Influence of subinhibitory concentrations of antibiotics on opsonization and phagocytosis of Pseudomonas aeruginosa by human polymorphonuclear leukocytes.

To determine whether pretreating Pseudomonas aeruginosa with antibiotics had an effect on phagocytosis, a serum-resistant clinical isolate was incubated with one-third of the minimum inhibitory concentrations of azlocillin, carbenicillin, cefoperazone, fosfomycin, netilmicin and piperacillin respectively prior to exposure to human polymorphonuclear leukocytes. The phagocytic process was measured by assaying radiolabeled bacteria. The uptake rates of untreated and antibiotic treated bacteria did not differ when normal human serum was used for opsonization. However, when the serum was heated to inactivate the complement system, its opsonic activity for untreated as well as for fosfomycin and netilmicin treated pseudomonas was removed and phagocytosis did not take place. In contrast, bacteria pretreated with the betalactam antibiotics still underwent phagocytosis, as also confirmed by electron microscopy. Even in the presence of rabbit immune serum untreated bacteria still required the participation of the complement system for optimal opsonization, whereas bacteria treated with beta-lactam antibiotics did not.