Genetic characterization of CTX-M-55-producing <i>Escherichia coli</i> isolated from commercial farms by integrated broiler chicken operations in Korea

Recently, the occurrence of β-lactamase CTX-M-55-harboring Escherichia coli has been rapidly increasing in poultry worldwide, including Korea. In this study, three CTX-M variants were identified within 466 β-lactam-resistant E. coli isolated from fecal and dust samples of 74 commercial farms, with CTX-M-55 (28.4%) having the highest prevalence in all operations. Among the 28 CTX-M-55-producing E. coli isolates, ST3285 (28.6%) and group B1 (60.7%) were the most prevalent sequence type (ST) and phylogenetic group, respectively. All CTX-M-55 producing E. coli isolates were resistant to ampicillin, and showed very high minimum inhibitory concentrations of ≥512 to cefazolin, the first-generation cephalosporins, and to cefotaxime and ceftiofur, which are third-generation cephalosporins, respectively. Moreover, 27 (96.4%) CTX-M-55-producing E. coli isolates showed multidrug resistance against four or more antimicrobial classes. The most common plasmid replicon type was the combination of FII and FIB (67.9%). Also, 10 (35.7%) CTX-M-55-producing E. coli isolates harbored other β-lactamase genes simultaneously, and 12 (42.9%) isolates were identified as avian pathogenic E. coli. This study suggests that comprehensive surveillance and continuous monitoring are necessary to prevent the spread of CTX-M-55-producing E. coli in the poultry industry , which poses a threat to public health.