In vitro susceptibility of Trichomonas vaginalis to metronidazole and treatment outcome in vaginal trichomoniasis.

We have identified Trichomonas vaginalis strains resistant in vitro to metronidazole, especially under aerobic conditions. Since little is known about the relationship of treatment outcome to metronidazole susceptibility of T. vaginalis, we studied the aerobic and anaerobic susceptibility to metronidazole of 310 clinical isolates of T. vaginalis. Of 199 patients with known outcomes after metronidazole treatment for vaginal trichomoniasis, the geometric mean minimal lethal concentration (MLC) under aerobic conditions for trichomonads associated with cases cured by a single 2-g dose was 24.1 micrograms/ml (n = 146), while that of treatment-resistant isolates (n = 53) was 195.5 micrograms/ml. The corresponding mean anaerobic MLC values were 1.6 and 5.05 micrograms/ml, respectively. The average aerobic:anaerobic MLC ratio was about twofold higher for the resistant isolates. Treatment resistance was more frequent at aerobic MLC values of greater than 25 micrograms/ml or anaerobic values of greater than 1.6 micrograms/ml. Although there was overlap of the metronidazole susceptibility distribution of susceptible and resistant isolates, significant resistance to treatment was common when isolates of T. vaginalis had aerobic MLC values of greater than 100 micrograms/ml or anaerobic MLC values of greater than 3.1 micrograms/ml.