The minimum inhibitory concentrations (MICs) and minimum mycoplasmacidal concentrations (MMCs) of danofloxacin, florfenicol, oxytetracycline, spectinomycin and tilmicosin against 62 recent British field isolates of Mycoplasma bovis were determined in vitro by a broth microdilution method. The isolates were most susceptible to danofloxacin with MIC90 and MMC90 values of 0.5 μg/ml and 1.0 μg/ml, respectively. They were less susceptible to florfenicol with a MIC90 of 16 μg/ml and MMC90 of 32 μg/ml. Oxytetracycline and spectinomycin had only a limited effect against the majority of isolates tested with MIC50s of 32 μg/ml and 4 μg/ml, respectively and MIC90s of 64 μg/ml and more than 128 μg/ml, respectively. Nearly 20 per cent of the isolates were highly resistant to spectinomycin, and tilmicosin was ineffective, with 92 per cent of the isolates having MIC values of 128 μg/ml or greater. There was no evidence of resistance by M bovis to danofloxacin.
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