This study examined how metronidazole inhibits hydrogen gas production in Trichomonas vaginalis, a human protozoan parasite, using membrane-inlet mass spectrometry to continuously monitor H₂, O₂, and CO₂ levels. The researchers compared drug-sensitive strain ATCC 30001 and drug-resistant strain 85. H₂ production only occurred under anaerobic conditions but was inhibited even at very low oxygen levels. Metronidazole inhibited both H₂ and CO₂ production in both strains, with the sensitive strain showing greater inhibition at lower drug concentrations (Ki = 0.16 mM) compared to the resistant strain (Ki = 1.0 mM). The inhibition occurred regardless of oxygen presence. Evidence suggests metronidazole's effect extends beyond simple competition for electrons; the drug or its reduced metabolites likely directly damage hydrogenosomal enzymes or electron carriers involved in H₂ production. Non-proliferating cells exposed to metronidazole at inhibitory concentrations lost motility completely and did not recover.

Key findings

• Metronidazole inhibited H₂ production in T. vaginalis at lower concentrations in drug-sensitive strain (Ki = 0.16 mM) than in resistant strain (Ki = 1.0 mM)
• H₂ production only occurred under anaerobic conditions but was inhibited even at undetectable oxygen levels (<0.25 µM)
• Metronidazole inhibition persisted in the presence of oxygen, suggesting the drug acts through direct interaction with hydrogenosomal electron transport components rather than simple electron competition
• Complete and irreversible loss of cell motility occurred in non-proliferating cells exposed to metronidazole at inhibitory concentrations
• CO₂ production was stimulated during aerobic conditions, indicating anaerobic electron flux is normally limited by hydrogenase activity