Antimicrobial Susceptibility of Microorganisms Isolated from the Mammary Glands of Dairy Heifers

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Abstract
Minimum inhibitory concentrations were determined with 1494 microorganisms isolated from the mammary glands of dairy heifers. The antimicrobial agents tested were penicillin, cloxacillin, cephapirin, ceftiofur, novobiocin, enrofloxacin, erythromycin, and pirlimycin. All minimum inhibitory concentrations were expressed as micrograms per milliliter. The isolates tested included 135 Staphylococcus aureus, 1222 Staphylococcus sp., 42 Streptococcus sp., 15 Enterococcus sp., 60 enteric species, and 20 miscellaneous organisms. The minimum inhibitory concentrations for 90% of isolates for the various antimicrobial agents with Staph. aureus were as follows: penicillin, .13; cloxacillin, .5; cephapirin, .5; ceftiofur, 1; novobiocin, .5; enrofloxacin, .5; erythromycin, .5, and pirlimycin, .5. In comparison, the minimum inhibitory concentrations 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 Streptococcus sp. were 2, 32, 2, 2, 8, 1, 64, and 32 for the respective antimicrobial agents; the minimum inhibitory concentrations for 90% of isolates were 4, 64, 32, 64, 4, 1, 4, and 4 for the enterococci. Against the Gram-negative enteric bacilli, only ceftiofur and enrofloxacin were active; minimum inhibitory concentrations for 90% of isolates were 1 microgram/ml for ceftiofur and .25 microgram/ml for enrofloxacin. Results indicated that the majority of staphylococcal strains were susceptible to the antimicrobial agents tested but that antimicrobial susceptibility varied for Streptococcus sp. Compounds currently available in intramammary infusion products demonstrated poor activity against the enteric organisms.