Seventy-seven S. aureus strains, isolated in milk samples obtained from 40 Holstein cows suffering from subclinical mastitis in a dairy herd in the state of São Paulo - Brazil, were undergone to PCR-ribotyping and in vitro susceptibility testing to 12 antimicrobial drugs. PCR-ribotyping revealed nine different patterns and presented a higher discriminatory power (D = 0.82) in comparison with the in vitro antibiotic susceptibility test (D = 0.42). Moreover,, the results showed four antibiotic resistance patterns, with the resistance to lincomycin being the most predominant and comprising 19 (24.7%) S. aureus strains. The other 58 (75.3%) isolates were sensitive to all antibiotics tested. Of the 19 isolates which presented resistance to antibiotics, 14 (73.7%) were grouped in three ribotypes, and 13 (92.9%) of them were resistant to eritromycin and/or lincomycin. The large number of ribotypes and antibiotic resistance patterns found in this single dairy herd showed that one must be careful when considering the results of in vitro susceptibility tests obtained from a few S. aureus strains, because there is a high heterogeneity among naturally occurring populations of S. aureus, which must be taken into consideration in control programs against bovine mastitis.
mastitis; Staphylococcus aureus; phenotypic and genotypic variabilities
