Effect of Metronidazole on the Growth of Vaginal Lactobacilli in vitro

Objective: To determine whether metronidazole has an adverse effect on the growth of Lactobacillus. Methods: Hydrogen peroxide- and bacteriocin-producing strains of Lactobacillus were used as test strains. Concentrations of metronidazole used ranged from 128 to 7000 μg/ml. Susceptibility to metronidazole was conducted by the broth microdilution method recommended by the National Committee for Clinical Laboratory Standards. Results: Growth of Lactobacillus was partially inhibited at concentrations between 1000 and 4000 μg/ml (p = 0.014). Concentrations ≥ 5000 μg/ml completely inhibited growth of Lactobacillus. Concentrations between 128 and 256 μg/ml stimulated growth of Lactobacillus (p = 0.025 and 0.005, respectively). Concentrations of metronidazole between 64 and 128 μg/ml or ≥ 512 μg/ml did not have an inhibitory or a stimulatory effect on the growth of Lactobacillus compared to the control. Conclusions: High concentration of metronidazole, i.e. between 1000 and 4000 μg/ml, partially inhibited the growth of Lactobacillus. Concentrations ≥ 5000 μg/ml completely suppressed the growth of Lactobacillus. Concentrations between ≥ 128 and ≤ 256 μg/ml stimulated the growth of Lactobacillus. Further investigation to determine the ideal concentration of metronidazole is needed in order to use the antimicrobial agent effectively in the treatment of bacterial vaginosis.