The Mode of Action of Metronidazole against Trichomonas vaginalis

This study investigated how metronidazole, an antiprotozoal drug, kills Trichomonas vaginalis. The researchers measured gas evolution from the organism and found that metronidazole rapidly inhibited hydrogen gas production (15-20 minutes) before affecting carbon dioxide evolution (60 minutes). By examining metabolic pathways, they determined that T. vaginalis uses a clostridial-type phosphoroclastic reaction to produce these gases, involving pyruvate conversion to acetyl phosphate, CO2, and H2. Metronidazole did not block acetyl phosphate synthesis but did inhibit hydrogen evolution, indicating the drug targets the hydrogenase enzyme component. Additional experiments with electron-acceptor dyes showed metronidazole interferes with electron transfer. Based on these findings, the authors propose that metronidazole acts as a potent electron acceptor competing with ferredoxin, a protein involved in electron transfer in anaerobic metabolism. The drug's redox potential (-0.56 V) is more negative than ferredoxin (-0.46 V), allowing it to effectively intercept electrons and disrupt the hydrogenase system. This mechanism explains why metronidazole selectively targets anaerobic organisms.

Key findings

• Metronidazole inhibits hydrogen gas evolution from T. vaginalis within 15-20 minutes, preceding inhibition of CO2 evolution by 45 minutes
• The organism uses a clostridial-type phosphoroclastic reaction for gas production, where metronidazole blocks the hydrogenase component but not acetyl phosphate synthesis
• Metronidazole inhibits electron transfer to artificial electron acceptors (methylene blue and DCPIP) in dose-dependent manner
• The drug likely acts as a superior electron acceptor compared to ferredoxin due to its more negative redox potential (-0.56 V vs -0.46 V)
• The selective toxicity to anaerobes results from metronidazole's competition for electrons in anaerobic metabolic pathways