Causes of Bacterial Vaginosis
Bacterial vaginosis results from the replacement of normal hydrogen peroxide-producing Lactobacillus species in the vagina with an overgrowth of anaerobic bacteria (including Prevotella and Mobiluncus species), Gardnerella vaginalis, and Mycoplasma hominis. 1, 2
Primary Pathophysiologic Mechanism
The fundamental cause of BV is a disruption of the normal vaginal ecosystem where protective Lactobacillus species are displaced by pathogenic organisms 3:
- Loss of protective Lactobacillus: The normal H₂O₂-producing Lactobacillus species that maintain vaginal health through lactic acid production, hydrogen peroxide generation, and bacteriocin secretion are replaced 1, 4
- Anaerobic bacterial overgrowth: High concentrations of anaerobic bacteria (Prevotella bivia, Prevotella disiens, Porphyromonas species, Mobiluncus species, Peptostreptococcus species) proliferate in the absence of protective lactobacilli 3, 4
- pH elevation: The loss of lactic acid-producing bacteria allows vaginal pH to rise above 4.5, creating an environment favorable for pathogenic organisms 5
Contributing Factors and Risk Associations
While the exact etiology remains incompletely understood, several factors are associated with BV development 3, 6:
Sexual Activity
- Multiple sexual partners are strongly associated with BV, though it remains unclear whether BV results from acquisition of a sexually transmitted pathogen 3
- Women who have never been sexually active are rarely affected 3
- Treatment of male sexual partners has not been beneficial in preventing BV recurrence, suggesting it may not be a classic sexually transmitted infection 3, 2, 7
Hormonal and Menstrual Factors
- Menstrual cycle changes can trigger BV through hormonal alterations that affect cervical barriers and reduce the bacteriostatic effect of cervical mucus 7
- Retrograde menstruation may facilitate bacterial ascent to upper genital tract 7
Microbiological Considerations
- The rectum may serve as a reservoir for BV-associated flora, similar to urinary tract infection pathogenesis 4
- Non-peroxide-producing lactobacilli replace peroxide-producing strains, allowing anaerobic overgrowth 5
Bacterial Mechanisms
The pathogenic bacteria produce specific enzymes that contribute to BV manifestations 5:
- Aminopeptidases degrade proteins
- Decarboxylases convert amino acids to amines, which raise vaginal pH and produce the characteristic fishy odor 5
- Bacteria attach to epithelial cells creating the diagnostic "clue cells" 5
Clinical Pitfalls
Important caveats to recognize 3, 2:
- Up to 50% of women with BV are asymptomatic, so absence of symptoms does not exclude the diagnosis 3, 2
- BV is not simply a sexually transmitted infection, despite its association with sexual activity 3
- The condition has a high recurrence rate (50-80% within one year), likely because beneficial Lactobacillus species fail to recolonize after antibiotic treatment 2, 8