Activity of Selected Antimicrobial Agents Against Strains of Staphylococcus aureus Isolated from Bovine Intramammary Infections that Produce β-Lactamase

Abstract

Listen

Translate article

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 β-lactamase. A total of 107 S. aureus strains (70 that were positive for β-lactamase and 37 that were negative for β-lactamase) were used in the study. Production of β-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 β-lactam compounds tested, penicillin and ampicillin were most affected by β-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 β-lactamase. Erythromycin and pirlimycin demonstrated good activity against the S. aureus strains that were negative for β-lactamase; 90% of the isolates had an MIC of ≤0.5μg/ml (MIC90). The MIC90 for erythromycin and pirlimycin for strains that were positive for β-lactamase was >64.0μg/ml. However, 8 strains, in addition to producing β-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 β-lactamase (MIC90 ≤0.5μg/ml).