Gardnerella vaginalis is a bacterium found in the vaginas of roughly 87% of women, including those with no symptoms or infection. It becomes a problem not because it’s present, but because something shifts the vaginal environment and allows it to multiply unchecked. That shift, from harmless resident to dominant overgrowth, is what triggers bacterial vaginosis (BV), the most common vaginal infection in reproductive-age women.
The Role of Lactobacillus and Vaginal pH
A healthy vaginal environment is kept acidic (below pH 4.5) by Lactobacillus bacteria, which produce lactic acid. Lactic acid is particularly effective at killing Gardnerella at low pH levels, and this acidic environment keeps Gardnerella’s numbers low even though the bacterium is almost always present. When Lactobacillus populations decline for any reason, pH rises above 4.5, and Gardnerella gains a growth advantage it didn’t have before.
The causes of Lactobacillus decline aren’t always clear, but several well-documented triggers exist: douching, which physically washes away protective bacteria and disrupts vaginal chemistry; exposure to new sexual partners, which introduces unfamiliar bacteria; having multiple sex partners; and not using condoms. BV rarely affects people who have never had sex, though it can occur without sexual activity.
How Gardnerella Takes Over
Once conditions shift in its favor, Gardnerella doesn’t just multiply freely in the vaginal fluid. It attaches directly to the vaginal lining and builds a biofilm, a structured community of bacteria encased in a protective matrix of carbohydrates, proteins, and DNA. This biofilm is central to why Gardnerella causes persistent problems. Compared to other anaerobic bacteria, Gardnerella has stronger adhesion to epithelial cells and higher toxicity to those cells.
Gardnerella produces a pore-forming toxin that punches holes in vaginal epithelial cells, damaging the tissue and helping the bacteria anchor themselves. It also produces an enzyme called sialidase that breaks down the protective mucus layer lining the vagina. With the mucus barrier degraded, bacteria can attach to the exposed cell surfaces more easily. When enough bacteria coat a single epithelial cell, the result is what clinicians call a “clue cell,” one of the hallmark signs of BV under a microscope.
The biofilm also contains extracellular DNA that holds the structure together. In lab experiments, applying an enzyme that breaks down DNA reduced bacterial counts within the biofilm roughly fivefold, showing how critical that structural scaffold is to Gardnerella’s survival strategy.
Gardnerella Doesn’t Work Alone
Gardnerella acts as the initial colonizer, building the biofilm scaffold that other bacteria then latch onto. One of the most important partners in this process is Atopobium vaginae, which was found alongside Gardnerella in 99.5% of samples in one study. When both bacteria are embedded in a biofilm together, the odds of having BV are dramatically higher than when either is present alone.
The relationship between Gardnerella and these secondary bacteria goes beyond just sharing space. Gardnerella releases large amounts of pyruvic acid and amino acids as it grows but cannot produce the amines responsible for the fishy odor associated with BV. Other bacteria in the community consume Gardnerella’s metabolic byproducts and convert them into those amines. This symbiotic arrangement means the characteristic smell of BV is actually a product of bacterial teamwork, not Gardnerella acting alone.
Known Risk Factors
The CDC identifies several factors that increase the likelihood of developing BV:
- Douching disrupts the balance of vaginal bacteria and washes out protective Lactobacillus.
- New or multiple sexual partners introduce bacteria that can shift the vaginal community.
- Not using condoms allows greater bacterial exchange during sex.
Researchers still don’t fully understand why some women develop symptomatic overgrowth while others carry the same bacteria without any issues. Genetics, immune response, and the specific strains of Gardnerella involved all play a role. Not all Gardnerella strains are equally aggressive. Some produce more biofilm, more toxin, or more sialidase than others, which likely explains part of the variation in who develops symptoms.
Why It Keeps Coming Back
One of the most frustrating aspects of Gardnerella overgrowth is recurrence. More than 50% of women who are treated for BV experience a return of symptoms within six months. Several factors contribute to this pattern.
The biofilm Gardnerella builds is itself resistant to standard antibiotic treatment. Even when symptoms resolve, remnants of the biofilm can persist on the vaginal lining and serve as a launchpad for regrowth. Certain Gardnerella strains and their partner bacteria, particularly Atopobium vaginae, show intrinsic resistance to metronidazole, the most commonly prescribed antibiotic for BV. Recurrence rates are higher when Atopobium is present alongside Gardnerella.
Reinfection from an untreated sexual partner is another suspected driver. If a partner harbors the same strains, reintroduction during sex can restart the cycle even after successful treatment. The failure to re-establish a healthy Lactobacillus population after antibiotics also leaves the vagina vulnerable. One Lactobacillus species, L. iners, has even been shown to absorb metronidazole in lab settings, reducing the drug’s effectiveness against Gardnerella.
How BV Is Diagnosed
Because Gardnerella is present in most women regardless of infection status, simply detecting the bacterium isn’t enough for diagnosis. Instead, clinicians look for signs that the overall vaginal ecosystem has shifted. The most common approach uses three or four clinical findings: a thin, grayish-white discharge; vaginal pH above 4.5; a fishy odor when a chemical solution is added to a sample; and the presence of clue cells under a microscope. Meeting at least three of these four criteria confirms BV.
A lab-based method scores a stained vaginal smear based on the relative abundance of Lactobacillus, Gardnerella, and curved bacteria. Scores of 0 to 3 are normal, 4 to 6 are intermediate, and 7 or above indicate BV.
Pregnancy Risks
BV during pregnancy carries meaningful risks. Women with BV have roughly a two- to fourfold increased risk of preterm delivery compared to women without it. In one study, the rate of delivery before 34 weeks was 22.7% in women with BV versus 6.2% in those without. A high bacterial load of Gardnerella specifically has been associated with a nearly fourfold increased hazard of preterm birth. These elevated risks persisted even when women received treatment, suggesting that early detection and management matter more than treatment alone in reducing complications.
Treatment Overview
Standard treatment involves either oral antibiotics or vaginal creams or gels, typically taken for five to seven days. Both metronidazole and clindamycin are used, in either oral or vaginal forms. The choice between them depends on individual factors like pregnancy status and side effect tolerance. Treatment clears symptoms in most cases initially, but the high recurrence rate means many women cycle through multiple rounds of therapy. Reducing known risk factors, particularly douching and unprotected sex with new partners, remains the most practical way to lower the chances of both initial infection and recurrence.