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 microg/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 microg/ml (p = 0.014). Concentrations > or = 5000 microg/ml completely inhibited growth of Lactobacillus. Concentrations between 128 and 256 microg/ml stimulated growth of Lactobacillus (p = 0.025 and 0.005, respectively). Concentrations of metronidazole between 64 and 128 microg/ml or > or = 512 microg/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 microg/ml, partially inhibited the growth of Lactobacillus. Concentrations > or = 5000 microg/ml completely suppressed the growth of Lactobacillus. Concentrations between > or = 128 and < or = 256 microg/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.