Impact of Low and High Doses of Marbofloxacin on the Selection of Resistant Enterobacteriaceae in the Commensal Gut Flora of Young Cattle: Discussion of Data from 2 Study Populations.

In the context of requested decrease of antimicrobial use in veterinary medicine, our objective was to assess the impact of two doses of marbofloxacin administered on young bulls (YBs) and veal calves (VCs) treated for bovine respiratory disease, on the total population of Enterobacteriaceae in gut flora and on the emergence of resistant Enterobacteriaceae. In two independent experiments, 48 YBs from 6 commercial farms and 33 VCs previously colostrum deprived and exposed to cefquinome were randomly assigned to one of the three groups LOW, HIGH, and Control. In LOW and HIGH groups, animals received a single injection of, respectively, 2 and 10 mg/kg marbofloxacin. Feces were sampled before treatment, and at several times after treatment. Total and resistant Enterobacteriaceae enumerating were performed by plating dilutions of fecal samples on MacConkey agar plates that were supplemented or not with quinolone. In YBs, marbofloxacin treatment was associated with a transient decrease in total Enterobacteriaceae count between day (D)1 and D3 after treatment. Total Enterobacteriaceae count returned to baseline between D5 and D7 in all groups. None of the 48 YBs harbored marbofloxacin-resistant Enterobacteriaceae before treatment. After treatment, 1 out of 20 YBs from the Control group and 1 out of 14 YBs from the HIGH group exhibited marbofloxacin-resistant Enterobacteriaceae. In VCs, the rate of fluoroquinolone-resistant Enterobacteriaceae significantly increased after low and high doses of marbofloxacin treatment. However, the effect was similar for the two doses, which was probably related to the high level of resistant Enterobacteriaceae exhibited before treatment. Our results suggest that a single treatment with 2 or 10 mg/kg marbofloxacin exerts a moderate selective pressure on commensal Enterobacteriaceae in YBs and in VCs. A fivefold decrease of marbofloxacin regimen did not affect the selection of resistances among commensal bacteria.