Antimicrobial Resistance Profiles and Molecular Characteristics of Extended-Spectrum β-Lactamase-Producing Escherichia coli Isolated from Healthy Cattle and Pigs in Korea

Antimicrobial-resistant bacteria isolated from food animals pose a major health threat to the public on this planet. This study aimed to determine the susceptibility profiles of Escherichia coli isolated from cattle and pig fecal samples and investigate the molecular characteristics of extended-spectrum beta-lactamase (ESBL)-producing E. coli using gene identification, conjugation, and Southern blot approach. Overall 293 E. coli were recovered from cattle (120 isolates) and pigs (173 isolates) in 7 provinces of Korea during 2017-2018. Ampicillin, chloramphenicol, streptomycin, and sulfisoxazole resistance rates were the highest in pigs' isolates (> 60%, p <= 0.001) compared to that in cattle (3-39%). Multidrug resistance (MDR) was higher in pig isolates (73%) than in cattle (31%), and the MDR profile usually includes streptomycin, sulfisoxazole, and tetracycline. Resistance to critically important antimicrobials such as ceftiofur, colistin, and ciprofloxacin was higher in weaners than those from finishers in pigs. The qnrS gene was detected in 13% of the pig isolates. Eight isolates from pigs and one isolate from cattle were identified as ESBL-producers and ESBL genes belonged to bla(CTX-M-55) (n = 4), bla(CTX-M-14) (n = 3), and bla(CTX-M-65) (n = 2). Notably, the bla(CTX-M-65) and qnrS1 genes were found to be carried together in an identical plasmid (IncHI2) in two isolates from finisher pigs. The bla(CTX-M)-carrying isolates belonged to phylogenetic groups B1 (n = 4), B2 (n = 2), A (n = 2), and D (n = 1). The bla(CTX-M) genes and non-beta-lactam resistance traits were transferred to the E. coli J53 recipient from seven bla(CTX-M)-positive strains isolated from pigs. The bla(CTX-M) genes belonged to the IncI1 alpha, IncFII, and IncHI2 plasmids and are also associated with the ISEcp1, IS26, IS903, and orf477 elements. These findings suggested the possibility of bla(CTX-M)-carrying E. coli transmission to humans through direct contact with cattle and pigs or contamination of food products.