The antibacterial activity of benzylpenicillin against Staphylococcus aureus ingested by granulocytes

Granulocytes that had ingested pre-opsonized Staphylococcus aureus for 3 min at 37 degrees C and then had been washed twice, were incubated in the presence of benzylpenicillin. The antibiotic showed a concentration-dependent effect on cell-associated S. aureus between 0.005 and 0.1 mg/l benzylpenicillin. The antibacterial effect on cell-associated S. aureus was equal to that on opsonized S. aureus in suspension. Microscopic studies with lysostaphin showed that the number of extracellular bacteria was negligible and that the decrease in the number of cell-associated bacteria reflected the true rate of intracellular killing of S. aureus. Contrary to the generally accepted opinion that benzylpenicillin does not act intracellularly in granulocytes, we were able to show intracellular antibacterial activity of benzylpenicillin against S. aureus under the experimental conditions we used.