Effects of antimicrobial administration on the prevalence of antimicrobial-resistant Escherichia coli in broiler flocks.

Increase of antimicrobial-resistant (AMR) bacteria caused by antimicrobial usage is a public health problem. We investigated the proportion of cephalexin (LEX)-resistant bacteria in fresh feces obtained from antimicrobial-free broilers in three flocks at <15, 15-40, and >40 days of age. DHL agar plates containing 25 μg/mL LEX (DHL-L) showed LEX-resistant bacteria in all flocks at <15 days of age and in one flock at >40 days of age. The bacterial counts on DHL and DHL-L were 105 -108 colony forming units (CFU)/g feces and <102 -105 CFU/g feces, respectively. We also assessed the proportion of AMR bacteria in feces collected at 5, 12, 19, 26, 33, and 40 days of age from two flocks treated with amoxicillin at 5-7 days of age and co-trimoxazole at 24-26 days of age. The proportion of ampicillin (AMP)-resistant bacteria was elevated at 12 and 26-33 days of age on DHL containing 50 μg/mL AMP while no increase in LEX-resistant bacteria was observed on DHL-L. All isolates tested exhibited AMP resistance at 12 days of age while most exhibited resistance to both AMP and trimethoprim/sulfamethoxazole at 26-33 days of age. Our results suggested that antimicrobial administration influenced the selection of AMR bacteria with cross- and coresistance.