High prevalence of blaCTX-M-55-carrying Escherichia coli in both ceftiofur-use and non-use pig farms

Abstract

The emergence of third-generation cephalosporin-resistant bacteria poses a significant human health hazard worldwide. We aimed to investigate the prevalence of extended-spectrum β-lactamase (ESBL)/AmpC β-lactamase-producing third-generation cephalosporin-resistant Escherichia coli in ceftiofur-use and non-use pig farms. A total of 585 E. coli isolates were collected from pig feces (n = 354) and farm environments (n = 231) across various farms between 2022 and 2023 in South Korea. Among them, 61% of E. coli from pigs and 22.9% from the environment were resistant to ceftiofur, with significantly higher rates in ceftiofur-use farms. The most common ESBL gene detected was blaCTX-M-55 (59.9%), followed by blaCTX-M-15 (12.3%) and blaCTX-M-14 (7.4%). Resistance to non-beta-lactam antimicrobials varied, with blaCTX-M-55-carrying E. coli showing high resistance. Gene transfer via conjugation was observed, particularly for blaCTX-M-1 (85.7%) and blaCTX-M-55 (46.6%). Of note, seven isolates co-harboring mcr-1 and blaCTX-M-55 were found in pigs. Replicon type IncFIB (85.3%) was mainly found in blaCTX-M-55-carrying plasmids and IncI1 (91.7%) in blaCTX-M-1. The most frequently observed sequence types were ST457, ST410, ST101, ST75, and ST10, which comprised 38.4% of the isolates. The transposable elements IS26-ISEcp1-blaCTX-M-55-orf477 (76%), ISEcp1-blaCTX-M-55-orf477 (22.7%), blaCTX-M-65-IS903 (50%), and blaCTX-M-14-IS903 (100%) were predominantly detected. This study implies that ceftiofur use, clonal, and horizontal transfer might cause a high prevalence of ESBLs in pig farms. Thus, these findings emphasize the importance of prudent antimicrobial use and biosecurity measures to combat antimicrobial resistance.