Minimum inhibitory and minimal lethal concentration against Chlamydia trachomatis dependent on the time of addition and the duration of the presence of antibiotics

The purpose of this study was to investigate the properties of several anti microbial agents found to be effective against Chlamydia trachomatis and to verify the eradication therapy schedule. The in vitro activities of two qu inolones (sparfloxacin, ofloxacin), of three macrolides (azithromycin, eryt hromycin, clarithromycin) and of a tetracycline (doxycycline) against C. tr achomatis were evaluated by several methods for the determination of the mi nimum inhibitory concentration (MIC) and minimal lethal concentration (MLC) . MLC of azithromycin was only 2 times higher than that of MIG. On the othe r hand, MLCs of other antibiotics were 4-16 times higher than their respect ive MICs. When all antimicrobial agents were added to the infected culture at different times, we found that the quinolones even at a concentration of 64 mu g/ml could not inhibit the formation of inclusion if they were added after 20 h from the start of infection. The corresponding period for macro lides and doxycycline was 24 h. When the antibiotics were removed at 8 h af ter the start of the infection, all antibiotics except azithromycin and cla rithromycin were needed at a concentration much higher than their MLCs to i nhibit the formation of inclusion. We consider macrolides, especially azith romycin, to be an excellent anti-C. trachomatis drug because of its lower M ICs and MLCs values which were also closer together.