Interaction between human monocytes and penicillin G in relation to the antibacterial effect on Staphylococcus aureus

The influence of the presence of monocytes on the effect of penicillin G on Staphylococcus aureus was studied. Conditions were varied in such a way that phagocytosis and intracellular killing of S. aureus by monocytes did not occur, was short lasting and limited, or long lasting and extensive. Synergism between penicillin G and monocytes was observed in the absence of and during limited phagocytosis and intracellular killing by the monocytes. When phagocytosis and intracellular killing were optimal, addition or antagonism between penicillin G and monocytes was observed. Enhancement of the antibacterial effect of penicillin G was also obtained in the presence of monocyte supernatant (the cell-free medium in which monocytes had been incubated for 3 hr). This indicates that monocytes secrete a factor that enhances the antibacterial activity of penicillin G. Serum inhibits the activity of this factor.