Effect of Sub-Minimal Inhibitory Concentrations of Ciprofloxacin and Fleroxacin on the Bacterial Capsular Antigen and Opsonophagocytosis by Human Polymorphonuclear Leukocytes

We have studied the effect of sub-minimal inhibitory concentrations (sub-MIC) of ciprofloxacin and fleroxacin on capsulated (K⁺) and non-capsulated (K^-) Gram-negative bacilli (Escherichia coli Ol:K1, O7:K1, O1:K-, O7:K-, and Klebsiella oxytoca) as well as on Straphylococcus aureus and we investigated the interaction of antibiotic pretreated bacteria with human serum and polymorphonuclear leukocytes (PMN). Following overnight growth in the presence of 1/2 MIC of the antibiotics, bacteria were opsonized in human serum and incubated with PMN. Opsonophagocytosis was quantified as the ratio of uptake by PMN of radioactively labeled bacteria. Ciprofloxacin and fleroxacin enhanced the phagocytosis rate of E. coli K⁺ strains (control 5–10%; 112 MIC of ciprofloxacin and fleroxacin 70–80%) of K. oxytoca (control 0–2%; 112 MIC of ciprofloxacin and fleroxacin 35–40%) as well as of Staph. aureus (control 5–10%; 1/2 MIC of the antibiotics 35–40%). Opsonophagocytosis of K^- strains was not altered. The enhancement of opsonophagocytosis was a complement dependent process. Exposure of capsulated E. coli to ciprofloxacin as well as to fleroxacin resulted in decreased amounts of the capsular antigen.