Fecal carriage, transferable β-lactam resistance, and efflux pump contribution in third-generation cephalosporin-resistant Escherichia coli from small-scale farm animals and workers in northern Thailand.

BACKGROUND AND AIM: Food-producing animals are recognized reservoirs of antimicrobial-resistant bacteria with zoonotic potential. Third-generation cephalosporin-resistant (3GC-R) Escherichia coli is of particular public health concern due to its association with extended-spectrum β-lactamase (ESBL) and plasmid-mediated AmpC enzymes. This study aimed to determine the prevalence, resistance mechanisms, transferability of resistance genes, efflux pump contribution, and genetic relatedness of 3GC-R E. coli isolated from farm animals and workers on a small-scale farm in lower northern Thailand.

MATERIALS AND

METHODS

A total of 265 fecal samples were collected from laying hens (n = 210), cattle (n = 33), swine (n = 19), and farm workers (n = 3). Isolation of 3GC-R E. coli was performed using cefotaxime-supplemented selective media. Antimicrobial susceptibility was determined by disk diffusion and minimum inhibitory concentration (MIC) assays. The presence of bla CTX-M and bla CMY-2 genes was identified by polymerase chain reaction and sequencing. Conjugation assays and plasmid replicon typing assessed gene transferability. Efflux pump involvement in ceftazidime resistance was evaluated using phenylalanine-arginine β-naphthylamide. Genetic relatedness was analyzed by pulsed-field gel electrophoresis.

RESULTS

Overall, 15.8% of samples yielded 3GC-R E. coli, with the highest prevalence observed in swine (47.4%), followed by cattle (27.3%) and laying hens (10.5%). Multidrug resistance was detected in 57.1% of isolates. The majority carried bla CTX-M alone (69.0%) or in combination with bla CMY-2 (21.4%), and both genes were transferable via IncF and IncI1-I plasmids. A ≥4-fold reduction in ceftazidime MICs in the presence of an efflux pump inhibitor was observed in 38.7% of isolates. While genetically identical strains were detected among different animal species, no clonal transmission between animals and workers was identified.

CONCLUSION

Small-scale farms in Thailand harbor 3GC-R E. coli with transferable resistance determinants and multiple resistance mechanisms, underscoring their role in antimicrobial resistance dissemination. These findings highlight the need for strengthened biosecurity, antimicrobial stewardship, and integrated One Health surveillance in rural farming systems.