Figure 1. Scanning electron micrograph (SEM) of Trichomonas vaginalis in vivo. The parasite attaches to the host epithelial cell (E) via cytoplasmic extensions indicated by the arrow. AF, anterior flagella; RF, recurrent flagellum. Bars, 5 m. Photographs kindly supplied by Dr M. A. Vannier-Santos (Universidade Federal do Rio de Janeiro, Brazil).

Figure 2. Nitroimidazoles used as antitrichomonad agents. (a) Metronidazole was the first nitroimidazole shown to have antimicrobial activity, and to date is the only drug approved in the USA for treatment of human trichomoniasis. (b) Dimetridazole and (c) ipronidazole are used for the treatment of bovine trichomoniasis. The presence of the nitro group on the 5¢ position (bold) of the imidazole ring confers specificity to the drug because of the low redox potential required to activate the nitro group when in this position.

Figure 3. Pathway of pyruvate metabolism by Trichomonas vaginalis hydrogenosomes. Pyruvate is metabolized to acetate and electrons are shuttled via ferredoxin to protons, forming hydrogen. The high energy bond of acetyl-coenzyme A (CoA) is used to drive the formation of ATP from ADP. (1) Pyruvate:ferredoxin oxidoreductase. (2) Succinyl-CoA synthetase. (3) Succinate thiokinase. (4) Adenylate kinase. (5) Hydrogenase. (6) Metronidazole (MTZ) diverts electrons from hydrogenase, inhibiting hydrogen production and undergoing the stepwise reduction to first the nitro-anion free radical (MTZ*). (7) Three further electrons are sequentially accepted, forming the nitroso, nitroso-free radical and hydroxylamine (HNOH). (8) Oxygen can oxidize the nitro-free radical of metronidazole, resulting in futile cycling of the first step of metronidazole reduction.