Penicillinase-resistant penicillin/gentamicin synergism. Effect in patients with Staphylococcus aureus bacteremia.

Gentamicin sulfate has been shown to enhance the effects of penicillinase-resistant penicillins against clinical isolates of Staphylococcus aureus in vitro, but the relevance of this observation to bacteremic patients is unclear. Therefore, serum samples from 14 patients with staphylococcal bacteremia were tested for growth inhibitory and bactericidal effects against the patients' own pathogen while they received a penicillinase-resistant penicillin alone, and again when gentamicin was added to the therapeutic regimen. Addition of gentamicin in vivo was associated with slight improvement in growth inhibitory activity but it caused profound increases in serum bactericidal activity. This effect could not be attributed to higher serum levels of penicillinase-resistant penicillin during gentamicin administration. Addition of gentamicin to penicillinase-resistant penicillin can lead to marked improvement in a patient's serum bactericidal activity against his own staphylococcal pathogen, and should be considered for any patient who does not respond to more conventional therapy.