Antibacterial effect of telithromycin (HMR 3647) and comparative antibiotics against intracellular Legionella pneumophila

The activity of the ketolide telithromycin (HMR 3647) against intracellular Legionella pneumophila strain L-1033 was compared with the activities of e rythromycin and levofloxacin. To assay intracellular antibacterial activity , human monocytes were allowed to adhere to wells in 24-well tissue culture plates and were then exposed to L. pneumophila cells for 1 h to allow phag ocytosis to occur. Antibiotics were added to the wells after removal of unp hagocytosed bacteria. Quantitative bacterial cell counts were made from lys ed monocytes at 0, 24, 48, 72 and 96 h. The antibacterial effects of antibi otics against intracellular L. pneumophila L-1033 were concentration and ti me dependent; at 10 x MIC the activity of telithromycin was greater than th at of erythromycin and was less than that of levofloxacin (P < 0.01); telit hromycin-rifampicin combinations showed no synergy or interference; and rem oval of telithromycin from assays at 24 h did not affect its intracellular antibacterial activity. In conclusion, the ketolide telithromycin has excel lent activity against intracellular L. pneumophila strain L-1033 and should be evaluated for therapy of legionnaires' disease.