ANTIMICROBIAL SUSCEPTIBILITY OF MICROORGANISMS ISOLATED FROM THE MAMMARY-GLANDS OF DAIRY HEIFERS

Minimum inhibitory concentrations were determined with 1494 microorgan isms isolated from the mammary glands of dairy heifers. The antimicrob ial agents tested were penicillin, cloxacillin, cephapirin, ceftiofur, novobiocin, enrofloxacin, erythromycin, and pirlimycin. All minimum i nhibitory concentrations were expressed as micrograms per milliliter. The isolates tested included 135 Staphylococcus aureus, 1222 Staphyloc occus sp., 42 Streptococcus sp., 15 Enterococcus sp., 60 enteric speci es, and 20 miscellaneous organisms. The minimum inhibitory concentrati ons for 90% of isolates for the various antimicrobial agents with Stap h. aureus were as follows: penicillin, .13; cloxacillin, .5; cephapiri n, .5; ceftiofur, 1; novobiocin, .5; enrofloxacin, .5; erythromycin, . 5, and pirlimycin, .5. In comparison, the minimum inhibitory concentra tions for 90% of isolates for the Staphylococcus sp. were 1, 1, .5, 1, .5, .5, 1, and .5 for penicillin, cloxacillin, cephapirin, ceftiofur, novobiocin, enrofloxacin, erythromycin, and pirlimycin, respectively. The minimum inhibitory concentrations for 90% of isolates for the Str eptococcus sp. were 2, 32, 2, 2, 8, 1, 64, and 32 for the respective a ntimicrobial agents; the minimum inhibitory concentrations for 90% of isolates were 4, 64, 32, 64, 4, 1, 4, and 4 for the enterococci. Again st the Gram-negative enteric bacilli, only ceftiofur and enrofloxacin were active; minimum inhibitory concentrations for 90% of isolates wer e 1 mu g/ml for ceftiofur and .25 mu g/ml for enrofloxacin. Results in dicated that the majority of staphylococcal strains were susceptible t o the antimicrobial agents tested but that antimicrobial susceptibilit y varied for Streptococcus sp. Compounds currently available in intram ammary infusion products demonstrated poor activity against the enteri c organisms.