Prevalence and characteristics of ESBL-producing Escherichia coli in clinically healthy pigs: implications for antibiotic resistance spread in livestock.

This study focused on characterizing antibiotic resistance profiles and extended-spectrum beta-lactamase (ESBL) genes, particularly blaCTX-M variants, in Escherichia coli isolated from healthy finishing pigs fed diets with or without antibiotics on Brazilian farms. Among 27 ceftiofur-resistant multidrug-resistant E. coli isolates, a high prevalence of ESBL production (85%) and various resistance genes—including blaTEM, blaCTX-M-1, blaCTX-M-2, blaCTX-M-8,25, sul2, ant(3″)-I, tetA, and mcr-1—were detected, with nine isolates demonstrating horizontal transfer of blaCTX-M via conjugative plasmids confirmed by Southern blot hybridization. The findings reveal that commensal E. coli in swine serve as reservoirs of mobile genetic elements carrying ESBL genes, regardless of antibiotic exposure, underscoring the potential for dissemination of antimicrobial resistance within livestock microbiota and raising concerns for public health.