What Causes Bacterial Vaginosis
Bacterial vaginosis is fundamentally caused by the loss of protective hydrogen peroxide (H₂O₂)-producing Lactobacillus species in the vagina, which allows overgrowth of anaerobic bacteria including Gardnerella vaginalis, Bacteroides species, Mobiluncus species, Prevotella species, Porphyromonas species, Peptostreptococcus species, and Mycoplasma hominis. 1, 2
Primary Pathophysiologic Mechanism
BV represents an ecological disruption rather than infection with a single pathogen—it is a syndrome of vaginal dysbiosis, not a traditional infection. 2
The normal vaginal pH of 3.8-4.2 becomes elevated to >4.5, which both results from and perpetuates the loss of Lactobacillus dominance. 2, 3
This microbial shift involves replacement of the normal H₂O₂-producing Lactobacillus flora with high concentrations of predominantly anaerobic organisms. 1, 4, 5
Risk Factors and Triggers
Sexual Activity
BV is strongly associated with sexual activity: women who have never been sexually active are rarely affected, and acquisition correlates with having multiple sex partners. 1, 2
However, BV is not considered exclusively a sexually transmitted disease because treating male partners does not prevent recurrence in women. 1, 2
Interestingly, BV often regresses spontaneously after unprotected sexual intercourse (9 of 13 spontaneous resolutions occurred within 48 hours of unprotected intercourse in one study). 6
Chemical and Mechanical Irritants
Avoiding chemical or mechanical irritation from products like regular soap, douches, and other vaginal products is critical, as they can disrupt the protective vaginal ecosystem and trigger the microbial shift. 2, 3
Regular soap can cause vulvar irritation that mimics infection symptoms without actual pathogens present. 3
Menstrual Cycle Patterns
BV arises most often in the first 7 days of the menstrual cycle and resolves spontaneously most often in mid-cycle. 6
Recurrences often follow episodes of candidiasis (BV appeared after candida on 9 of 11 episodes in one longitudinal study). 6
Clinical Context
Prevalence and Demographics
BV affects 9-23% of pregnant women in academic medical centers and public hospitals, with higher rates among African-American women than Caucasian women. 1
It is most common in women of childbearing age but may also occur in menopausal women, and is rare in children. 4
Asymptomatic Disease
- Up to 50% of women meeting clinical criteria for BV are completely asymptomatic, meaning absence of symptoms does not exclude the diagnosis or eliminate the risk of complications. 1, 2
Important Clinical Pitfall
The exact cause of the microbial alteration is not fully understood despite decades of research. 1, 7 BV likely results from complex interactions between multiple factors including the vaginal microbial ecosystem components, bacteriophages, host immune response, and environmental triggers—none of which alone can reliably explain all epidemiological data. 7, 5