- Metronidazole is drug of choice for bacterial vaginosis ( with or without pregnancy )- Alternative to metronidazole is clindamycin.- Rovamycin i.e. spiramycin (which is a macrolide) is drug of choice for toxoplasmosis in pregnancy. Metronidazole -* It is the prototype nitroimidazole having broad-spectrum cidal activity against anaerobic protozoa, including Giardia lamblia , trichomonas and amoeba. * Many anaerobic and microaerophilic bacteria, such as Bact. fragilis, Fusobacterium, Clostridium perfringens, Cl. difficile, Helicobacter pylori, Campylobacter, peptococci, spirochetes and anaerobic Streptococci are sensitive. * Metronidazole does not affect aerobic bacteria. Mechanism of action- After entering the cell by diffusion, its nitro group is reduced by ceain redox proteins operative only in anaerobic microbes to a highly reactive nitro radical which exes cytotoxicity. The nitro radical of metronidazole acts as an electron sink which competes with the biological electron acceptors of the anaerobic organism for the electrons generated by the pyruvate : ferredoxin oxidoreductase (PFOR) enzyme pathway of pyruvate oxidation. The energy metabolism of anaerobes that have no mitochondria is thus, disrupted. Aerobic environment attenuates cytotoxicity of metronidazole by inhibiting its reductive activation. Moreover, O2 competes with the nitro radical of metronidazole for the free electrons generated during energy metabolism of anaerobes. Metronidazole, in addition, has been found to inhibit cell mediated immunity, to induce mutagenesis and to cause radiosensitization. Uses of metronidazole- o Amoebiasiso Giardiasiso Trichomonas vaginitiso Bacterial vaginosiso Anaerobic bacterial infection o Pseudomembranous enterocolitiso Acute necrotising ulcerative gingivitiso Helicobacter pylori gastritis o Guinea worm infestation