What causes bacterial vaginosis (BV)?

Causes of Bacterial Vaginosis

Bacterial vaginosis (BV) is caused by a replacement of the normal H₂O₂-producing Lactobacillus species in the vagina with high concentrations of anaerobic bacteria (e.g., Prevotella sp. and Mobiluncus sp.), Gardnerella vaginalis, and Mycoplasma hominis. 1, 2, 3

Microbial Alteration

• BV represents a complex disturbance of the normal vaginal flora characterized by an overgrowth of anaerobic and other microorganisms with a corresponding decrease in beneficial lactobacillus species 4
• The normal vaginal microbiota is dominated by Lactobacillus species, which maintain vaginal health by producing:
  • Lactic acid (creating an acidic environment)
  • Hydrogen peroxide (H₂O₂)
  • Bacteriocins 5
• In BV, this protective Lactobacillus-dominant environment is disrupted, allowing overgrowth of:
  • Anaerobic bacteria (e.g., Prevotella sp. and Mobiluncus sp.)
  • Gardnerella vaginalis
  • Mycoplasma hominis
  • Atopobium vaginae 1, 2, 5

Risk Factors and Associations

• BV is associated with having multiple sexual partners, although it's unclear whether BV results directly from acquisition of a sexually transmitted pathogen 1, 3
• Women who have never been sexually active are rarely affected by BV 1
• Treatment of male sexual partners has not been shown to be beneficial in preventing BV recurrence, suggesting complex transmission dynamics 1, 3, 6
• Hormonal changes during menstruation may trigger BV by:
  • Altering cervical barriers that normally prevent bacterial ascent
  • Reducing the bacteriostatic effect of cervical mucus
  • Potentially allowing retrograde menstruation that facilitates bacterial movement 6

Pathophysiology

• Anaerobic bacteria produce enzymes that contribute to BV symptoms:
  • Aminopeptidases (degrade proteins)
  • Decarboxylases (convert amino acids to amines) 7
• These bacterial metabolic products:
  • Raise vaginal pH above the normal 4.5
  • Produce the characteristic fishy odor
  • Contribute to vaginal discharge 7
• The excessive bacteria attach to epithelial cell surfaces, resulting in the characteristic "clue cells" seen on microscopic examination 1, 7

Clinical Significance

• BV is the most prevalent cause of vaginal discharge and malodor, though approximately 50% of women with BV are asymptomatic 1, 3
• BV is associated with serious health complications:
  • Increased risk of preterm birth
  • Pelvic inflammatory disease (PID)
  • Increased susceptibility to sexually transmitted infections, including HIV
  • Endometritis and vaginal cuff cellulitis after invasive procedures 1, 8, 5
• BV has high recurrence rates, with 50-80% of women experiencing recurrence within a year after antibiotic treatment 8

Diagnostic Considerations

• BV is diagnosed using either clinical (Amsel) criteria or Gram stain (Nugent) criteria
• Amsel criteria require three of the following:
  • Homogeneous, white, non-inflammatory discharge coating vaginal walls
  • Presence of clue cells on microscopic examination
  • Vaginal fluid pH greater than 4.5
  • Fishy odor before or after addition of 10% KOH (whiff test) 1, 2, 3

The exact etiology of BV remains not fully understood despite its prevalence and clinical significance, making it an ongoing area of research 4, 8.