Comparison of methods for the determination of antimicrobial resistance in Staphylococcus aureus from bovine mastitis.

The results of three standard methods (broth dilution, agar dilution, disk diffusion) and an experimental modification of the microdilution method for determination of resistance to ampicillin, cephalotin, cloxacillin, neomycin, novobiocin, penicillin and streptomycin were compared using 151 Staphylococcus aureus isolates obtained from cases of mastitis. The accuracy of the dilution methods was compared by determination of minimum inhibition concentrations (MIC, MIC50, MIC90 and modal MIC) and by assessment of the agreement within the tolerance of +/-1 dilution step in 2-fold dilution series. The results of the dilution methods were further compared with those of the reference disk diffusion method and the strains were classified as sensitive or resistant using the interpretation criteria for human strains. The comparisons indicated that MIC characteristics and the final classification as sensitive or resistant were method-dependent. Resistance to aminoglycoside antibiotics was observed more often when using broth dilution methods, especially when the broth was supplemented with lactose.