Antibiotic Classes: Metronidazole and Gentamicin
Metronidazole is a nitroimidazole antibiotic, and gentamicin is an aminoglycoside antibiotic. 1, 2
Metronidazole: Nitroimidazole Class
Metronidazole belongs to the nitroimidazole class of antimicrobials, which are synthetic agents with a unique mechanism of action. 1, 3
Mechanism: Nitroimidazoles work through toxic metabolites that cause DNA strand breakage, making them bactericidal against susceptible organisms. 4
Spectrum of activity: Metronidazole has a limited but highly specific spectrum encompassing anaerobic bacteria (both Gram-negative and Gram-positive) and certain protozoa including Entamoeba histolytica, Giardia lamblia, and Trichomonas vaginalis. 5
Key anaerobic coverage: It is highly effective against Gram-negative anaerobes (Bacteroides and Fusobacterium species) and most Gram-positive anaerobes (Peptostreptococcus and Clostridium species), though resistance is more common in the latter. 5
Clinical positioning: Metronidazole is considered the "gold standard" antibiotic for anaerobic infections and is the only agent rapidly bactericidal against the Bacteroides fragilis group. 3, 5
Gentamicin: Aminoglycoside Class
Gentamicin is a water-soluble aminoglycoside antibiotic derived from Micromonospora purpurea. 2
Clinical use in combination therapy: Guidelines consistently recommend gentamicin as part of combination regimens for intra-abdominal infections, typically paired with metronidazole or ampicillin to provide coverage against aerobic Gram-negative bacteria while metronidazole covers anaerobes. 6
Dosing: Current guidelines recommend gentamicin 5-7 mg/kg IV once daily for adults, with serum drug-concentration monitoring for dosage individualization. 6, 7
Important safety consideration: Aminoglycosides should be administered after fluid resuscitation to restore adequate visceral perfusion and reduce nephrotoxicity risk. 7
Positioning in guidelines: Gentamicin appears as a second-line option for surgical prophylaxis and as part of combination therapy for severe intra-abdominal infections, particularly in settings with high quinolone resistance. 6