Several fastidious bacteria have been associated with bacterial vaginosis (BV) using broad-range bacterial PCR methods such as consensus sequence 16S rRNA gene PCR, but their role in BV remains poorly defined. We describe changes in vaginal bacterial concentrations following metronidazole therapy for BV. Vaginal swabs were collected from women with BV diagnosed using Amsel clinical criteria, and vaginal fluid was assessed by Gram stain to generate Nugent scores. Follow-up swabs were collected 1 month after a 5-day course of vaginal 0.75% metronidazole gel and analyzed for 24 subjects with cured BV and 24 subjects with persistent BV. Changes in bacterial concentrations were measured using eight bacterium-specific 16S rRNA gene quantitative PCR assays. DNA from several fastidious BV-associated bacteria (BVAB) were present at high concentrations in the vagina prior to treatment. Successful antibiotic therapy resulted in 3- to 4-log reductions in median bacterial loads of BVAB1 (P = 0.02), BVAB2 (P = 0.0004), BVAB3 (P = 0.03), a Megasphaera-like bacterium (P < 0.0001), Atopobium species (P < 0.0001), Leptotrichia/Sneathia species (P = 0.0002), and Gardnerella vaginalis (P < 0.0001). Median posttreatment bacterial levels did not change significantly in subjects with persistent BV except for a decline in levels of BVAB3. The presence or absence of BV is reflected by vaginal concentrations of BV-associated bacteria such as BVAB1, BVAB2, Leptotrichia/Sneathia species, Atopobium species, Gardnerella vaginalis, and a Megasphaera-like bacterium, suggesting that these bacteria play an important role in BV pathogenesis and may be suitable markers of disease and treatment response. Copyright © 2009, American Society for Microbiology. All Rights Reserved.