Protection of Phagocytized Bacteria Against Antibiotics

Antibiotic‐sensitive Staphylococcus aureus phagocytized by human neutrophil granulocytes were exposed to a mixture of penicillin G and streptomycin in concentrations from 25 U (25 μ g/ml) to 1 000 U (1 000 μ g/ml). Phagocytized bacteria were effectively protected from the antibacterial effect of the antibiotics. Considerable numbers of intracellular bacteria remained viable after 20 h exposure to concentrations of antibiotics that killed more than 99% of extracellular bacteria in less than 30 min. Killing of phagocytized bacteria by granulocyte enzymes was effectively inhibited by phenylbutazone. A new method is described for the evaluation of the number of viable intracellular bacteria, using phenylbutazone for the inhibition of intracellular killing of bacteria and high concentrations of antibiotics for the control of extracellular organisms. This method facilitates an in vitro evaluation of the phagocytic and bactericidal activities of neutrophil granulocytes.