Relative susceptibilities of Gardnerella vaginalis (Haemophilus vaginalis), Neisseria gonorrhoeae, and Bacteroides fragilis to Metronidazole and its two major metabolites.

The susceptibilities of strains of Gardnerella vaginalis (Haemophilus vaginalis), Neisseria gonorrhoeae, and Bacteroides fragilis to metronidazole and its principal oxidative metabolites (1-[2-hydroxyethyl]-2-hydroxymethyl-5-nitroimidazole) ("hydroxy" metabolite) and 1-acetic acid-2-methyl-5-nitroimidazole ("acid" metabolite), were compared by determinations of the minimal inhibitory concentrations (MICs) of these compounds. Against ten strains of G. vaginalis, the hydroxy metabolite was the most active (median MIC, 2 microgram/ml); the median MICs of metronidazole and of the acid metabolite were 8 and 64 microgram/ml, respectively. The hydroxy metabolite was also the most active against 15 strains of N. gonorrhoeae (median MIC, 32 microgram/ml). In contrast, metronidazole was the most active against ten strains of B. fragilis (median MIC, 1 microgram/ml); the hydroxy and acid metabolites had median MICs of 2 and 64 micrograms/ml, respectively. These results indicate that in the treatment of G. vaginalis-associated vaginitis with metronidazole, the hydroxy metabolite may contribute a significant antimicrobial effect, in view of its excellent activity in vitro.