In vitro antimicrobial inhibition of Mycoplasma bovis isolates submitted to the Pennsylvania Animal Diagnostic Laboratory using flow cytometry and a broth microdilution method

Mycoplasma bovis is an important pathogen of cattle, causing mastitis, pneumonia, conjunctivitis, otitis, and arthritis. Currently there are only a few reports of sensitivity levels for M. bovis isolates from the United States. Mycoplasma bovis isolates submitted to the Pennsylvania Animal Diagnostic Laboratory between December 2007 and December 2008 (n = 192) were tested for antimicrobial susceptibility to enrofloxacin, erythromycin, florfenicol, spectinomycin, ceftiofur, tetracycline, and oxytetracycline using a broth microdilution method. The most effective antimicrobials against M. bovis determined by using the broth microdilution method were florfenicol, enrofloxacin, and tetracycline with minimum inhibitory concentration (MIC) ranges of 2–32 µg/ml, 0.1–3.2 µg/ml, and 0.05 to >12.8 µg/ml, respectively. Spectinomycin, oxytetracycline, and tetracycline showed a wide-ranging level of efficacy in isolate inhibition with broth microdilution with MIC ranges of 4 to >256 µg/ml, 0.05 to >12.8 µg/ml, and 0.05 to >12.8 µg/ml, respectively. A significant difference in the susceptibility levels between quarter milk and lung isolates was found for spectinomycin. When MIC values of a subset of the M. bovis isolates (n = 12) were tested using a flow cytometric technique, the MIC ranges of enrofloxacin, spectinomycin, ceftiofur, erythromycin, tetracycline, oxytetracycline, and florfenicol ranges were 0.1–0.4 µg/ml, 4 to >256 µg/ml, >125 µg/ml, >3.2 µg/ml, 6.4 µg/ml, 0.8 to >12.8 µg/ml, and