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1 Animal Health Discovery Research, Pharmacia & Upjohn, Inc., Kalamazoo, MI 49001
The activity of selected antimicrobial agents was determined against strains of Staphylococcus aureus that were isolated from bovine intramammary infections and that were positive or negative for ß-lactamase. A total of 107 S. aureus strains (70 that were positive for ß-lactamase and 37 that were negative for ß-lactamase) were used in the study. Production of ß-lactamase was determined using a chromogenic cephalosporin disk method. Minimum inhibitory concentrations (MIC) for each test strain were determined using a commercially available microdilution panel. The following compounds were tested: penicillin, ampicillin, oxacillin, cephapirin, ceftiofur, penicillin plus novobiocin, erythromycin, and pirlimycin. Of the five ß-lactam compounds tested, penicillin and ampicillin were most affected by ß-lactamase activity, but oxacillin, cephapirin, and ceftiofur were not affected. Penicillin plus novobiocin also demonstrated excellent activity against strains of S. aureus that were both positive and negative for ß-lactamase. Erythromycin and pirlimycin demonstrated good activity against the S. aureus strains that were negative for ß-lactamase; 90% of the isolates had an MIC of 
0.5 µg/ml (MIC90). The MIC90 for erythromycin and pirlimycin for strains that were positive for ß-lactamase was >64.0 µg/ml. However, 8 strains, in addition to producing ß-lactamase, were also resistant to macrolides and lincosaminides. Recalculation of the MIC90 without these 8 strains yielded equivalent values for both erythromycin and pirlimycin with strains that were positive or negative for ß-lactamase (MIC90 0.5 µg/ml).
Key Words: minimum inhibitory concentration • ßbeta;-lactamase • Staphylococcus aureus
Submitted on April 15, 1996
Accepted on August 1, 1996
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