Minimum inhibitory concentration breakpoints and disk diffusion inhibitory zone interpretive criteria for tilmicosin susceptibility testing against Pasteurella spp. associated with bovine respiratory disease.

Tilmicosin is a novel macrolide antibiotic developed for exclusive use in veterinary medicine. The first tilmicosin-containing product was approved to treat bovine respiratory disease associated with pasteurellae. The development of antimicrobial susceptibility testing guidelines for tilmicosin was predicated on the relationship of clinical efficacy studies that demonstrated a favorable therapeutic outcome, on pharmaco-kinetic data, and on in vitro test data, as recommended by the National Committee for Clinical Laboratory Standards (NCCLS). The NCCLS-approved breakpoints for the MIC dilution testing are resistant > or = 32 micrograms/ml, intermediate 16 micrograms/ml, and susceptible < or = 8 micrograms/ml. The zone of inhibition interpretive criteria for disk diffusion testing with a 15-micrograms disk are resistant < or = 10 mm, intermediate 11-13 mm, and susceptible > or = 14 mm.