Nafcillin and oxacillin: comparative antistaphylococcal activity in mice

The therapeutic properties of nafcillin, oxacillin and erythromycin were determined in mice infected with a strain (Evans) of Staphylococcus aureus shown to be tolerant to the bactericidal action of penicillinase-resistant derivatives of penicillin. The therapeutic activity of these agents was also correlated with the extent of colonization of kidneys by resistant clones of S. aureus Evans. The CD50 values or potency ratios proved that nafcillin was highly active against this organism, whereas oxacillin and erythromycin were capable of protecting the animals to a lesser degree. Of the agents studied, only nafcillin was capable of preventing or interfering with the colonization of the kidneys by S. aureus Evans. Although the exact interpretation and application of these data in the management of clinical problems remains to be determined, the observations suggest important differences between nafcillin and oxacillin in vivo and that it is difficult to predict the antibacterial efficacy of these drugs solely from MIC and MBC data.