What causes bacterial vaginosis?

Causes of Bacterial Vaginosis

Bacterial vaginosis (BV) is caused by a disruption of the normal vaginal flora, where hydrogen peroxide-producing lactobacilli are replaced by high concentrations of anaerobic bacteria including Prevotella species, Mobiluncus species, Gardnerella vaginalis, and Mycoplasma hominis. 1, 2

Pathophysiology of Bacterial Vaginosis

BV represents a significant shift in the vaginal microbiome:

• Normal vaginal flora is dominated by Lactobacillus species that produce hydrogen peroxide (H₂O₂)
• In BV, these beneficial lactobacilli are replaced by:
  • Anaerobic bacteria (e.g., Prevotella sp., Mobiluncus sp.)
  • Gardnerella vaginalis
  • Mycoplasma hominis
  • Other facultative anaerobes 1, 3

This microbial alteration leads to the characteristic symptoms and diagnostic findings of BV, including:

• Elevated vaginal pH (>4.5)
• Fishy odor (especially after adding KOH)
• Presence of clue cells on microscopic examination
• Homogeneous vaginal discharge 1, 4

Risk Factors and Associations

While the exact cause of the microbial alteration is not fully understood, several risk factors have been identified:

• Multiple sexual partners 1, 2
• Douching 1
• Lack of vaginal lactobacilli 1
• Sexual activity (women who have never been sexually active are rarely affected) 1

It's important to note that while BV is associated with sexual activity, it is not classified as a conventional sexually transmitted disease. The CDC guidelines clarify that treatment of male sex partners has not been beneficial in preventing BV recurrence 1, 2.

Clinical Significance

Understanding the cause of BV is important because:

• BV is the most prevalent cause of vaginal discharge or malodor 1
• Up to 50% of women with BV may be asymptomatic 1
• BV is associated with serious reproductive health outcomes:
  • Increased risk of preterm birth
  • Higher susceptibility to sexually transmitted infections
  • Pelvic inflammatory disease
  • Complications after gynecologic procedures 5, 3

Biofilm Formation

A key aspect of BV pathogenesis involves biofilm formation:

• G. vaginalis is the predominant microbe in polymicrobial biofilms
• These biofilms can protect BV-associated microbes from host defense mechanisms and antibiotics
• This may explain the high recurrence rate (50-80% within a year after antibiotic treatment) 5, 3

Diagnostic Considerations

The microbial shift in BV can be confirmed through:

• Clinical criteria (Amsel criteria): requires 3 of 4 findings (homogeneous discharge, clue cells, pH >4.5, positive whiff test)
• Gram stain (Nugent score): shows marked reduction in Lactobacillus morphology and predominance of Gardnerella morphotype 1, 4

It's crucial to rule out other pathogens commonly associated with vulvovaginitis, including Trichomonas vaginalis, Chlamydia trachomatis, Neisseria gonorrhoeae, Candida albicans, and Herpes simplex virus 4.

Prevention Insights

Understanding the cause of BV helps inform prevention strategies:

• Avoiding douching
• Using condoms
• Considering hormonal contraception
• Smoking cessation
• Maintaining vaginal Lactobacillus through probiotic approaches 5

The complex and recurrent nature of BV has led to ongoing research into alternative approaches beyond antibiotics, including probiotics containing vaginal Lactobacillus species, vaginal microbiome transplantation, pH modulation, and biofilm disruption strategies 5, 6.