Susceptibility of Gardnerella vaginalis to metronidazole, its bioactive metabolites, and tinidazole.

The susceptibilities of 510 clinical isolates of Gardnerella vaginalis to metronidazole, its principal oxidative metabolites, and tinidazole were determined by an agar dilution method. The hydroxy metabolite was the most active, with an MIC90 value (minimum concentration that inhibited 90% of the strains) of 1.12 mg/L (5.51 mumol/L). Tinidazole and metronidazole were somewhat less active, with MIC90s of 4.09 mg/L (23.9 mumol/L) and 4.44 mg/L (18.0 mumol/L), respectively. The acid metabolite was inactive, with an MIC90 value of 226.55 mg/L (1.22 mmol/L). These results suggest that the hydroxy metabolite of metronidazole may contribute significantly to the antimicrobial effect of the parent drug in G. vaginalis-associated infections.