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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 10 5 -10 8 colony forming units (CFU)/g feces and 10 2 -10 5 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.

Effects of Antimicrobial Administration on the Prevalence of Antimicrobial-Resistant <i>Escherichia coli</i> in Broiler Flocks