Microbiologic response to treatment of bacterial vaginosis with topical clindamycin or metronidazole

To compare the frequencies, concentrations, and antimicrobial susceptibilities of vaginal microbes isolated from women with bacterial vaginosis (BV) before and after therapy, 119 nonpregnant women aged 18 to 45 with clinical and Gram stain evidence of BV were randomized to receive intravaginal clindamycin or metronidazole. Vaginal swabs were collected at baseline and 7 to 12 days, 35 to 45 days, and 70 to 90 days following therapy for quantitative vaginal culture. For the 99 women completing all four visits, statistical analyses were performed comparing differences in vaginal microflora between the two treatment arms and between visits in the same treatment group. Antimicrobial susceptibility testing using the agar dilution method was performed for anaerobic gram-negative rods. Although both therapies resulted in decreased colonization by Gardnerella vaginalis and Mycoplasma hominis, only metronidazole treatment resulted in a significant decrease in the frequency and concentration of Prevotella bivia and black-pigmented Prevotella species. Of the 865 anaerobic gram-negative rods evaluated for susceptibility, only 3 (0.3%) were resistant to metronidazole, whereas clindamycin resistance increased significantly for P. bivia and black-pigmented anaerobic gram-negative rods persisting following clindamycin therapy. Clindamycin-resistant subpopulations of P. bivia and black-pigmented Prevotella species emerged 7 to 12 days after therapy even among women colonized initially by clindamycin-susceptible strains. These resistant subpopulations persisted at high frequencies (42 to 50%) 70 to 90 days following therapy. The two topical agents for treatment of BV have differing microbiologic effects on the vaginal microflora. The emergence of clindamycin-resistant anaerobic gram-negative rods following therapy is of concern.