Bacterial vaginosis (BV) is caused by a shift in the balance of bacteria inside the vagina. Normally, beneficial bacteria called Lactobacillus dominate the vaginal environment and keep it acidic (below pH 4.5). When those protective bacteria decline and get replaced by a mix of other organisms, BV develops. It’s the most common vaginal condition in reproductive-age women, and while researchers still don’t fully understand why the shift happens in some people and not others, several clear triggers and risk factors have been identified.
How the Vaginal Ecosystem Normally Works
A healthy vagina is dominated by Lactobacillus bacteria. These bacteria feed on a sugar called glycogen that vaginal cells naturally produce, and they convert it into lactic acid. That lactic acid lowers the vaginal pH, creating an environment that’s inhospitable to most harmful bacteria. Lactobacillus species also produce hydrogen peroxide and natural antimicrobial compounds called bacteriocins, which directly inhibit the growth of other organisms. Together, these defenses keep the bacterial community stable.
When Lactobacillus populations drop, the vaginal pH rises above 4.5 and the environment becomes far more welcoming to organisms that would otherwise struggle to gain a foothold. That’s the core mechanism behind BV: the loss of the protective acid-producing bacteria and a takeover by a diverse mix of anaerobic organisms.
Which Bacteria Take Over
BV isn’t caused by a single “bad” bacterium. It’s a polymicrobial condition, meaning a whole community of organisms is involved. The most prominent player is Gardnerella vaginalis, which appears to be the initiator. Gardnerella attaches to the vaginal lining and builds a structured biofilm, a thin, sticky layer of bacteria that clings to the tissue surface. That biofilm then acts as a scaffold, allowing other species to join and grow within it.
One important partner is Atopobium vaginae, which rarely shows up in BV without Gardnerella already being present. Lab research has shown that A. vaginae struggles to survive on its own: nearly 92% of its cells lose viability within 48 hours when grown alone. But when Gardnerella is present, A. vaginae thrives, incorporating itself into the existing biofilm and making up as much as 20% of the total biofilm cells. Other anaerobic bacteria, including Prevotella species and genital mycoplasmas, also join this community.
The biofilm is a big reason BV is so persistent. Studies have found biofilm on 90% of vaginal tissue samples from people with BV. Bacteria living inside a biofilm are significantly more tolerant of both hydrogen peroxide and lactic acid compared to free-floating bacteria, which means even if some Lactobacillus return, the biofilm can resist their natural defenses. This also helps explain why BV recurs so frequently after treatment.
Known Risk Factors and Triggers
The CDC identifies several factors that upset the normal balance of vaginal bacteria and increase BV risk:
- New or multiple sexual partners. BV occurs most often in people who are sexually active, and exposure to a new partner is one of the strongest risk factors. Sexual contact can introduce unfamiliar bacteria into the vaginal environment or transfer biofilm-forming organisms between partners. Researchers have found that BV may be sexually transmissible specifically when dense clusters of Gardnerella are present, though BV is not classified as a sexually transmitted infection in the traditional sense.
- Not using condoms. Unprotected intercourse allows direct exchange of bacteria and exposes the vaginal environment to semen, which has a higher pH and can temporarily raise vaginal acidity.
- Douching. Vaginal douching physically washes out Lactobacillus bacteria, even temporarily. Research from Fred Hutch Cancer Center has shown that vinegar-based douches are not a substitute for the lactic acid that Lactobacillus naturally produces. Acetic acid (from vinegar) does not replicate the protective effects of lactic acid, so douching removes the body’s defenses without replacing them.
Other factors that may play a role include smoking, using scented soaps or hygiene products near the vagina, and having an IUD, though these are less well established than the three main risk factors above.
Why Sexual Activity Matters but BV Isn’t an STI
This is one of the most confusing aspects of BV. It clearly occurs more often in sexually active people, and new partners are a well-documented trigger. Yet BV can also develop in people who have never had sex, and it doesn’t behave like a classic STI where a specific pathogen passes from one person to another. The current understanding is that sexual activity disrupts the vaginal ecosystem, making it easier for the bacterial shift to happen, rather than transmitting a single infectious agent. The distinction matters because treating a partner the way you would for chlamydia or gonorrhea hasn’t consistently been shown to prevent recurrence.
Why BV Keeps Coming Back
Recurrence is one of the most frustrating aspects of BV, and the biofilm is a major reason. Even after antibiotic treatment clears symptoms, the Gardnerella-dominated biofilm can persist on vaginal tissue. That residual biofilm resists the protective compounds that returning Lactobacillus produce, giving BV-associated bacteria a head start if conditions shift again. Certain species, like Peptoniphilus, have been isolated from over a third of persistent BV cases, suggesting that specific bacterial communities may be harder to eradicate than others.
The biofilm also sheds. What doctors have traditionally called “clue cells,” the hallmark finding on a BV lab slide, are actually vaginal lining cells coated in biofilm that have peeled off the surface. This shedding can spread the biofilm to new areas of the vaginal lining and may play a role in transmission between partners.
What Happens if BV Goes Untreated
BV sometimes resolves on its own, but leaving it untreated carries real risks. The disrupted vaginal environment makes you more susceptible to sexually transmitted infections, including HIV, chlamydia, and gonorrhea, because the protective acid barrier is weakened. During pregnancy, untreated BV is associated with preterm birth and low birth weight. It can also increase the risk of infection after gynecological surgeries.
Common symptoms include a thin grayish-white discharge, a noticeable fishy odor (especially after sex), and mild irritation, though roughly half of people with BV have no symptoms at all. Diagnosis typically involves checking vaginal pH (above 4.5 suggests BV) and examining a sample under a microscope for clue cells and the absence of Lactobacillus.