Bacterial vaginosis (BV) is still somewhat of a mystery. It's a really common vaginal infection and refers to a state of dysbiosis (imbalance) in the vaginal microbiome. While researchers have identified a few main culprits, the exact cause is still poorly understood. Let's dive into what bacteria cause BV (that we know of), and how to treat it. 

What is bacterial vaginosis?

Bacterial vaginosis is the most common vaginal infection in women of reproductive age. It's a condition that occurs when there's an imbalance in the natural bacteria found in the vagina. 

Normally, "good" bacteria, primarily Lactobacilli, outnumber the "bad" bacteria, helping to keep the vaginal environment healthy and slightly acidic. However, bacterial vaginosis can develop when the balance shifts and the "bad" bacteria become more prevalent.

Researchers are still trying to understand what exactly causes the overgrowth of bacteria associated with bacterial vaginosis. But certain things can either increase or decrease your risk of this imbalance in vaginal bacteria. Risk factors for bacterial vaginosis include:

Bacterial vaginosis doesn't always cause noticeable symptoms, but when it does, telltale BV symptoms are a thin, gray vaginal discharge and a strong, fishy vaginal odor.

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What bacteria causes BV?

BV is caused by an overgrowth of certain types of anaerobic bacteria, which thrive in low-oxygen environments. Here’s a (non-exhaustive) list of bacteria that have been linked to bacterial vaginosis:

Gardnerella 

Gardnerella is the most common bacteria linked to bacterial vaginosis. It's present in up to 95% of bacterial vaginosis cases. Gardnerella is commonly found in the normal vaginal flora, and while it usually coexists peacefully with other microbes, it has the potential to disrupt the natural balance in the vagina and contribute to infections like bacterial vaginosis.

There are four species of Gardnerella known to cause BV:  

  • Gardnerella leopoldii
  • Gardnerella swidsinskii
  • Gardnerella piotii
  • Gardnerella vaginalis. 

Gardnerella is known for creating a biofilm (a three-dimensional structure that cells can live in). The interesting thing about bacterial vaginosis is that Gardnerella doesn’t build a structure for itself, but it gets a lot of its friends to live there too. Like A. vaginae, Mobiluncus, Prevotella, and others. This is what is called a “polymicrobial biofilm” and is part of the reason diagnosing and treating bacterial vaginosis is so challenging. 

Atopobium vaginae

Atopobium vaginae is a type of bacteria commonly found in women with bacterial vaginosis. Recently it was renamed to Fannyhesae vaginae (so you might see it called both names in the literature). 

It’s a natural part of the vaginal flora in some women. However, when it is part of a biofilm and becomes more prevalent than the "good" bacteria, it can contribute to the development of bacterial vaginosis.

This bacterium often coexists with Gardnerella vaginalis and is often resistant to certain antibiotics commonly used to treat bacterial vaginosis, such as metronidazole, making it harder to treat and more likely to come back. 

Understanding the role of Atopobium vaginae in bacterial vaginosis helps healthcare providers tailor treatments more effectively, especially in cases where standard treatments have failed.

Mobiluncus 

Mobiluncus species, including Mobiluncus curtisii and Mobiluncus mulieris, play a big role in causing bacterial vaginosis. 

Mobiluncus species are tied to several key aspects of bacterial vaginosis. They make the vaginal environment less acidic and more alkaline, which lets other harmful bacteria thrive. This imbalance leads to the typical BV symptoms. Plus, these bacteria can worsen inflammation and disrupt the vaginal mucosa, making the condition even more challenging.

Prevotella

Prevotella species are really important when it comes to understanding bacterial vaginosis. These bacteria thrive in low-oxygen environments and are actually a normal part of the human microbiome. However, when the balance of bacteria present in your vagina gets thrown off, Prevotella can start to overgrow and cause bacterial vaginosis.

Prevotella species, like Prevotella bivia, produce enzymes and compounds that break down proteins and mucins in the vaginal lining. This not only affects the integrity of the vaginal walls but also creates an environment that helps harmful bacteria grow. When Prevotella species overgrow, it leads to an increase in vaginal pH, making it more alkaline and less welcoming to the beneficial lactobacilli that are normally dominant in a healthy vaginal ecosystem.

