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, <0.025 to >6.4 μg/ml, 0.8 to >12.8 μg/ml, and <2-4 μg/ml, respectively. Flow cytometry offers potential in clinical applications due to high-throughput capability, quick turnaround time, and the objective nature of interpreting results.