Antimicrobial susceptibility and beta-lactamase testing of staphylococci isolated from dairy herds.

Evaluation of 722 staphylococcal isolates from seven dairy herds revealed marked herd differences in antimicrobial susceptibility patterns. A low overall resistance to penicillin was noted for Staphylococcus aureus, although isolates from three herds had greater than or equal to 16% penicillin resistance. Other staphylococcal species demonstrated a higher resistance to penicillin than did S. aureus. The predominant staphylococci other than S. aureus were S. hyicus and S. chromogenes for six of the seven herds while S. simulans and S. epidermidis predominated in one herd. beta-Lactamase testing using a commercially available chromogenic cephalosporin was an accurate predictor of penicillin resistance for S. aureus. Staphylococcus hyicus, S. chromogenes, and S. epidermidis that were penicillin resistant by susceptibility testing demonstrated negative beta-lactamase results of 35, 11, and 9%, respectively. Retesting of these isolates after exposure to penicillin resulted in positive beta-lactamase results, indicating induction of beta-lactamase. These results indicate the variability in antimicrobial susceptibility patterns for staphylococci and suggest the importance of epidemiologic knowledge of a dairy herd to guide initial therapy.