Molecular Epidemiology of the blaCTX-M Gene in Escherichia coli from a Pig Farm: Antimicrobial Resistance Profiles, Genetic Background, and Its Horizontal Transfer and Environmental Dissemination

This study investigated the epidemiology, antimicrobial resistance, and transmission risks of β-lactamase, cefotaxime-hydrolyzing, Munich (blaCTX-M)-positiveEscherichia coli(CTX-M-EC) in large-scale pig farms in Jiangxi Province (China). In total, 278 samples (manure, wastewater, drinking water, and flies) were collected. CTX-M-EC strains were isolated and analyzed using antimicrobial susceptibility testing, resistance gene profiling, multilocus sequence typing, and genetic environment analysis with gene transfer assessed by transduction experiments. Twenty-seven CTX-M-EC strains (9.71%) were isolated, all exhibiting multi-drug resistance with 100% resistance to cefotaxime, ciprofloxacin, and tetracycline, and >90% resistance to ceftazidime, florfenicol, and trimethoprim-sulfamethoxazole. FourblaCTX-Msubtypes were identified.blaCTX-M-55was the predominant subtype (70.37%) and was distributed across diverse sequence types and serotypes. Each strain harbored multiple antibiotic resistance genes, plasmids, and virulence genes. Mobile elements such as ISEcp1and IS26were detected surrounding theblaCTX-Mgene, and 96.29% of strains successfully transferred theblaCTX-Mgene via transduction. Clones highly homologous to pig manure strains were detected in flies and sewage, suggesting that this resistance gene can spread between animals, the environment, and vectors. These findings highlight the high transmission risk ofblaCTX-Mand underscore the need for rational antibiotic use, waste management, and vector control within a One Health framework.