Antimicrobial drug susceptibility of Staphylococcus aureus strains isolated from bovine milk.

The minimum inhibitory concentration (MIC) of ten antimicrobial drugs for 287 S. aureus strains recently isolated from bovine mastitic milk in different herds all over Sweden was determined. The minimum bactericidal concentration (MBC) of benzylpenicillin for 20 strains was determined. Thirty strains (10%) produced beta-lactamase. All strains were susceptible to oxacillin and neomycin, and more than 90% to streptomycin, trimethoprim-sulphamethoxazole chloramphenicol, erythromycin and tetracycline, whereas all were resistant to sulphamethoxazole. None of 20 strains investigated was tolerant to benzylpenicillin. However, S. aureus strains, isolated from bovine milk, should be tested for beta-lactamase production prior to treating mastitic cases with beta-lactam drugs.