Comparative Activity of Ciprofloxacin, Ofloxacin, Levofloxacin, and Erythromycin Against Legionella Species by Broth Microdilution and Intracellular Susceptibility Testing in HL-60 Cells

Abstract

Animal lung macrophages or human peripheral blood mononuclear cells have been used for testing intracellular activity of anti- Legionella antibiotics; such studies are labor intensive such that comparative antibiotic studies for the many Legionella species are few. We evaluated a human monocyte cell line (HL-60) as an alternative model. HL-60 (1.5 × 10 6 cells/well) was differentiated into adherent cells and infected with 1.5 × 10 7 CFU of Legionella pneumophila ( L. pneumophila). Erythromycin and quinolones, ciprofloxacin, ofloxacin, and levofloxacin were added to cells at 1 and 8× MIC. Percent (%) inhibition ratios equal to total L. pneumophila with agent divided by L. pneumophila without agent × 100 were determined at 48 h; lower ratios implied greater potency. By broth dilution in buffered yeast extract broth, the most potent agents against L. pneumophila were (MIC): ciprofloxacin (0.015–0.03), ofloxacin (0.015–0.03), levofloxacin (0.015–0.03), erythromycin (0.125–1.0 μg/mL). In the intracellular model, the most potent inhibitors of L. pneumophila multiplication at 8× MIC were (in order of potency) levofloxacin (24.2%), ciprofloxacin (30.6%), ofloxacin (37.1%), and erythromycin (55.0%). All the quinolones were highly active and significantly more potent against L. micdadei and L. bozemanii when compared to L. pneumophila.