Synergism of oxacillin and gentamicin against enterococci

Strains of enterococci isolated from 34 patients were studied for susceptibility to oxacillin and gentamicin alone and in combination. The minimal bactericidal concentrations of oxacillin and gentamicin for these strains ranged from 16 to 250 mug/ml (median 32 mug/ml) for oxacillin and 12 to 48 mug/ml (median 24 mug/ml) for gentamicin. The minimal bactericidal concentration of oxacillin for 50% of strains in the presence of 12, 6, 3, and 1.5 mug of gentamicin per ml, respectively, was 2, 8, 16, and 32 mug/ml. The combination of oxacillin and gentamicin at clinically attainable serum levels was synergistically bactericidal against 27/34 (80%) of these strains using strict criteria. This report reconfirms the original observation that antibiotics which affect the synthesis of bacterial cell walls combine synergistically with aminoglycosides against enterococci. This is so even though enterococci are far more resistant to oxacillin than to penicillin or ampicillin. In addition, this report suggests that the combination of oxacillin and gentamicin administered in the usual dosages includes enterococci in its bactericidal spectrum.