Chlamydia trachomatis: in vitro susceptibility of genital and ocular isolates to some quinolones, amoxicillin and azithromycin.

The minimum inhibitory concentration (MIC) of ofloxacin, ciprofloxacin, norfloxacin, amoxicillin and a new erythromycin analogue (azithromycin or CP 62993) against Chlamydia trachomatis was determined. There was a large difference between the MICs (microgram/ml) of different quinolones (median of 3 independent measurements; range): ofloxacin (0.5; 0.5-1) less than ciprofloxacin (1; 1-2) less than norfloxacin (16; 16-32). The MIC of amoxicillin varied from 0.25 to 1 (median 0.5) in different experiments. The MIC of azithromycin (0.125; 0.063-0.25) was lower than that of erythromycin (0.25; 0.125-0.5). The minimum lethal concentration (MLC) of ofloxacin and azithromycin was determined with and without passage of the McCoy cells. Both methods gave the same results. Ofloxacin seemed to have a lethal effect on C. trachomatis, as the MIC and MLC were equal. In contrast, the effect of the MIC of azithromycin on C. trachomatis was bacteriostatic. The MLC of azithromycin was 2-4 times higher than the MIC (p less than 0.001).