[Interactions between macrophages and anti-infective agents]

Direct effects of antimicrobial agents on macrophages functions are not well precised. However, antimicrobials can enhance phagocytosis of bacteria by macrophages in vitro, at subinhibitory concentrations. This enhancement is related to the antibiotic effect. It is dependent of the nature of the antimicrobial agent, of the experimental procedure and some discrepancies appeared in the results. The precise mechanism, effect on the bacterial wall, or on the bacterial protein synthesis is uncertain. The third type of interaction concerns the intracellular penetration and activity of antimicrobial agents within macrophages. In a human monocyte-derived macrophage model, the intracellular growth of Legionella pneumophila was inhibited by erythromycin, rifampicin and fluoroquinolones at concentrations clearly lower than their MICs; doxycycline and cotrimoxazole were inhibitory at concentrations closed to their MICs. Cefoxitin was not inhibitory even at high concentrations despite a low MIC value. This confirms the good intracellular activity of macrolides and fluoroquinolones and the low intracellular activity of beta-lactams.