Antibiotic resistance of Staphylococcus aureus and coagulase-negative staphylococci isolated from bovine mastitis

The antibiotic susceptibility test was carried out on 103 Staphylococcus aureus and 136 coagulase-negative staphylococci (CNS) strains. Only 35 of the isolates were susceptible to all antibiotics tested, while the remaining 204 isolates were resistant at least to one of the antibiotics. Among staphylococci, 18 isolates of S. aureus and 31 isolates of CNS were found phenotypically resistant to methicillin and these isolates were also resistant to penicillin G, ampicillin, amoxycillin and cloxacillin. Out of 68 S. aureus strains 55.9% were beta-lactamase producers. beta-lactamase producer isolates were 21.1% resistant to methicillin, but were 100% susceptible to amoxycillin/clavulanic acid and 97.4% susceptible to ampicillin/sulbactam. The differences observed in the efficacy of the antibiotics tested against staphylococci show that the antibiotic susceptibility tests should be performed together with the identification of the bacterial agents.