Research Note: Epidemiological cutoff values and acquired resistance mechanisms of three veterinary antibiotics against Escherichia coli from chicken respiratory tract infections.

Florfenicol, apramycin, and danofloxacin are antibiotics approved only for veterinary use and that have good therapeutic effects on chicken respiratory infections caused by Escherichia coli. We established epidemiological cutoff values (ECV) for these antibiotics using 363 E. coli isolates from tracheal samples of chickens in 5 veterinary clinics in Guangdong Province, China. The minimum inhibitory concentrations (MIC) were determined using the agar dilution method as per Clinical and Laboratory Standards Institution guidelines. The ECV were then calculated using the statistical method and verified by normalized resistance interpretation and ECOFFinder software programs. The ECV of florfenicol, apramycin, and danofloxacin against E. coli were 16, 16, and 0.125 μg/mL, respectively. Susceptibility tests indicated that these isolates were resistant to florfenicol (66.7%), apramycin (22.3%), and danofloxacin (92.3%). Strains carrying floR were distributed in the range of MIC ≥32 μg/mL for florfenicol. Apramycin resistance was found in 77 strains (77/363, 21.1%), and isolates that carried aac(3)-IV were all in the range of MIC ≥512 μg/mL. Danofloxacin resistance was found in the range of MIC ≤0.125 μg/mL, but there were no mutations in the quinolone resistance-determining regions and plasmid-mediated quinolone resistance genes qnrA, qnrB, qnrC, qnrD, aac-(6')-Ib-cr, qep, and oqxB. The presence of the qnrS gene was verified in a few of the strains with an MIC of 0.06 μg/mL. The establishment of ECV was significant for monitoring of resistance development and therapy guidance.