Effects of Antimicrobial Administration on the Prevalence of Antimicrobial-Resistant Escherichia coli in Broiler Flocks

抄録

The increase in 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 old. DHL agar plates containing 25 μg/mL LEX (DHL-L) showed LEX-resistant bacteria in all flocks at ＜15 days old and in one flock at ＞ 40 days old. 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 old from two flocks treated with amoxicillin at 5–7 days old and co-trimoxazole at 24–26 days old. The proportion of ampicillin (AMP)-resistant bacteria was elevated at 12 and 26–33 days old 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 old, while most exhibited resistance to both AMP and trimethoprim/sulfamethoxazole at 26–33 days old. Our results suggest that antimicrobial administration influenced the selection of AMR bacteria with cross- and coresistance.