• Nitroimidazoles

  • Metronidazole

  Metronidazole Audio Overview

  • Metronidizole

   

  • Introduction

    • Metronidazole is a widely used nitroimidazole antibiotic and antiprotozoal agent with broad activity against anaerobic bacteria and certain protozoa/

    • Metronidizole is an important medication for treating anaerobic bacterial infections, protozoal infections and microaerophilic bacterial infections.¹  

      • This agent is cytotoxic to anaerobic microorganisms which can grow either in the presence or absence of oxygen (facultative anaerobic microorganisms).¹

      •  Specifically, metronidizole exhibits excellent activity against anaerobic bacteria such as Bacterioides, Closteridium, Fusobacterium, Peptococcus, Peptostreptococcus, and Eubacterium.²

        •  

          Bacterioides (top) and Closteridium (bottom)

          "Bacterioides biacutis-one of the many commensal anaerobic Bacterioides species in the gastrointestinal tract: cultured in blood agar medium for 48 hours." Attribution US gov, Public domain, via Wikimedia Commons Image credit CDC/Dr. V.R.Dowell, Jr, 1972 https://commons.wikimedia.org/wiki/File:Bacteroides_biacutis_01.jpg

        • "A photomicrographs of Clostridium botulinum bacteria. This is the photomicrographs of Clostridium botulinum stained with Gentian violet. The bacterium C. Botulinum produces an nerve toxin, which causes the rare but serious paralytic illness botulism." Attribution Content Providers: CDC, Public domain, via Wikimedia Commons (1979) https://commons.wikimedia.org/wiki/File:Clostridium_botulinum_01.png

        •  

          Fusobacterium novum (top) and Peptostreptococcus (bottom)

          "Fusobacterium novum after being cultured in a thioglycollate medium." Attribution Dr.V.R.Dowell, Public domain, via Wikimedia Commons (1972) https://commons.wikimedia.org/wiki/File:Fusobacterium_novum_01.jpg

        • "Gram-positive Peptostreptococcus species bacteria growing in chain formation." Attribution Content Providers: CDC Public Health Image Library (1972) Image credit: Gilda L Jones, 1972 https://commons.wikimedia.org/wiki/File:Peptostreptococcus_spp_01.jpg

      • Metronidizole shows lesser activity against Gardnerella and Helicobacter.²  

        • Gardnerella vaginalis (top) and Helicobacter bilis (bottom)

          "Microscopic view of Gardnerella vaginalis, magnified 400x" Attribution Dr. F.C. Turner, Public domain, via Wikimedia Commons May 12, 2010. https://commons.wikimedia.org/wiki/File:G.vaginalis.jpg

        • "Scanning electron micrograph of Helicobacter bilis bacteria." Attribution See page for author, Public domain, via Wikimedia Obtained from the CDC Public Health Image Library Image credit: CDC/Dr. Patricia Fields, Dr. Collette Fitzgerald, 2004. https://commons.wikimedia.org/wiki/File:Helicobacter_sp_01.jpg

      • Resistance to metronidizole is expected in Gram-positive anaerobes Actinomyces, Propionibacterium and Lactobacillus.²

    • In clinical practice (including primary care), metronidazole is valued for its bactericidal action, excellent tissue penetration, and oral availability.

  • Mechanism of Action

    • Metronidazole is a prodrug that becomes activated under anaerobic conditions to damage microbial DNA.²

      • After passive diffusion into susceptible organisms, its nitro group is reduced by anaerobic cellular enzymes (such as ferredoxin-linked pyruvate oxidoreductase) to form a highly reactive nitroso radical anion.³

        • The radical anion targets DNA and other macromolecules, causing strand breakage and loss of helical structure, ultimately inhibiting nucleic acid synthesis and resulting in cell death.¹ Other suggested mechanism of action for metronidazole involves an indirect inhibition of DNA synthesis and repair.⁵

          •  Reductive activation in this setting occurs only in low-oxygen environments because oxygen competes for the electrons; hence metronidazole is selectively active against anaerobic and microaerophilic organisms and has minimal effect on aerobic human cells.³ 

            • Here oxygen effectively outcompetes metronidazole for available electrons, thereby preventing the drug's activation.⁴   

              • This fundamental biochemical difference explains metronidazole's negligible effect on human cells and its lack of activity against aerobic bacteria, allowing it to be used safely in patients and in combination with other antibiotics to cover aerobes in mixed infections.

            • Anaerobes and certain protozoa “activate” metronidazole into a DNA-damaging agent, whereas human cells and aerobic bacteria do not.

