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)-positive Escherichia 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. Four blaCTX-M subtypes were identified. blaCTX-M-55 was 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 ISEcp1 and IS26 were detected surrounding the blaCTX-M gene, and 96.29% of strains successfully transferred the blaCTX-M gene 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 of blaCTX-M and underscore the need for rational antibiotic use, waste management, and vector control within a One Health framework.