Causes of Bacterial Vaginosis
Bacterial vaginosis is fundamentally caused by disruption of the normal vaginal ecosystem, where protective hydrogen peroxide-producing Lactobacillus species are replaced by an overgrowth of anaerobic bacteria including Gardnerella vaginalis, Prevotella species, Mobiluncus species, Bacteroides species, Peptostreptococcus species, and Mycoplasma hominis. 1, 2
Primary Pathophysiologic Mechanism
The core pathology is ecological disruption rather than infection with a single pathogen—it represents vaginal dysbiosis, not a traditional infection. 1 The normal vaginal environment is maintained by Lactobacillus species that produce lactic acid, hydrogen peroxide, and bacteriocins, creating an acidic pH of 3.8-4.2. 1 When these protective bacteria are lost, the vaginal pH rises above 4.5, which both results from and perpetuates the loss of Lactobacillus dominance. 1
The specific anaerobic bacteria that overgrow include: 3, 1
- Gardnerella vaginalis
- Mycoplasma hominis
- Bacteroides species
- Mobiluncus species
- Prevotella bivia and Prevotella disiens
- Porphyromonas species
- Peptostreptococcus species
Sexual and Behavioral Risk Factors
Sexual activity is strongly associated with bacterial vaginosis—women who have never been sexually active are rarely affected, and the condition correlates with having multiple sexual partners. 1, 2 However, a critical nuance exists: bacterial vaginosis is not considered exclusively a sexually transmitted disease because treating male partners does not prevent recurrence in women. 1, 2 This suggests that while sexual activity may trigger the microbial shift, the condition is not caused by transmission of a single pathogenic organism. 2
Additional behavioral risk factors include: 4
- Higher number of lifetime sexual partners
- Younger age at first intercourse
- Being unmarried
- Engagement in commercial sex work
Environmental and Chemical Triggers
Chemical or mechanical irritation from products like regular soap, douches, and other vaginal products can disrupt the protective vaginal ecosystem, potentially triggering the microbial shift. 1 Regular douching is specifically associated with increased risk of bacterial vaginosis. 4
Menstrual cycle changes can trigger bacterial vaginosis through hormonal alterations that affect cervical barriers and reduce the bacteriostatic effect of cervical mucus. 2
Host Factors
Women with underlying debilitating medical conditions such as uncontrolled diabetes or those on corticosteroid treatment do not respond as well to standard therapies, suggesting host immune factors play a role. 1
Critical Clinical Pitfalls
Up to 50% of women meeting clinical criteria for bacterial vaginosis are completely asymptomatic, meaning absence of symptoms does not exclude the diagnosis or eliminate the risk of complications. 1, 2 This is particularly important because the condition is associated with serious reproductive health outcomes including preterm birth, increased risk of sexually transmitted infections, and pelvic inflammatory disease. 5
The condition has a high recurrence rate of 50-80% within one year, likely because beneficial Lactobacillus species fail to recolonize after antibiotic treatment. 2, 5 This suggests that while antibiotics can eliminate the overgrown anaerobes, they do not address the underlying failure of the protective vaginal ecosystem to restore itself.