              • This mechanism yields rapid, concentration-dependent bactericidal activity against susceptible microbes.¹

              • "Metronidazole Mechanism Schematic"

                "An illustration of how metronidazole incorporates itself into the respiration of anaerobic organisms." Attribution Tom282f3, CC0, via Wikimedia Commons  (December 27, 2024) https://commons.wikimedia.org/wiki/File:Metronidazole_mechanism_schematic_2.svg

   

  • Antimicrobial Spectrum and Activity

    • Bacteria

      • Metronidazole’s antimicrobial spectrum is limited to obligate anaerobes and select microaerophiles.

        • It is potently active against Gram-negative anaerobes (e.g. Bacteroides fragilis, Fusobacterium, Prevotella, Porphyromonas, Bilophila wadsworthia) and Gram-positive anaerobes (e.g. Clostridium species including C. perfringens and C. difficile, Peptostreptococcus).¹ 

          • It also covers Gardnerella vaginalis (associated with bacterial vaginosis) and microaerophilic bacteria such as Helicobacter pylori.¹ 

          • Metronidazole is bactericidal to susceptible anaerobes.

            • Metronidizole administration kills B. fragilis and C. perfringens more rapidly than clindamycin at equivalent doses.¹

          • Metronidizole also exhibits excellent activity in anaerobic environments like abscess cavities.^1,3  

        • Metronidizole has no activity against aerobic bacteria (e.g. Enterobacteriaceae, Pseudomonas, streptococci, staphylococci) because these organisms cannot reduce the drug to its active form, noting that in aerobic bacteria higher oxygen levels not only reduce generation of activated metronidazole but also increase recycling back to the unactivated metronidazole.³

        • Metronidazole’s role in bacterial infections is therefore targeted to anaerobic infections such as:

          •  Intra-abdominal infections⁶

            • Complicated intra-abdominal infections are common and involve any mixture of anaerobic and aerobic bacteria.

              • Reasonable approaches may involve single-agent treatment with carbapenem as well as combination treatment with metronidazole and cephalosporin or fluorquinolone.

              • Despite long-term clinical use, generally resistance to metronidazole remains low(2015).^6,7

          • Aspiration pneumonia and lung abscess (anaerobic oral flora)⁸

            • In the case of lung abscess, the infection tends to be polymicrobial and if metronidazole is used pairing it with another agent (e.g. penicillin) would be likely required to obtain the necessary coverage in this setting.⁹

          • Deep soft tissue infections (e.g. diabetic foot with anaerobes)¹⁰

            • A number of antimicrobials that prove effective may be used in treating this condition.¹⁰

          • Brain abscess (often polymicrobial with anaerobes)¹¹

            • The use of cefotaxime, a third-generation cephalosporin, extends coverage compared to metronidazole alone to include Gram-positive, Gram-negative as well as anaerobic bacteria.¹²

          • Dental/orofacial infections (anaerobic oral organisms).¹³

        • It is also used for surgical prophylaxis in colorectal surgery to prevent anaerobic infections, often in combination regimens.¹

    • Protozoa

      • Metronidizole continues to be effective in treating Trichomonas vaginalis  and the nitroimidazoles represents the only class of medications with demonstrated efficacy against T. vaginalis infections.¹⁴ 

      • Metronidazole is effective in treating giardiasis, although antimicrobial resistance has been reported. Metronidazole is drug most commonly used in the United States for treating giardiasis.

        •  Another agent, tinidazole, also approved in the United States is the first-line drug outside the United States.¹⁵

        • Mechanistically, resistant protozoal strains often have decreased pyruvate-ferredoxin oxidoreductase activity or lower ferredoxin levels, which impairs metronidazole activation.³

      • Entamoeba histolytica has not shown significant resistance clinically.³  

  • Bacterial and Parasitic Resistance

    • Anaerobic Bacteria

      • Some Bacteroides and other anaerobes have acquired nim genes (nimA–nimK) encoding 5-nitroimidazole reductases that convert metronidazole to non-toxic amine derivatives before it can damage DNA.³

        • Strains harboring nim genes show decreased susceptibility or clinical resistance to nitroimidazoles.¹⁶ 

        • Other resistance mechanisms in bacteria include increased expression of drug efflux pumps, altered redox environment (elevated oxygen or altered electron transport that impairs drug activation), and biofilm formation reducing drug penetration.³

      • Clostridioides difficile has shown increasing treatment failure rates with metronidazole.

