In vitro susceptibilities of field isolates of Mycoplasma agalactiae to oxytetracycline, tylosin, enrofloxacin, spiramycin and lincomycin-spectinomycin.
The minimum inhibitory concentrations (MICs) of tetracycline, enrofloxacin, tylosin, spiramycin and a lincomycin:spectinomycin 1:2 combination, against 24 Sicilian isolates of Mycoplasma agalactiae, the causative organism of contagious agalactia were determined in vitro by a broth dilution method. Enrofloxacin was the most effective antimicrobial in vitro with a range of MIC values from 0.125 to 0.500 µg/ml and an MIC50 of 0.203 and MIC90 of 0.365 µg/ml. Using the MIC50 and MIC90 values the remaining four antimicrobials are ranked in order of in vitro effectiveness as follows: tylosin (MIC50 0.292; MIC90 0.525 µg/ml) was slightly more effective than tetracycline (MIC50 0.296; MIC90 0.533 µg/ml), followed by lincomycin:spectinomycin (MIC50 0.521; MIC90 0.938 µg/ml) and spiramycin (MIC50 1.583; MIC90 2.850 µg/ml). MIC values above 1.000 µg/ml were obtained using tetracycline, tylosin and spiramycin for some M. agalactiae isolates.