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Antimicrobial susceptibility of Mycoplasma bovis strains isolated from dairy cattle herds in Antimicrobial susceptibility of Mycoplasma bovis strains isolated from dairy cattle herds in Croatia

Mycoplasma bovis is the causative agent of severe clinical mastitis in several allied cattle herds in Croatia and appeared resistant to conventional antimicrobial therapy. Other clinical conditions including pneumonia, arthritis, otitis and reproductive problems are associated with this pathogen and are reported worldwide. In 2002 and between 2007 and 2009 eighty-seven Mycoplasma bovis isolates, recovered from milk, lungs, nasal swabs and joint fluid of Croatian dairy cows or calves were confirmed by PCR and antigen capture ELISA. Subsequently minimum inhibitory concentrations (MIC’s) for seven antimicrobials were determined using a microbroth dilution method. Antimicrobials were supplied freeze-dried at specified concentrations in ‘Sensititre’ plates. The plates were designed to provide doubling dilutions of the antimicrobials from 0.12 µg/ml to 32 µg/ml (or from 0.25 µg/ml to 128 µg/ml for tulathromycin). The inoculum was standardised at approx 5×105 CFU per ml and the tests read after 48 hours incubation at 37°C. The MIC50 for enrofloxacin, marbofloxacin, lincomycin, tulathromycin, florfenicol, oxytetracycline and tylosin were 0.25, 0.5, 1, 2, 8, 8 and >32 µg/ml, respectively. Lincomycin, tulathromycin, florfenicol, oxytetracycline and tylosin had a wide range of MIC values indicating the development of antimicrobial resistance and the involvement of different strains in outbreaks. In vitro susceptibility should give an indication of the antimicrobials effectiveness for treating animals in vivo, although many factors may affect their efficacy and the successful treatment of infected cattle. Our results provide an indication of the antimicrobials that are most likely to be effective. With the range of MIC’s obtained from the isolates tested we recommend testing M. bovis isolates to help select the best therapeutic treatment.

MIC; Mycoplasma bovis; antimicrobials

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