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 antimicrobial agents found to be effective against Chlamydia trachomatis and to verify the eradication therapy schedule. The in vitro activities of two quinolones (sparfloxacin, ofloxacin), of three macrolides (azithromycin, erythromycin, clarithromycin) and of a tetracycline (doxycycline) against C. trachomatis were evaluated by several methods for the determination of the minimum inhibitory concentration (MIC) and minimal lethal concentration (MLC). MLC of azithromycin was only 2 times higher than that of MIC. On the other hand, MLCs of other antibiotics were 4–16 times higher than their respective 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 μg/ml could not inhibit the formation of inclusion if they were added after 20 h from the start of infection. The corresponding period for macrolides and doxycycline was 24 h. When the antibiotics were removed at 8 h after the start of the infection, all antibiotics except azithromycin and clarithromycin were needed at a concentration much higher than their MLCs to inhibit the formation of inclusion. We consider macrolides, especially azithromycin, to be an excellent anti-C. trachomatis drug because of its lower MICs and MLCs values which were also closer together.