Background.  Bacterial vaginosis (BV) is a common polymicrobial disease associated with numerous negative reproductive health outcomes, including an increased risk of human immunodeficiency virus acquisition. BV is treatable with antibiotics, but relapse is common. A more detailed understanding of bacterial dynamics during antibiotic therapy for BV could identify conditions that favor establishment, maintenance, and eradication of BV-associated bacterial species, thereby improving treatment outcomes.

Methods.  We used mathematical models to analyze daily quantitative measurements of 11 key bacterial species during metronidazole treatment for 15 cases of BV.

Results.  We identified complete reorganization of vaginal bacterial composition within a day of initiating therapy. Although baseline bacterial levels predicted a longer time to clearance, all anaerobic species were eliminated rapidly within a median of 3 days. However, reemergence of BV-associated species was common following treatment cessation. Gardnerella vaginalis, a facultative anaerobe, was cleared more slowly than anaerobic BV-associated species, and levels of G. vaginalis often rebounded during treatment. We observed gradual Lactobacillus species growth, indicating that untargeted microbes fill the transient vacuum formed during treatment.

Conclusions.  Under antibiotic pressure, the human microbiome can undergo rapid shifts on a scale of hours. When treatment is stopped, BV-associated bacteria quickly reemerge, suggesting a possible role for intermittent prophylactic treatment.