        • Reduced susceptibility of C. difficile isolates has been reported, corresponding to higher recurrence rates.¹⁷ 

        • Because of this finding, recent guidelines have de-emphasized metronidazole monotherapy in C. difficile infection.

          • Metronidizole is no longer viewed as the first-line antibiotic choice for Clostridium difficile infection (vancomycin or fidaxomicin are now recommended over metronidazole for initial Clostridium difficile presentations.)^1,18

            • This assessment is based on the 2017 Update by the Infectious Diseases Society of (AmericaIDSA) and Society for Healthcare Epidemiology of America (SHEA)¹⁸ 

          • The status of metronidizole in terms of treating Clostridium difficile was further considered in the 2021 update to the IDSA/SHEA Clostridioides difficile guidelines.¹⁹

            • Not only is metronidizole describe is no longer a first-line antibiotic choice (1), the guidelines now prioritize the use of fidaxomicin over vancomycin for treating initial and recurrent Clostridioides difficile infection (CDI).²⁰

              • Fidaxomicin

                Structure of Fidaxomicin Attribution Yikrazuul, Public domain, via Wikimedia Commons https://commons.wikimedia.org/wiki/File:Fidaxomicin.svg (June 9 2013)

                 

              • Fidaxomicin (green) binding to Cdiff RNA polymerase. This binding inhibits RNA polymerase.

                Single particle cryo-electron microscopy was used to resolve the structure of Cdiff RNA polymerase (RNAP). The insert identifies the binding site of fidaxomicin (green). Attribution This figure corresponds to figure 1C in the reference below. (21) Cao X Boyac H Chen J Bao Y Landick R Campbell E Basis of neuro- spectrum activity of fidaxomicin on Clostridioides difficile. Nature. 2022 April 6;604(7906): 541-545. https://pmc.ncbi.nlm.nih.gov/articles/PMC9635844/#F1 (Reference 21)

            • Fidaxomicin (Fdx) inhibits RNA polymerase (RNAP), thus targeting Clostridium difficile (Cdiff) with limited effects on gut commensals which decreases the likelihood of Cdiff recurrence.²¹

              • Clostridioides difficile (Cdiff) Isaac Gram-positive, toxin-producin intestinal bacterium which upon infecting the human gut induces lethal diarrhea (Cdiff infections or CDIs).²¹

              • "With the alarming increase in affection with the alarming increase in infections caused by highly pathogenic variance, Cdiff has been designated an 'urgent threat' by the CDC.

                • "Broad-spectrum antibiotics like vancomycin and Metronidazolea used to treat CDIs, but these antibiotics decimate the normal gut microbiome, paradoxically priming the gastrointestinal tract become more prone to CDI recurrences."^{21 for the quotation, 23,24}

                  Antibiotic Resistance Threats in the United States (2019)²²

                  Antibiotic Resistance Threats in the United States (2019) https://www.cdc.gov/antimicrobial-resistance/media/pdfs/2019-ar-threats-report-508.pdf

        • H. pylori is another organism with notable metronidazole resistance with resistance rates exceeding 50-80% in some regions.¹⁹ 

          • In a recent cross-sectional study involving the Egyptian population, about 44% of isolates proved resistant to metronidizole with 12%, 4.5% and 55.4% exhibiting resistance to clarithromycin, amoxicillin and levofloxacin.²⁵

          • Helicobacter pylori is a microaerophilic (requiring reduced oxygen levels compared to atmospheric levels) Gram-negative bacterium associated with the human gastric mucosa (about 44% in adults in about 35% in children and adolescents). Warren and Marshall²⁷  identified H. pylori association with chronic gastritis and peptic ulcer disease²⁸  which dramatically changed clinical management.²⁶

    • Protozoa

      • Resistance in Trichomonas vaginalis and Giardia lamblia has been observed in some cases of treatment failure.14,29 

        • Mechanistically, resistant protozoal strains often have decreased pyruvate-ferredoxin oxidoreductase activity or lower ferredoxin levels, which impairs metronidazole activation.³⁰   Some T. vaginalis isolates also develop higher oxygen tolerance (reducing activation of the drug).³

        • These parasites may still respond to higher doses or longer courses of metronidazole, or to alternative nitroimidazoles (like tinidazole).^31,32

          • A number of alternative drugs and drug combinations to the nitroimidazoles in the case of resistance.

            • Some observational studies and some clinical studies describe available options.³³

        • Metronidazole resistant Entamoeba histolytica has been described as a developing concern, although metronidizole remains the most commonly used agents against amoebiasis.³⁴

August, 2025