Gardnerella vaginalis is a bacterium that naturally lives in the vagina. It’s present in roughly 87% of women without any infection at all, making it one of the most common residents of healthy vaginal flora. Problems arise not from its mere presence but from its overgrowth, which is the primary driver of bacterial vaginosis (BV), the most common vaginal infection in women of reproductive age.
A Normal Resident That Can Cause Problems
In a healthy vagina, the bacterial community is dominated by Lactobacillus species, which produce lactic acid and keep the environment acidic (typically below pH 4.5). Gardnerella vaginalis coexists alongside these protective bacteria at low levels without causing harm. One study of healthy women found G. vaginalis in nearly 87% of participants who had no signs of BV whatsoever.
The trouble starts when something disrupts this balance. When Lactobacillus populations decline, G. vaginalis can multiply rapidly and become the dominant species. This shift raises vaginal pH above 4.5, creating conditions that favor further overgrowth of other anaerobic bacteria. The result is bacterial vaginosis.
Scientists once thought Gardnerella vaginalis was a single species, but genomic analysis has revealed at least 13 distinct species within the Gardnerella genus. Three of these were formally named in 2019: G. leopoldii, G. piotii, and G. swidsinskii. Some of these species appear more strongly linked to infection than others, which may help explain why some women carry high levels of Gardnerella without ever developing symptoms.
How Gardnerella Triggers Infection
G. vaginalis has several tools that make it effective at taking over. One of the most important is its ability to form biofilms, structured communities of bacteria that attach to the vaginal lining and encase themselves in a protective matrix of carbohydrates, proteins, and genetic material. These biofilms are difficult for the immune system to clear and resistant to antibiotics, which is a major reason BV recurs so frequently.
The bacterium also produces an enzyme called sialidase that breaks down the protective mucus layer lining the vagina. It does this by stripping sugar molecules from the mucus, which thins it out and makes it easier for bacteria to attach directly to vaginal cells. Once attached, G. vaginalis produces a toxin that punches holes in vaginal epithelial cells, damaging tissue and helping the infection establish itself. This toxin specifically targets a receptor found on human cells, which is why it affects vaginal tissue without harming the bacteria themselves.
Recognizing the Symptoms
Many women with Gardnerella overgrowth have no symptoms at all. When symptoms do appear, the hallmark is a thin, grayish-white discharge with a distinct fishy smell. The odor is often most noticeable after sex or during menstruation, because semen and menstrual blood are alkaline, which triggers the release of the odor-causing compounds.
Unlike yeast infections, BV rarely causes significant itching, burning, or visible redness. The discharge tends to be uniform and watery rather than thick or clumpy. This difference matters because the two conditions require completely different treatments, and self-treating for the wrong one can delay relief.
How It’s Diagnosed
Clinicians typically diagnose BV using one of two methods. The first is a bedside evaluation that looks for four signs: thin grayish discharge, vaginal pH above 4.5, a fishy odor when a chemical solution is applied to a sample of the discharge, and the presence of “clue cells” under a microscope. Clue cells are vaginal skin cells so heavily coated with rod-shaped bacteria that their edges appear blurred or stippled. At least three of these four signs need to be present for a diagnosis.
The second method is a lab-based scoring system that examines a stained vaginal smear under a microscope and counts three types of bacteria: Lactobacillus, Gardnerella, and curved gram-negative rods. Each type receives a score, and the totals are added together on a scale of 0 to 10. A score of 0 to 3 is normal, 4 to 6 is intermediate, and 7 or higher confirms BV. This method is considered more objective but takes longer because it requires lab processing.
Treatment and the Recurrence Problem
BV is treated with antibiotics, either taken orally or applied as a vaginal gel or cream. A typical course lasts five to seven days. These treatments are effective at clearing symptoms initially, but recurrence is the defining challenge of this infection. More than 50% of women relapse within three to six months of successful treatment, and up to 66% experience a recurrence within a year.
The biofilms that G. vaginalis forms are a major reason for this pattern. Antibiotics can kill the free-floating bacteria and resolve symptoms, but they often fail to fully penetrate the biofilm. The surviving bacteria within the biofilm then repopulate the vagina after treatment ends.
For women with recurrent BV, the American College of Obstetricians and Gynecologists now recommends considering treatment for male sexual partners as well. Updated guidance from ACOG suggests concurrent partner therapy with a combination of oral and topical antibiotics for male partners when BV keeps coming back. For same-sex partners or first-time BV episodes, the decision is based on shared discussion between the patient and provider. This is a significant shift, as partner treatment was previously not part of standard BV management.
Risks During Pregnancy
Gardnerella overgrowth during pregnancy carries specific risks. Research has found that high levels of G. vaginalis in the vaginal tract are significantly predictive of spontaneous preterm birth. One study found that women with the highest levels of G. vaginalis had over ten times the risk of preterm delivery compared to those with the lowest levels.
The mechanism involves inflammation. When BV-associated bacteria invade the tissue surrounding the uterus and placenta, they release toxins that trigger the immune system to produce inflammatory compounds. These compounds can stimulate premature contractions and weaken the membranes surrounding the fetus, potentially leading to premature rupture of membranes or preterm labor. The resulting inflammation may also interfere with proper placental development early in pregnancy.