Bacterial vaginosis (BV) is driven primarily by Gardnerella, a group of bacterial species that has been linked to the condition since the 1950s. But BV isn’t a simple one-bug infection. It develops when the normal balance of vaginal bacteria shifts: protective species decline, and a mix of anaerobic bacteria overgrow in their place. Globally, BV affects roughly 23 to 29% of women of reproductive age.
Gardnerella: The Central Player
Gardnerella is considered a “keystone species” in BV, meaning it plays a disproportionately large role in triggering the condition. Women with BV consistently show higher levels of Gardnerella in the vagina compared to women without it. Importantly, research published in The Journal of Infectious Diseases found that having multiple Gardnerella species present at the same time was necessary for symptomatic BV to develop. A single species alone may not be enough.
What makes Gardnerella particularly significant is its ability to form biofilms, sticky layers of bacteria that attach to the vaginal lining. These biofilms create a sheltered environment where other harmful bacteria can thrive. Biofilm formation increases as vaginal pH rises above the normal acidic range, which is exactly what happens during BV. Once established, these biofilms are difficult to eliminate, which is a major reason BV recurs so frequently after treatment.
Other Bacteria Involved
Once Gardnerella establishes a biofilm, several other anaerobic species flourish within it. The most commonly identified include:
- Fannyhessea vaginae (formerly called Atopobium vaginae), which integrates into Gardnerella biofilms and is strongly associated with recurrent BV
- Prevotella bivia, an anaerobe that grows readily in the higher-pH environment BV creates
- Mobiluncus species, curved bacteria that appear in lab analysis of vaginal samples and are one of the key markers used in scoring systems to diagnose BV
- Peptostreptococcus anaerobius, another anaerobe found in multi-species BV biofilms
None of these bacteria act alone. BV is a polymicrobial condition, meaning it results from the collective overgrowth of many species rather than infection by a single organism. This is part of what makes it different from conditions like a yeast infection or chlamydia, which involve one identifiable pathogen.
What Normally Keeps These Bacteria in Check
A healthy vagina is dominated by Lactobacillus bacteria, which maintain an acidic environment with a pH between 3.8 and 4.5. These bacteria produce hydrogen peroxide and other antimicrobial substances that suppress the growth of Gardnerella and other anaerobes. The most protective species are Lactobacillus crispatus and Lactobacillus jensenii, both strong hydrogen peroxide producers.
The difference is striking. Women who carry hydrogen peroxide-producing Lactobacillus have BV rates around 3%. Women colonized by Lactobacillus strains that don’t produce hydrogen peroxide have rates around 25%. And women whose vaginal bacteria are dominated by other species entirely have rates as high as 46%. When these protective bacteria decline for any reason, the door opens for Gardnerella and its associated anaerobes to take over.
How the Shift Happens
BV develops when something disrupts the Lactobacillus-dominant environment. Acquiring new Gardnerella species through sexual activity is one well-documented pathway. As Gardnerella colonizes the vaginal lining and forms biofilms, the local pH rises above 4.5, which further favors biofilm growth and suppresses Lactobacillus. Other anaerobic bacteria then join the biofilm community, amplifying the imbalance.
This cascade explains why BV often follows a pattern: an initial trigger (frequently sexual exposure to new bacteria) leads to biofilm formation, pH elevation, Lactobacillus decline, and further anaerobic overgrowth. Each step reinforces the next, making the condition self-sustaining once it takes hold.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV, and the biofilm is largely to blame. Standard antibiotics can kill free-floating bacteria effectively, but they struggle to penetrate the dense biofilm structure that Gardnerella builds on the vaginal wall. Lab studies have shown inconsistent results when antibiotics are tested against established biofilms. Some early experiments suggested complete biofilm destruction, but more recent studies using realistic multi-species biofilms found that standard treatments were far less effective.
Reinfection through sexual contact adds another layer. Even after successful treatment, exposure to a partner carrying Gardnerella can restart the entire cycle. This combination of biofilm persistence and reintroduction of bacteria explains why up to half of women treated for BV experience it again within months.
Health Risks of the Bacterial Imbalance
BV sometimes resolves on its own, but leaving it untreated carries real risks. The disrupted vaginal environment makes it easier to acquire other sexually transmitted infections, including HIV, chlamydia, and gonorrhea. These infections can in turn lead to pelvic inflammatory disease, which can affect fertility.
During pregnancy, BV increases the likelihood of preterm birth and low birth weight. The inflammatory environment created by BV-associated bacteria can weaken the membranes surrounding the fetus, which is why screening and treatment during pregnancy are taken seriously.
How Doctors Identify the Bacteria
Diagnosing BV doesn’t typically involve culturing individual species. Instead, doctors look at the overall pattern. Clinical diagnosis requires at least three of four criteria: a thin, milky vaginal discharge; vaginal pH above 4.5; a fishy odor; and the presence of “clue cells” under a microscope (vaginal cells visibly coated with bacteria).
Lab-based scoring offers another approach, evaluating a vaginal sample under a microscope for the relative amounts of three bacterial types: large rods (Lactobacillus), small rods (Gardnerella and Bacteroides), and curved rods (Mobiluncus). The fewer Lactobacillus and the more Gardnerella and Mobiluncus visible, the higher the score and the more certain the diagnosis. This scoring captures exactly the bacterial shift that defines BV: protective bacteria disappearing while harmful ones multiply.