Effect of subinhibitory antibiotic concentrations on the phagocytosis of Staphylococcus aureus

The effect of subinhibitory antibiotic concentrations on the phagocytosis of Staphylococcus aureus was studied by pretreating 3H-thymidine labelled bacteria with one-third the minimal inhibitory concentration of clindamycin, doxycyclin, cefotiam, vancomycin, piperacillin and penicillin G, respectively. Pretreatment with clindamycin and doxycyclin resulted in enhanced uptake of the bacteria by polymorphonuclear leukocytes compared to the untreated control. The augmented phagocytosis was still observed at 1/32 the MIC of clindamycin and 1/64 the MIC of doxycyclin, and when the serum was diluted to a concentration of 1%. Pretreatment of the bacteria with penicillin, cefotiam, piperacillin and vancomycin had no effect on phagocytosis. Inhibitors of bacterial protein synthesis induce alterations of Staphylococcus aureus leading to increased phagocytosis, whereas antibiotics acting on cell wall synthesis are without effect.