What Is the Link Between the Vaginal Microbiome and STIs?

The vaginal microbiome is the complex ecosystem of bacteria, fungi, and other organisms that live in your vagina. Its main role is protecting you from infections like bacterial vaginosis (BV) and yeast infections, but researchers are learning that it does so much more. 

The health of the vaginal microbiome has been linked to fertility issues, pregnancy outcomes, your risk of developing certain cancers, and even your likelihood of getting a sexually transmitted infection (STI). 

While the research on this connection is somewhat mixed (and varies depending on the specific infection), it's clear that the vaginal microbiome plays a significant role. So, what is the link between the vaginal microbiome and STIs? Keep scrolling to learn more. 

Viral Infections

When it comes to STIs caused by viruses — like human immunodeficiency virus (HIV), human papillomavirus (HPV), herpes, and hepatitis B — the vaginal microbiome can impact them in two main ways.

The first way is by altering the mucus that lines the vaginal walls. Vaginal mucus keeps the vaginal tract clean and lubricated and helps trap harmful organisms. Some microbes, like Lactobacillus crispatus produce molecules that make the mucus stickier, offering protection against viruses. However, other vaginal microbes like Lactobacillus iners, Gardnerella vaginalis, and Prevotella amnii can eat their way through the mucus, making it thinner and easier for viruses to penetrate. 

The second way your vaginal microbiome can influence your likelihood of viral STIs is through inflammation. Inflammation has become a bit of a buzzword in the wellness world, but it’s a necessary function. Think of it as your immune system kicking into action. It's like when you get a cut on your skin and the area gets red and inflamed, or when you catch a cold and have a runny nose. Your immune system works hard to fight off infection and begin the healing process. Similarly, when you contract an STI, your vagina can experience the same kind of process. 

Inflammation in the vagina can have both positive and negative effects on the immune response. Microbes in the vagina can actively stimulate inflammation or do the opposite and produce molecules that suppress inflammation. 

Although inflammation plays a vital role in protecting the body against infections, too much inflammation can actually make it easier for viruses like HIV to spread. HIV needs immune cells to replicate, and if there is a higher level of inflammation in the vagina due to a disrupted microbiome, more immune cells are present in that area. Unfortunately, this gives HIV more opportunities to establish itself within those cells and reproduce. 

These two factors are connected and can result in a downward spiral. The mucosal barrier acts as a shield, preventing harmful microbes from interacting with the cells of your vaginal wall and causing inflammation. Unfortunately, when this barrier is weakened, the cells lining your vagina become more susceptible to sensing these microbes, which can trigger inflammation and promote the growth of mucous-eating microbes. 

This is a nightmare chicken-and-the-egg situation for researchers to dissect because it’s not always clear which causes which, but they're working hard to get to the bottom of it. But, to put it simply, having a diverse microbiome that isn't dominated by Lactobacillus and has a higher abundance of mucus-eating microbes can increase your risk of acquiring HPV, HIV, and herpes.

Chlamydia and gonorrhea 

STIs caused by bacteria like chlamydia and gonorrhea also interact with the vaginal microbiome.

Your vaginal health relies heavily on lactobacilli, the “good” bacteria that act as guardians of your vaginal microbiome. They keep your vaginal pH low, block harmful pathogens, and produce natural antibiotics to keep inflammation down. Low levels of lactobacilli can lead to vaginal dysbiosis (the technical term for an imbalanced microbiome), increasing the risk of infections such as BV, yeast infections — and, potentially, some STIs.

Having a microbiome teeming with Lactobacillus bacteria can help protect against infections like chlamydia and gonorrhea, two of the most common STIs worldwide. These good bacteria produce lactic acid, which creates an acidic environment that prevents the growth of harmful bacteria like Chlamydia trachomatis and Neisseria gonorrhoeae, the bacteria that cause chlamydia and gonorrhea respectively. 

On the other hand, some bacteria commonly present in the microbiome, such as Prevotella and Peptoniphilus, can support the growth of C. trachomatis and N. gonorrhoeae by producing a molecule called indole. Luckily, some Lactobacilli bacteria can absorb indole and counteract its effect.

Maintaining a healthy vaginal microbiome is important in preventing these infections, and an easy way to look after your microbiome and reduce your risk of STIs is by using a barrier method like a condom or dental dam when you have sex.

Trichomoniasis 

Trichomoniasis, also known as "trich", is an infection caused by a microscopic parasite known as Trichomonas vaginalis that is transmitted sexually.

Trichomoniasis often appears alongside BV, but we're not sure if BV increases the risk of trich, or vice versa. However, we do know that trichomoniasis can deplete the levels of Lactobacilli. If the levels of Lactobacilli are already low, this can increase the risk of contracting trich later by up to nine times.

Another microbe, called Mycoplasma hominis, is common in women of reproductive age and is considered neutral on its own. However, it has been linked to trichomoniasis and has been shown to help T. vaginalis attach to human cells. 

Additionally, Mycoplasma girerdii also helps T. vaginalis attach to human cells. This means that having both types of Mycoplasma in your vaginal microbiome could increase your susceptibility to trichomoniasis. However, more studies are needed to evaluate the state of the microbiome before infection versus the microbiome changes caused by the infection itself. 

Understanding how the vaginal microbiome and STIs are related is super important for coming up with effective prevention and treatment strategies. It seems that having a healthy vaginal microbiome (in this case, one dominated by Lactobacillus) can lower the chances of getting STIs and improve your overall reproductive health. However, we still need more research to figure out all the details, and whether this link can help create solutions to prevent and treat STIs.