Metronidazole Drug Classification
Metronidazole is a nitroimidazole antimicrobial agent with selective activity against anaerobic bacteria and certain protozoa. 1, 2
Drug Class and Mechanism
- Metronidazole belongs to the nitroimidazole class of antimicrobials, which are the only drugs useful for oral or parenteral therapy of certain protozoal infections like trichomoniasis 3
- The drug is both an antiprotozoal and antibacterial agent with a unique mechanism involving toxic metabolites that cause DNA strand breakage in susceptible organisms 1, 4
- It functions as a bactericidal agent at low concentrations, typically achieving a 2 to 5 log decrease in bacterial colony counts within one hour 2
Spectrum of Activity
Protozoal Coverage
- Highly effective against Trichomonas vaginalis, Entamoeba histolytica, and Giardia lamblia 5, 6
- The CDC confirms metronidazole is the only readily available FDA-approved agent in the United States for treating trichomoniasis 3
Bacterial Coverage
- Most active antimicrobial agent against Bacteroides fragilis, the most resistant of anaerobic bacteria 2
- Covers most Gram-negative and Gram-positive anaerobic bacteria, including Fusobacterium species, peptostreptococci, and Clostridium species 5
- Also effective against Gardnerella vaginalis (bacterial vaginosis) and Clostridium difficile 6
- Lacks any activity against aerobic bacteria, requiring combination with other agents like aminoglycosides for mixed infections 2, 7
Clinical Implications
- Metronidazole must be combined with other antimicrobials when treating mixed aerobic-anaerobic infections, as it has no aerobic coverage 2
- The drug demonstrates concentration-dependent bactericidal activity with a post-antibiotic effect extending beyond 3 hours after concentrations fall below the MIC 4
- Topical formulations are ineffective for systemic or deep tissue infections, achieving cure rates <50% for conditions like trichomoniasis 3, 8