Susceptibility of Mycoplasma bovis field isolates to antimicrobial agents

The purpose of this study was to determine the antibacterial susceptibility of field isolates of Mycoplasma bovis originating from the upper respiratory tract of cattle of different ages. Bacteriological examination of 90 nasal swabs collected from calves at three months of age identified M. bovis in 31 (34.44%) samples. Seventeen (18.88%) of these animals still housed M. bovis in their nasal cavity at nine months and five animals (5.55%) still at seventeen months of age. M. bovis were confirmed by biochemical and antigenic methods. To confirm that these belonged to the M. bovis species isolated mycoplasmas were tested using the PCR method. Fifteen field strains of M. bovis isolated from the same cattle at three, nine and seventeen months (five strains from each age group) were selected for antibacterial susceptibility testing against six groups of antimicrobial agents using an agar dilution method. The MIC90 ranges established for tylosin, tulathromycin, enrofloxacin, florfenicol and lincomycin were 0.39-0.78 µg/ml, 0.50-1.00 µg/ml, 0.78-1.56 µg/ml, 3.12 µg/ml and 0.39-0.78 µg/ml, respectively. The range of MIC90 for oxytetracycline was from 50 to 100 µg/ml. Preliminary examination of the antimicrobial susceptibility of field strains of M. bovis did not reveal significant differences between different age groups of cattle. After evaluation of the MIC90 data with the SPSS 13.0 statistical package it was found that M. bovis isolates from animals at three, nine and seventeen months were similarly susceptible to tylosin and tulathromycin. Statistically significant differences in susceptibility of M. bovis isolated from cattle of different ages were found to florfenicol compared with tulathromycin (P < 0.01), lincomycin (P < 0.01) and enrofloxacin (P < 0.05). The susceptibility of all M. bovis isolates to oxytetracycline and penicillin G significantly differed from the sensitivity to all other antimicrobial agents used in the present study (P < 0.05). The in vitro susceptibility test showed that field isolates of M. bovis isolated from cattle of different ages were similarly sensitive to tylosin, tulathromycin, lincomycin and enrofloxacin. It was also determined that the field strains are resistant to oxytetracycline.