An in vitro metronidazole susceptibility test for trichomoniasis using the InPouch TV test

An efficient anaerobic culture system, the InPouch TV test, was used to determine the susceptibility of Trichomonas vaginalis to metronidazole. Glacial acetic acid was employed as a solvent for metronidazole.

T vaginalis isolates were cultured from 16 symptomatic female patients. The 11 who responded to oral metronidazole, 250 mg tid for 7 days, were considered as having sensitive trichomonads; the 5 who remained infected after treatment were considered to have resistant organisms. All isolates were cultured for minimum lethal concentrations (MLC). Metronidazole was added to a series of pouches; two-fold dilution of the most concentrated was 50 micrograms/ml and the least was 0.4 micrograms/ml. The inoculum of viable trichomonads was 1 x 105/ml in each pouch. Pouches were incubated at 37 degrees C for 48 h, examined microscopically for motile trichomonads, and then 0.5 ml was subcultured to drug free pouches. After 5 days incubation at 37 degrees C, each subculture and culture were examined microscopically for viable trichomonads.

Eleven isolates of T vaginalis from patients responding to metronidazole treatment had MLC between 0.4 to 3.1 micrograms/ml. The MLC from the 5 treatment failure patients were between 12.5 to 50 micrograms/ml.

For the 16 patients in this study, the MLC values determined with the InPouch TV test differentiated between infection caused by metronidazole sensitive and resistant trichomonads. The mean MLC of clinically resistant isolates was approximately eleven fold higher than the mean MLC of clinically sensitive isolates (15 micrograms/ml vs 1.32 micrograms/ml). There was a significant difference between clinically resistant and sensitive isolates (t = 5.47, p < 0.0005). This study suggests that the InPouch TV test could be used for distinguishing between metronidazole resistant and sensitive isolates.