Microdilution antimicrobial susceptibilities of selected gram-negative veterinary bacterial isolates

Gram-negative bacterial isolates (635) obtained from routine submissions to the Oklahoma Animal Disease Diagnostic Laboratory during 1983-1987 were tested for antimicrobial susceptibility. Minimal inhibitory concentrations (MICs) were determined for the following antimicrobials using commercially prepared microdilution assay materials: ampicillin, cephalothin, chloramphenicol, erythromycin, gentamicin, kanamycin, oxytetracycline, penicillin G, spectinomycin, sulfachlorpyridazine, sulfadimethoxine, and tylosin. Results for isolates from cattle, dogs, horses, and pigs are presented. In only a few instances were differences in MICs apparent among bacterial isolates from different tissues. Aminocyclitol MICs for equine uterine isolates of Klebsiella pneumoniae differed from MICs for isolates from other tissues, and ampicillin, kanamycin, and spectinomycin MICs for bovine fecal isolates of Escherichia coli differed from MICs for isolates obtained from other tissues. In several instances, bimodal distribution of susceptibilities was apparent for ampicillin, kanamycin, and/or oxytetracycline. There was also a bimodal distribution pattern for erythromycin against Pasteurella haemolytica of bovine origin.