Phagocytosis and killing of A-protein positive Staphylococcus aureus in the presence of low doses of antibiotics

An A-protein positive strain of Staphylococcus aureus was grown in varying concentrations of erythromycin, clindamycin and miocamycin at sub-MIC levels and incubated in purified human polymorphonuclear leukocytes (PMNs). The presence of A-protein produced resistance of the above strain to phagocytosis and killing by PMNs and a slight resistance to opsonization by normal serum. The addition of antibiotics, which inhibit protein biosynthesis, caused significant changes in the ability of this bacterium to resist opsonization by serum complement. The antimicrobial agents produced a significant enhancement of bacterial killing and phagocytosis. In particular, clindamycin and erythromycin, at sub-MIC concentrations, modified the hair-like structure of S. aureus, which contains A-protein, and were able to affect the interaction of the strain with phagocytic cells.