Antimicrobial susceptibility of Mycoplasma bovis isolates from veal calves and dairy cattle in the Netherlands.

Control of Mycoplasma bovis infections depends on good husbandry practices and antibiotic treatment. To allow more prudent use of antimicrobial drugs, there is a need for information on the susceptibility profile of this pathogen. The objective of the present study was to analyse the in vitro antimicrobial susceptibility of clinical M. bovis isolates in the Netherlands. The collection comprised 95 bovine isolates, originating from lungs (n=56), mastitis milk (n=27), and synovial fluid (n=12), collected between 2008 and 2014. Minimal inhibitory concentrations (MICs) were assessed by broth microdilution, both by using in-house prepared MIC plates and by using commercially available MIC plates. For each antimicrobial agent, the range of MIC results, the MIC50, and MIC90 values were calculated. M. bovis strains recently isolated in the Netherlands appeared to be characterized by relatively high MIC values for antimicrobial agents that, until now, have been recommended by the Dutch Association of Veterinarians for treating pneumonia caused by Mycoplasma species. Fluoroquinolones appeared to be the most efficacious in inhibiting M. bovis growth, followed by tulathromycin and oxytetracycline. The highest MIC values were obtained for erythromycin, tilmicosin, and tylosin. Future studies should be done on determining M. bovis specific clinical breakpoints, standardization of methods to determine MIC values as well as molecular studies on detection of antimicrobial resistance mechanisms of M. bovis isolates to develop PCR assays for determining resistance.