A study on the in vitro susceptibility of Trichomonas vaginalis to metronidazole

The susceptibility of 32 clinical isolates of Trichomonas vaginalis to metronidazole has been studied under aerobic and anaerobic conditions. Of 32 patients with vaginal trichomoniasis, 27 (84%) were cured by a standard metronidazole treatment regimen (200 mg thrice daily for seven days). The geometric means of minimum inhibitory concentration (MIC) under aerobic and anaerobic conditions for these isolates were 2.0 and 0.4 micrograms/ml, respectively; the geometric means of minimum lethal concentration (MLC) under aerobic and anaerobic conditions were 7.4 and 2.0 micrograms/ml, respectively. However, for those five patients with treatment failure, the geometric means of MIC for these isolates under aerobic and anaerobic conditions were 6.8 and 1.1 micrograms/ml, respectively; the geometric means of MLC under aerobic and anaerobic conditions were 18 and 5.5 micrograms/ml, respectively. Trichomonads reisolated from patients after treatment failure had similar susceptibility to metronidazole as before treatment. However, all five women were cured by a second course of metronidazole treatment. Although primary treatment failure was common when isolates of T. vaginalis had aerobic MLC values of > 18 micrograms/ml or anaerobic MLC values > 5.5 micrograms/ml, two cases with isolates having high MLC values (aerobic: 20 micrograms/ml, anaerobic: 5 micrograms/ml) responded well to the standard treatment. It was evident that no metronidazole-resistant trichomonads were found in this study.