MICRODILUTION ANTIMICROBIAL SUSCEPTIBILITIES OF SELECTED GRAM-NEGATIVE VETERINARY BACTERIAL ISOLATES

Gram-negative bacterial isolates (635) obtained from routine submissio ns to the Oklahoma Animal Disease Diagnostic Laboratory during 1983-19 87 were tested for antimicrobial susceptibility. Minimal inhibitory co ncentrations (MICs) were determined for the following antimicrobials u sing commercially prepared microdilution assay materials: ampicillin, cephalothin, chloramphenicol, erythromycin, gentamicin, kanamycin, oxy tetracycline, penicillin G, spectinomycin, sulfachlorpyridazine, sulfa dimethoxine, and tylosin. Results for isolates from cattle, dogs, hors es, and pigs are presented. In only a few instances were differences i n MICs apparent among bacterial isolates from different tissues. Amino cyclitol MICs for equine uterine isolates of Klebsiella pneumoniae dif fered from MICs for isolates from other tissues, and ampicillin, kanam ycin, and spectinomycin MICs for bovine fecal isolates of Escherichia coli differed from MICs for isolates obtained from other tissues. In s everal instances, bimodal distribution of susceptibilities was apparen t for ampicillin, kanamycin, and/or oxytetracycline. There was also a bimodal distribution pattern for erythromycin against Pasteurella haem olytica of bovine origin.