Farm-level evaluation of enrofloxacin resistance in Escherichia coli isolated from broiler chickens during a rearing period

In this study, enrofloxacin resistance was investigated in commensal Escherichia coli isolated from broiler chickens using phenotypic and genotypic assays. Five E. coli isolates were recovered from the cloacal swabs of 15 broiler chickens in the farms at the ages of 1 day (stage 1), 30 days (stage 2), and 1 day before slaughter (stage 3). Minimum inhibitory concentration results for enrofloxacin showed that the prevalence of resistance significantly increased (P < 0.05) during the rearing period (43, 80, and 86%, respectively, for stages 1, 2, and 3, respectively). Relation of enrofloxacin resistance with gyrA and parC mutations was evaluated by sequencing and PCR-RFLP. Wild-type isolates (without mutations in gyrA and parC) and isolates with a single mutation in Ser-83 were higher in early days of the rearing period of chickens in comparison with the second and third stages. Mutations at both Ser-83 and Asp-87 of gyrA and at Ser-80 of parC were higher in the second and third stages of rearing period. No qnrA, qnrB, and qnrS determinants were detected among these E. coli isolates. Evaluation of different risk factors showed that factors related to antimicrobial exposure such as antibiotic consumption in previous stocking, occurrence of colibacillosis and respiratory viral infections, and antibiotic usage in recent stocking significantly increased the rate of enrofloxacin resistance (P < 0.05).