Bacterial Vaginosis is Not Caused by a Single Bacterium
Bacterial vaginosis is a polymicrobial syndrome caused by the replacement of normal hydrogen peroxide-producing Lactobacillus species with an overgrowth of multiple anaerobic bacteria, including Gardnerella vaginalis, Prevotella species, Mobiluncus species, and Mycoplasma hominis 1.
The Polymicrobial Nature of BV
BV is fundamentally a disruption of the vaginal ecosystem rather than an infection by a single pathogen. The condition involves:
Loss of Protective Flora
- Normal vaginal flora is dominated by H₂O₂-producing Lactobacillus species (particularly L. crispatus and L. jensenii)
- These protective lactobacilli are depleted or replaced in BV 1, 2
Overgrowth of Multiple Anaerobes
The bacteria that proliferate in BV include 1, 3:
- Anaerobic bacteria: Prevotella species, Bacteroides species, Mobiluncus species, Porphyromonas species, Peptostreptococcus species
- Gardnerella vaginalis: Present in high concentrations, though also found in 50% of healthy women
- Mycoplasma hominis: A mycoplasma species associated with the dysbiotic state
- Novel species: Molecular diagnostics have identified additional organisms like Atopobium vaginae 4
Important Clinical Caveats
G. vaginalis is NOT the Sole Cause
A critical pitfall is assuming Gardnerella vaginalis alone causes BV. While historically implicated and present in high concentrations during BV, culture of G. vaginalis is not recommended for diagnosis because it lacks specificity—it can be isolated from vaginal cultures in approximately 50% of normal women 3. The condition requires the entire polymicrobial shift, not just the presence of this single organism 5.
The Biofilm Component
Recent evidence suggests that G. vaginalis and Atopobium vaginae may form biofilms coating the vaginal epithelium, which contributes to the persistence and recurrence of BV 2. This biofilm formation helps explain why antibiotic treatment often fails to provide long-term cure, with recurrence rates of 50-80% within one year 6.
Not Exclusively a Sexually Transmitted Infection
While BV is associated with sexual activity and multiple partners, it is not considered exclusively an STI 1, 3. Women who have never been sexually active are rarely affected, but the exact mechanism of microbial alteration remains incompletely understood. Recent evidence does support sexual transmission of BV-associated bacteria between partners 7.
The Microbiome Perspective
Modern molecular methods have revealed that BV involves complex bacterial communities with many fastidious species, particularly from the phyla Bacteroidetes and Actinobacteria 2. The microbiology is heterogeneous between patients, meaning the exact bacterial composition varies from woman to woman, though the pattern of Lactobacillus depletion with anaerobic overgrowth remains consistent 2, 8.