This study used electron spin resonance spectroscopy to detect metronidazole radical anions generated in vivo within Trichomonas vaginalis, a sexually transmitted human parasite. Researchers compared a drug-sensitive strain (ATCC 30001) with a metronidazole-resistant strain (85) and exposed both to varying oxygen concentrations. Under anaerobic conditions, both strains produced metronidazole radicals. However, when exposed to oxygen, the resistant strain lost its radical signal at lower oxygen partial pressures (above 6 kPa) compared to the sensitive strain, which retained detectable radicals at higher oxygen levels (above 8 kPa). Using chromium oxalate as a spin-broadening agent, researchers confirmed that radicals remained intracellular and were not released into the medium. These findings suggest that the metronidazole-resistant strain has defective oxygen scavenging mechanisms, allowing intracellular oxygen to accumulate and quench the drug's active radical form more efficiently than in sensitive strains. This represents a novel explanation for clinical drug resistance in this organism.

Key findings

• Metronidazole radical anions were directly detected in T. vaginalis cells under anaerobic conditions using electron spin resonance spectrometry.
• The drug-resistant strain lost radical signals at lower oxygen partial pressures (≥6 kPa) compared to the sensitive strain (≥8 kPa), indicating greater oxygen quenching capacity.
• Intracellular oxygen scavenging systems appear to be defective in the resistant strain, allowing higher intracellular oxygen accumulation.
• Drug-resistant and sensitive strains showed similar drug uptake and metabolic activation rates, suggesting resistance is not due to reduced drug activation.
• The resistance mechanism appears linked to compromised oxygen scavenging capacity rather than altered drug metabolism or transport.