Prevotella are also thought to be one of the main contributors to vaginal odor.

Other BV-associated bacteria 

This is just a short list, lots of other bacteria are associated with bacterial vaginosis, including: 

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  • Megasphaera
  • Mycoplasma
  • Finegoldia
  • Fusobacterium
  • Peptoniphilus
  • Porphyromonas
  • Ureaplasma
  • Veillonella
  • Sneathia. 

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There are even certain bacteria that have only been discovered in people suffering from bacterial vaginosis; they earned the name BV-associated bacteria (BVAB). The identities of these bacteria have only recently been discovered (one as recently as 2023). They include Candidatus Lachnocurva vaginae (BVAB-1), Amygdalobacter indicium (BVAB-2), and Mageeibacillus indolicus (BVAB-3).

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Less is known about how these organisms contribute to bacterial vaginosis, but we will keep you updated as we learn more. 

Bacterial vaginosis treatment 

When it comes to treating bacterial vaginosis, antibiotics are usually the go-to. They can be taken orally or applied vaginally in the form of a gel or suppository and help bring back the natural balance of bacteria in the vagina.

Common antibiotics for this include clindamycin and metronidazole, but there are other options like tinidazole or secnidazole. These antibiotics work for about 80% of people with bacterial vaginosis within a month, but the annoying thing is that bacterial vaginosis can come back. Up to half of the people treated will have symptoms return within six months. 

Researchers are still figuring out why bacterial vaginosis tends to come back so often. One possibility is that the antibiotic prescribed might not be effective against the specific bacteria causing the problem.

Common antibiotics like clindamycin and metronidazole are usually the first choices for treating bacterial vaginosis, but each one may work differently on certain bacteria. For example, metronidazole might not be as effective as clindamycin in treating Gardnerella and Atopobium vaginae, while it could be more effective against Prevotella. If the antibiotics don't completely get rid of the BV-associated bacteria, they can grow back after initial treatment. 

Ideally, your healthcare provider should diagnose bacterial vaginosis with a vaginal swab test before prescribing treatment. Swab tests involve taking a sample of vaginal fluid with a swab, which then gets analyzed in a lab and detects which bacteria are causing the infection. Evvy provides an at-home vaginal swab test option (with all testing and follow-up care managed by providers) if you're interested in testing from home.

Your vagina naturally has a balance that keeps everything in check, but sometimes opportunistic bacteria can overgrow and cause issues. Understanding the bacteria that cause bacterial vaginosis and recognizing its symptoms can help you take control of your vaginal health. An Evvy vaginal microbiome test can uncover over 700 bacteria in your microbiome and help you get to the bottom of recurrent symptoms.

While bacterial vaginosis isn’t typically considered a serious condition, it can increase your risk of serious complications. Untreated bacterial vaginosis increases the risk of: 

How to prevent bacterial vaginosis

There are also a few things you can do to reduce your likelihood of developing bacterial vaginosis or preventing bacterial vaginosis from coming back: 

  • Don’t douche
  • Use condoms and dental dams during sex
  • Cover sex toys with a condom during partnered sex, and wash them after each use
  • Quit smoking
  • Change your period products within the proper time frame.

FAQ

What bacteria is responsible for causing BV?

Bacterial vaginosis is caused by an imbalance in the normal bacterial flora of the vagina rather than by a single specific bacterium. In healthy vaginas, Lactobacillus species are predominant and help maintain a low, acidic pH that protects against harmful bacteria. However, in bacterial vaginosis, the levels of Lactobacillus decrease, and an overgrowth of various other anaerobic bacteria occurs, including Gardnerella, Prevotella, Mobiluncus, and Atopobium vaginae. 

Can a man get BV from a woman?

No, men can’t get bacterial vaginosis. However, they can carry bacteria linked to bacterial vaginosis in their penile microbiomes (yes, penises have a microbiome, too!) and pass it on to their female sexual partner during unprotected sex. This is one of the reasons why having unprotected sex can put you at an increased risk of developing BV.

What antibiotics treat BV?

Common antibiotics for this include clindamycin, metronidazole, tinidazole, and secnidazole. How effective an antibiotic will be depends largely on which bacteria are causing the infection, because not all antibiotics are effective against the bacteria linked to bacterial vaginosis.