The in vitro activity of ofloxacin (DL8280), a new pyridone carboxylic acid, against Chlamydia trachomatis serotype E was assessed by growth of chiamydia in cycloheximide-treated McCoy cells. Chlamydia-infected monolayers were exposed to the drug for 48 and 96 h and for 48 h followed by 48 h of further incubation after withdrawal of ofloxacin. Inclusions were visualized by indirect immunofluorescence, and the effect of the drug was assessed both by inclusion development and by the number of inclusion-forming units generated in the monolayers. The action of ofloxacin was completely lethal when monolayers were exposed to a concentration of 1 ,ug/ml throughout one chlamydial developmental cycle. In contrast, tetracycline, when examined by similar techniques, inhibited inclusion development at a concentration of 0.3 ,ug/ml, but was only lethal at 2.4 pug/ml. Ofloxacin may prove to be a useful new antichlamydial agent.

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