Intraphagocytic killing of gram-positive bacteria by ciprofloxacin

The intraphagocytic killing of Staphylococcus aureus, Streptococcus pyogenes, and Corynebacterium group D2 by ciprofloxacin (0.1, 1 and 5 mg/L) within human neutrophils was determined. The organisms showed different susceptibility to neutrophil killing mechanisms. The neutrophils with intact and impaired (by phenylbutazone treatment) O2-dependent killing mechanisms were studied. The minimum concentrations of ciprofloxacin to kill 90% of phagocytosed bacteria within untreated neutrophils after 2 h were 1 mg/L for S. aureus and Corynebacterium group D2, and 0.1 mg/L for S. pyogenes. In contrast, exposure for 3 h was required to achieve similar cidal effects within phenylbutazone treated neutrophils. Synergic interaction between ciprofloxacin and the O2-dependent mechanisms of phagocytes was found. The reactive oxygen metabolites produced in the respiratory burst did not affect the intraphagocytic activity of ciprofloxacin. Phenylbutazone treatment of phagocytes would be a good experimental model to study intraphagocytic killing by drugs in situations where the oxidative mechanisms of neutrophils are impaired (for example AIDS and chronic granulomatous disease).