Metronidazole requires anaerobic activation to exert its bactericidal effect. The key activation step is:

• A Protonation of the nitro group under acidic lysosomal pH forming a bactericidal acid
• B Oxidation by bacterial catalase producing H2O2-mediated DNA strand breaks
• C Hydrolysis of the imidazole ring generating formaldehyde that alkylates DNA
• D Reduction of the nitro group by electron carriers (ferredoxin/nitroreductase) to reactive nitroso/hydroxylamine intermediates that damage DNA ✓

Explanation

Metronidazole is a prodrug. Under low redox potential conditions present in anaerobic bacteria and certain parasites (Trichomonas, Giardia, Entamoeba), electron carriers like ferredoxin or NADH oxidoreductases transfer electrons to the nitro group, reducing it to nitro-radical anion and then to reactive nitroso and hydroxylamine intermediates. These short-lived reactive species cause single-strand DNA breaks and inhibit DNA repair, leading to bactericidal effect. Aerobic bacteria lack the low-redox electron carriers needed for this activation, explaining selectivity.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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Written and medically reviewed by the StethoPrep medical team.