What is the cause of bacterial vaginosis?

Cause of Bacterial Vaginosis

Bacterial vaginosis is caused by an imbalance in the normal vaginal bacterial flora characterized by a decrease in Lactobacilli and an increase in anaerobic bacteria including Gardnerella vaginalis, Mycoplasma, and other anaerobes. 1, 2, 3

Microbial Imbalance

The normal vaginal microbiome is typically dominated by Lactobacillus species, which:

• Maintain vaginal health by producing lactic acid
• Generate hydrogen peroxide (H₂O₂)
• Produce bacteriocins
• Help maintain a healthy acidic vaginal pH (≤4.5)

In bacterial vaginosis, this balance is disrupted, leading to:

• Marked reduction or absence of beneficial Lactobacillus species
• Overgrowth of anaerobic bacteria, particularly:
  • Gardnerella vaginalis (predominant)
  • Atopobium vaginae
  • Mycoplasma species
  • Other anaerobic bacteria 2, 4, 5

Diagnostic Features

The microbial imbalance in bacterial vaginosis results in characteristic clinical findings:

• Homogeneous, white, thin discharge adhering to vaginal walls
• Fishy or musty odor, especially after addition of 10% KOH (positive "whiff test")
• Vaginal fluid pH greater than 4.5
• Presence of "clue cells" (epithelial cells covered with bacteria) on microscopic examination 2, 3

Risk Factors and Potential Transmission

While the exact etiology remains incompletely understood, several factors are associated with bacterial vaginosis:

• Sexual activity: Observational evidence suggests possible sexual transmission, though BV is not classified as a sexually transmitted infection 6
• Smoking: Associated with increased risk and recurrence
• Lack of condom use: Consistent condom use may help prevent recurrence
• Hormonal factors: Hormonal contraception may offer some protection 2, 4
• Racial differences: Studies show higher prevalence among African-American women compared to Caucasian women 1

Clinical Significance

Understanding the cause of bacterial vaginosis is important due to its association with:

• Preterm delivery (relative risk 1.4 to 6.9)
• Preterm premature rupture of membranes (RR 2.0 to 7.3)
• Spontaneous abortion (RR 1.3 to 2.0)
• Preterm labor (RR 2.0 to 2.6)
• Increased risk of sexually transmitted infections, including HIV
• Pelvic inflammatory disease 1, 4

Treatment Challenges

The complex microbial imbalance in bacterial vaginosis presents treatment challenges:

• Standard antibiotic treatments (metronidazole, clindamycin) provide short-term cure but have high recurrence rates
• 50-80% of women experience recurrence within one year of treatment
• Beneficial Lactobacillus species often fail to recolonize the vagina after antibiotic treatment
• Biofilm formation by BV-associated bacteria may contribute to treatment resistance and recurrence 4, 6, 7

The cause of bacterial vaginosis is multifactorial, involving a complex disruption of vaginal microbiota. While treatment can temporarily restore balance, the high recurrence rate suggests that underlying factors driving this microbial imbalance are not fully addressed by current therapeutic approaches.