Flagyl (Metronidazole) Antimicrobial Coverage
Flagyl covers anaerobic bacteria (including Bacteroides fragilis, Clostridium species, Peptostreptococcus, and Peptococcus) and specific protozoans (Trichomonas vaginalis, Entamoeba histolytica, and Giardia lamblia). 1
Bacterial Coverage
Gram-Negative Anaerobes
- Bacteroides fragilis group (B. fragilis, B. distasonis, B. ovatus, B. thetaiotaomicron, B. vulgatus) - metronidazole is the most active agent against B. fragilis, the most resistant anaerobe 1, 2
- Fusobacterium species 1
- Prevotella species (P. bivia, P. buccae, P. disiens) 1
Gram-Positive Anaerobes
- Clostridium species (including C. perfringens and C. difficile) 1, 2
- Peptococcus species 1
- Peptostreptococcus species 1
- Eubacterium species 1
Important caveat: Resistant isolates among gram-positive anaerobes are encountered more frequently than with gram-negative anaerobes 3. Metronidazole demonstrates rapid bactericidal activity with a 2-5 log decrease in colony forming units within one hour 2.
Other Susceptible Bacteria
- Gardnerella vaginalis (associated with bacterial vaginosis) 3
- Helicobacter pylori (when used in combination regimens) 3
Protozoal Coverage
Metronidazole is FDA-approved and highly effective against:
- Trichomonas vaginalis - cure rates of 90-95% 4
- Entamoeba histolytica (amebiasis) 1
- Giardia lamblia (giardiasis) 3, 5
Important Resistance Patterns
Organisms NOT Covered
Metronidazole has no clinically relevant activity against:
- Facultative anaerobes 1
- Obligate aerobes 1
- Actinomyces species 6
- Arachnia species 6
- Propionibacterium species 2, 6
- Occasional anaerobic cocci 2
- Some non-sporulating gram-positive bacilli 2
Clinical Implications
Because metronidazole lacks aerobic coverage, it must be combined with other antimicrobials (typically aminoglycosides or cephalosporins) when treating mixed aerobic-anaerobic infections 7, 2. This is particularly critical in intra-abdominal infections, where combination therapy with cefotaxime/ceftriaxone plus metronidazole is recommended for non-severe cases 7.
FDA-Approved Indications
Per the drug label, Flagyl is indicated for 1:
- Intra-abdominal infections (peritonitis, abscess, liver abscess)
- Skin and skin structure infections
- Gynecologic infections (endometritis, tubo-ovarian abscess)
- Bacterial septicemia
- Bone and joint infections (adjunctive)
- CNS infections (meningitis, brain abscess)
- Lower respiratory tract infections (pneumonia, empyema, lung abscess)
- Endocarditis
- Trichomoniasis (symptomatic and asymptomatic)
- Amebiasis
Mechanism and Pharmacology
Metronidazole is a nitroimidazole that requires reduction in the anaerobic environment to become active 1. The reduced form interacts with DNA, leading to inhibition of DNA synthesis and bacterial death 1. This selective activation explains its specificity for anaerobic organisms while sparing normal aerobic flora 6.