ACTIVITY OF SELECTED ANTIMICROBIAL AGENTS AGAINST STRAINS OF STAPHYLOCOCCUS-AUREUS ISOLATED FROM BOVINE INTRAMAMMARY INFECTIONS THAT PRODUCE BETA-LACTAMASE

The activity of selected antimicrobial agents was determined against s trains of Staphylococcus aureus that were isolated from bovine intrama mmary infections and that were positive or negative for beta-lactamase . A total of 107 S. aureus strains (70 that were positive for beta-lac tamase and 37 that were negative for beta-lactamase) were used in the study. Production of beta-lactamase was determined using a chromogenic cephalosporin disk method. Minimum inhibitory concentrations (MIC) fo r each test strain were determined using a commercially available micr odilution panel. The following compounds were tested: penicillin, ampi cillin, oxacillin, cephapirin, ceftiofur, penicillin plus novobiocin, erythromycin, and pirlimycin. Of the five beta-lactam compounds tested , penicillin and ampicillin were most affected by beta-lactamase activ ity, but oxacillin, cephapirin, and ceftiofur were not affected. Penic illin plus novobiocin also demonstrated excellent activity against str ains of S. aureus that were both positive and negative for beta-lactam ase. Erythromycin and pirlimycin demonstrated good activity against th e S. aureus strains that were negative for beta-lactamase; 90% of the isolates had an MIC of less than or equal to 0.5 mu g/ml (MIC90). The MIC90 for erythromycin and pirlimycin for strains that were positive f or beta-lactamase was >64.0 mu g/ml. However, 8 strains, in addition t o producing beta-lactamase, were also resistant to macrolides and linc osaminides. Recalculation of the MIC90 without these 8 strains yielded equivalent values for both erythromycin and pirlimycin with strains t hat were positive or negative for beta-lactamase (MIC90 less than or e qual to 0.5 mu g/ml).