 2019 to 2022. Furthermore, we determined their genetic relatedness to human-derived isolates registered in PubMLST using multilocus sequence typing (MLST). All isolates harbored various virulence and survival genes ( flh A, cad F, cdt A, cdt C, cme A, and sod B) out of 17 tested genes, as confirmed via polymerase chain reaction analysis. Adherence factor gene vir B11 was not detected in any isolate. All isolates harbored 12 or more virulence and survival genes. Antimicrobial susceptibility testing indicated that ciprofloxacin resistance was the most prevalent (84.0%), followed by nalidixic acid (82.0%) and tetracycline (52.0%) resistance. MLST analysis of the isolates revealed 18 sequence types (STs), including four new ones. Overlapping STs between chicken slaughterhouse and human-derived isolates included ST42, ST45, ST50, ST137, ST354, and ST464. Our study identified 11 clonal complexes (CCs), with CC-21 being the most prevalent in both human and chicken slaughterhouse-derived isolates. This study provides comprehensive insights into recent C. jejuni isolates from chicken slaughterhouses, including data on quinolone resistance and virulence factors. The MLST-based genetic relatedness between isolates from humans and chicken slaughterhouses in this study suggests the potential of C. jejuni transmission from chickens to humans through the food chain. This study suggests the need for improved management practices in chicken slaughterhouses to reduce the transmission of chicken slaughterhouse-derived C. jejuni to humans.

Kang J, et al. J Vet Sci. 2020 Mar;21(2):e37. https://doi.org/10.4142/jvs.2020.21.e37