Use of susceptibility testing in veterinary medicine.

There has been increased interest in optimizing treatment protocols for antimicrobial agents, with substantial reliance on susceptibility testing of bacterial pathogens isolated from diseased cattle. Antimicrobial susceptibility testing of bovine bacterial pathogens has traditionally used the agar diffusion (Kirby-Bauer) method, which was designed to reflect the antibiotic concentration in serum and interstitial fluid of human patients. The validity of agar diffusion susceptibility breakpoints derived from humans to the treatment of mastitis, diarrhea and respiratory disease in cattle has not been established. The use of susceptibility testing to guide treatment decisions for individual cattle is not recommended until the breakpoints have been validated as being predictive of treatment outcome.