Enhanced Susceptibility of Penicillin-Resistant Staphylococci to Phagocytosis after in Vitro Incubation with Low Doses of Nafcillin

Abstract

Incubation of penicillin-resistant staphylococci with a subinhibitory concentration of nafcillin increased the susceptibility of the organisms to in vitro phagocytosis by normal mouse peritoneal exudate cells (macrophages). Methicillin and oxa-cillin were effective as inhibitors of bacterial growth only when used in relatively large doses; subinhibitory doses did not significantly affect subsequent phagocytosis by the PE cell. Penicillin G had no detectable effect on the staphylococci used in this system, either as a direct inhibitor of microbial growth or by influencing phagocytosis. The synergistic effect described here between nafcillin and macrophages may be related to the clinical efficacy reported for this antibiotic even when present only in low doses.