Metronidazole is a nitroimidazole, a family of compounds with activity against bacteria, fungi, and parasites. The antibacterial action requires reduction of the nitro group under anaerobic conditions, which explains the limitation of its activity to bacteria that prefer anaerobic or at least microaerophilic growth conditions. The reduction products act on the cell at multiple points; the most lethal of these effects is induction of breaks in DNA strands.
Metronidazole is active against a wide range of anaerobes, including Bacteroidesfragilis. Clinically, it is useful for any infection in which anaerobes may be involved. Be-cause these infections are typically polymicrobial, a second antimicrobial (eg, β-lactam) is usually added to cover aerobic and facultative bacteria.
Rifampin binds to the -subunit of DNA-dependent RNA polymerase, which prevents the initiation of RNA synthesis. This agent is active against most Gram-positive bacteria and selected Gram-negative organisms, including Neisseria and Haemophilus but not members of the Enterobacteriaceae. The most clinically useful property of rifampin is its antimy-cobacterial activity, which includes Mycobacterium tuberculosis and the other species that most commonly infect humans. Because resistance by mutation of the polymerase readily occurs, rifampin is combined with other agents in the treatment of active infections. It is used alone for chemoprophylaxis.