Bacterial vaginosis (BV) is not caused by a single bacterium. It results from a shift in the vaginal microbiome where protective bacteria decline and a mix of anaerobic organisms overgrow. The bacterium most closely linked to BV is Gardnerella vaginalis, but it works alongside many other species, and its presence alone doesn’t always cause symptoms. BV affects roughly 20 to 30 percent of women of reproductive age worldwide, making it the most common vaginal infection.
Gardnerella Vaginalis: The Central Player
Gardnerella vaginalis has been associated with BV since the 1950s, when researchers first identified it (under the name Haemophilus vaginalis) as the apparent sole cause of what was then called “non-specific vaginitis.” That initial picture turned out to be too simple. In early experiments where Gardnerella was introduced into the vaginas of healthy women, only 1 out of 13 developed both a positive culture and clinical signs of infection. The rest either cleared the bacterium quickly or carried it without symptoms.
Today, scientists recognize that Gardnerella plays both direct and indirect roles. It produces enzymes that break down the vaginal lining, creating the characteristic “clue cells” (vaginal cells coated in bacteria) that clinicians look for under a microscope. It also appears to pave the way for other harmful bacteria. In mouse studies, animals colonized with Gardnerella developed more severe uterine infections from Prevotella bivia than animals exposed to Prevotella alone. So Gardnerella doesn’t just cause damage on its own; it amplifies the harm done by its bacterial partners.
The Full Cast of BV-Associated Bacteria
The list of bacteria linked to BV has grown substantially as genetic sequencing tools have improved. Beyond Gardnerella, the key players include:
- Atopobium vaginae: frequently found alongside Gardnerella and strongly associated with recurrent BV
- Prevotella species: anaerobic bacteria that thrive once the vaginal environment becomes less acidic
- Mobiluncus: curved, rod-shaped bacteria often seen on vaginal smears from women with BV
- Sneathia and Leptotrichia: less well-known anaerobes that appear consistently in BV-associated communities
- Mycoplasma: tiny bacteria lacking a cell wall, linked to both BV and other reproductive tract infections
- BVAB1 through BVAB3: relatively recently identified organisms in the Clostridiales order that were discovered through DNA sequencing rather than traditional culture methods
No single species is present in every case of BV. The specific mix of bacteria varies from person to person, which is part of why the condition can look and behave differently across individuals.
What Happens to the Vaginal Microbiome
A healthy vaginal microbiome is dominated by Lactobacillus species, which produce lactic acid and hydrogen peroxide. The lactic acid keeps vaginal pH below 4.5, creating an environment that most anaerobic bacteria can’t tolerate. Hydrogen peroxide, produced by most Lactobacillus species (with the notable exception of Lactobacillus iners), generates free radicals that directly kill oxygen-sensitive organisms.
When Lactobacillus populations drop, vaginal pH rises above 4.5, and anaerobic bacteria that were previously held in check begin to multiply rapidly. This isn’t a simple infection where one “bad” organism invades. It’s more like an ecosystem collapse: the protective species lose their foothold, and a diverse community of anaerobes fills the gap. What triggers this initial Lactobacillus decline isn’t fully understood, though douching, new sexual partners, and antibiotic use are known risk factors.
How Biofilms Drive Recurrence
One of the most frustrating aspects of BV is how often it comes back. Recurrence rates are high even after successful antibiotic treatment, and bacterial biofilms are a major reason why. Gardnerella vaginalis forms the initial scaffold of these biofilms, creating a structured community on the vaginal wall that other species then attach to and grow within.
These biofilms act as a physical shield. Antibiotics that would easily kill free-floating bacteria struggle to penetrate the dense matrix of the biofilm, allowing bacteria deep inside to survive a full course of treatment. The biofilm also helps bacteria evade the immune system. Recent research has shown that other bacteria, particularly Group B Streptococcus, can actively enhance Gardnerella’s biofilm production through chemical signaling molecules. When Group B Strep detects increasing Gardnerella density, it ramps up production of a signaling molecule that causes Gardnerella to build thicker, more resilient biofilms. This cross-species communication helps explain why BV so often returns within weeks or months of treatment.
How BV Is Identified
Because BV involves a community of bacteria rather than a single pathogen, diagnosis doesn’t rely on identifying one specific organism. Instead, clinicians look at the overall pattern. The most common bedside approach uses four criteria: a thin, gray-white vaginal discharge; a vaginal pH above 4.5; a fishy odor released when a chemical solution is added to a sample; and the presence of clue cells under a microscope. Meeting three of these four criteria confirms the diagnosis.
A more standardized lab method scores a vaginal smear based on the relative numbers of three bacterial types visible under the microscope: Lactobacillus (the protective bacteria you want to see), Gardnerella-like organisms, and curved gram-negative rods like Mobiluncus. High numbers of Lactobacillus score low, while high numbers of the other two score high. A combined score of 7 or above indicates BV, while 4 to 6 falls into an intermediate category.
Symptoms and What BV Looks Like
Many women with BV have no symptoms at all. When symptoms do appear, the most recognizable is a thin white or gray vaginal discharge with a strong fishy smell, particularly noticeable after sex. The odor comes from amines, chemicals produced by anaerobic bacteria as they break down organic compounds in the vaginal environment. The alkaline pH of semen makes these amines more volatile, which is why the smell often intensifies after unprotected intercourse.
Unlike yeast infections, BV rarely causes significant itching or irritation. The discharge tends to be thinner and more uniform rather than thick and clumpy. This distinction matters because the two conditions require completely different treatment, and misidentifying one for the other is common among women who self-diagnose.