The in vitro activity of penicillins(penicillin G, ampicillin, carbenicillin, methicillin and cloxacillin) and gentamicin to Staphylococcus aureus and coagulase-negative staphylococci isolated from bovine mastitic milk samples was determined. The grow...
The in vitro activity of penicillins(penicillin G, ampicillin, carbenicillin, methicillin and cloxacillin) and gentamicin to Staphylococcus aureus and coagulase-negative staphylococci isolated from bovine mastitic milk samples was determined. The growth of all isolates of staphylococci tested was inhibited by cloxacillin and methicillin at a concentration of $0.78{\mu}g/ml$ and by gentamicin at a minimum inhibitory concentration (MIC) of $1.56{\mu}g/ml$. Of the 140 strains of Staphylococcus aureus isolated 99(70.7%) gave positive reactions for penicillinase on the starch-iodine test. Of 121 isolates of coagulase-negative staphylococci, 58(47.9%) showed penicillinase production, but all the Staph. xylosus lacked the ability to produce penicillinase. MIC of penicillin G of the penicillinase-positive strains was $0.2{\mu}g/ml$ or more. The combination of cloxacillin with gentamicin showed a synergistic effect by inhibiting regrowth of the tested organism.