Bactericidal effect of polymorphonuclear leukocytes on Pseudomonas aeruginosa pre-incubated in ofloxacin

The in vitro effect of human polymorphonuclear leukocytes on sub-MIC-ofloxacin-treated Pseudomonas aeruginosa was studied. Two bacteria strains were selected: one serum-sensitive and one serum-resistant. Exposure of the bacteria to subinhibitory concentrations of ofloxacin caused the bacteria to elongate into filaments. Pseudomonas aeruginosa was incubated with polymorphonuclear leukocytes and the bactericidal effect was quantitated by the reduction of CFUs. The killing of untreated and ofloxacin-treated bacteria did not differ when normal human serum was used for opsonization. However, when heat-inactivated serum or buffer was used, the killing of ofloxacin-treated bacteria was increased from 40% to 63% (P less than 0.05). Kinetic studies showed that this effect increased with prolonged incubation time. In addition, polymorphonuclear leukocytes incubated with ofloxacin had an increased bactericidal activity, probably due to intracellular uptake of the drug. Phagocytic ingestion of the bacteria was not influenced by ofloxacin-treatment. These findings support other investigations with DNA-gyrase inhibitors, which show that exposed bacteria become more susceptible to killing and that antibiotic-treated polymorphonuclear leukocytes react more strongly.