Gardnerella is a type of bacterium that naturally lives in the vagina. In small numbers, it’s completely normal. Problems start when Gardnerella overgrows and crowds out the healthy bacteria that keep the vaginal environment balanced, leading to a condition called bacterial vaginosis (BV). BV is the most common vaginal disorder in women of reproductive age, affecting roughly 27% of women in North America.
How Gardnerella Causes Problems
A healthy vagina is home to large populations of Lactobacillus bacteria, which produce acid and keep the vaginal pH below 4.5. Gardnerella is what scientists call “gram-variable,” meaning it has an unusual cell wall that doesn’t fit neatly into the standard categories of bacteria. It thrives when the acidic environment weakens. Once Gardnerella gains a foothold, it lowers acidity further, creating conditions where other harmful bacteria also multiply.
One of the reasons Gardnerella is so persistent is its ability to form biofilms: thin, sticky layers of bacteria that coat the vaginal lining. These biofilms act like a shield, making it harder for the immune system or antibiotics to fully clear the infection. Other bacteria can also boost Gardnerella’s biofilm production through chemical signaling molecules, which helps explain why BV involves a whole community of microbes rather than a single culprit. Gardnerella is considered the dominant player, but species like Mobiluncus, Mycoplasma hominis, and various anaerobic bacteria are typically involved too.
Symptoms of Gardnerella Overgrowth
Many people with BV have no symptoms at all. When symptoms do appear, the most recognizable one is a thin vaginal discharge that may look gray, white, or greenish. The discharge often has a distinctive “fishy” smell that can become stronger after sex. Some people also notice vaginal itching, though this is less common than the odor and discharge.
BV does not usually cause the burning, redness, or thick cottage-cheese-like discharge associated with yeast infections. If your main symptom is a persistent fishy odor with thin discharge, BV is more likely than a yeast infection.
How BV Is Diagnosed
Diagnosis typically relies on a set of four clinical signs known as the Amsel criteria. A healthcare provider checks for at least three of the following:
- Thin, homogeneous discharge that is white, gray, or yellowish
- Vaginal pH above 4.5, tested with a simple strip of litmus paper
- Clue cells on microscopy, which are vaginal skin cells so heavily coated with bacteria that their edges look blurred or stippled under a microscope
- A positive “whiff test,” where adding a chemical solution to a sample of discharge releases a fishy odor
Clue cells are the hallmark finding. They look distinctly different from normal vaginal cells because bacteria cling to their surface in thick clusters, giving them a grainy, almost peppered appearance. Some clinics also use molecular tests that detect Gardnerella DNA, which can be more sensitive than microscopy alone.
Treatment Options
BV is treated with antibiotics, either taken by mouth or applied inside the vagina. The CDC’s recommended options are oral metronidazole taken twice daily for seven days, metronidazole vaginal gel applied once daily for five days, or clindamycin vaginal cream applied at bedtime for seven days. All three are considered equally effective for an initial episode.
Male sexual partners do not need treatment. Current CDC guidelines are clear on this point: treating a male partner has not been shown to reduce recurrence or improve outcomes.
Why BV Keeps Coming Back
Recurrence is the single most frustrating aspect of BV. Between 50% and 80% of women experience a return of symptoms within a year of completing antibiotic treatment. The biofilms Gardnerella builds are a major reason. Antibiotics can kill the free-floating bacteria and reduce symptoms, but bacteria embedded in biofilms survive at much higher rates. Once antibiotic treatment ends, the surviving bacteria repopulate the vaginal lining.
Factors that increase the risk of recurrence include having a new sexual partner, douching, and not using condoms. Some research suggests that reintroducing Lactobacillus through probiotic supplements or suppositories may help restore the vaginal environment after antibiotics, though evidence is still mixed on how well this works in practice.
Risks During Pregnancy
BV affects an estimated 6 to 16% of pregnant women and carries meaningful risks. Research shows that women with BV have significantly higher rates of preterm birth before 34 weeks: about 23% compared to 6% of women without BV. A high load of Gardnerella bacteria specifically has been linked to a nearly fourfold increase in the risk of delivering early.
Other potential complications include premature rupture of membranes (water breaking too early), infection of the amniotic fluid, and postpartum uterine infection. For the newborn, these complications can lead to respiratory problems and NICU admission. Pregnant women who notice symptoms of BV should bring them up at their next prenatal visit, since treatment during pregnancy can reduce these risks.
What Raises Your Risk
BV is not classified as a sexually transmitted infection, but sexual activity is one of the strongest risk factors. Having multiple partners or a new partner increases the likelihood of disruption to vaginal bacteria. Other risk factors include douching, which washes away protective Lactobacillus, using scented soaps or products inside the vagina, and having an IUD in some cases.
Prevalence varies significantly by population. In young South African women aged 15 to 24, BV prevalence reaches 42%. In North America, it’s closer to 27%. These differences likely reflect a combination of biological, behavioral, and environmental factors rather than any single cause. BV can also occur in people who have never been sexually active, reinforcing that it’s ultimately a shift in microbial balance rather than a traditional infection passed between partners.