You've probably heard about the microbiome in your gut.

It's the collection of trillions of tiny microorganisms thought to have a powerful effect on your health and wellbeing.

But did you know that a similarly complex and important ecosystem of bacteria, fungi and viruses exists in the female reproductive tract?

It's known as the vaginal microbiome, and it plays a key role in keeping women healthy.

So how does it work?

A balance of bacteria

Just like the gut, the vagina consists of trillions of microorganisms — mostly bacteria, plus some fungi and viruses. Together, these populations make up the vaginal microbiome.

The primary colonising bacteria of a healthy vagina is called lactobacillus. It helps to keep the environment acidic and discourage other bacteria, yeast, and viruses from thriving.

"Particular types of lactobacilli have functions that keep the environment very acidic, that interact with our host cells and keep the right mucus production going," says Associate Professor Willa Huston from the University of Technology Sydney.

"This is what we would consider to be the healthiest type of microbiota for the vagina."

But the relationship is complicated: not all types of lactobacilli are good, and there is at least one strain that puts women at a higher risk of acquiring a sexually transmitted infection.

On the other hand, women who lack a dominant lactobacillus in their microbiome are also at a higher risk of acquiring STIs and other conditions including bacterial vaginosis.

"It depends on the type of lactobacillus, it might depend on the function of that lactobacillus … and perhaps how it's interplaying with the genome type of the woman," Dr Huston says.

"But everyone is different and there's no one size fits all for the best or healthiest microbiota."

Disruptions to the microbiome

The vaginal environment experiences all kinds of disturbances on a regular basis, which in turn disrupts the microbiome.

"It's a really dynamic space," Dr Huston says.

Sexual activity, lubricants, and semen can all change the composition of the microbiome, as well as hormonal contraceptives, menstruation and antibiotics.

"Antibiotics, just like in gut bacteria, can basically eliminate lactobacillus and give the opportunity to other bacteria that are not very welcome to grow and thrive," Professor Jacques Ravel from the University of Maryland told The Health Report.

Disruption of the microbiome can cause an imbalance of bacteria, which may lead to bacterial vaginosis. It can also trigger the overgrowth of the fungus, Candida.

"If you lower the number of bacteria, you open the ecological space for Candida to grow and thrive, and that's when [thrush] develops."

The impact of diet is less clearly understood, Professor Ravel says, but it can also cause changes.

"It's something that we are still investigating. We don't fully understand the link that exists between the bacteria that lives in our gut and those that live in a vagina, but there is certainly a link between the two," he says.

The ever-changing vaginal microbiome is also largely age-dependent, Dr Huston says.

"As your reproductive status changes and your reproductive physiology changes, there's a lot of evidence that the microflora, as well as other functions, change in the reproductive tract," she says.

Connection to disease and fertility

There is an established link between a woman's vaginal microbiome and her risk of acquiring sexually transmitted infections and other conditions.

"We know that microbiotas that have particular types of lactobacillus, or don't have a good lactobacillus composition, are more at risk of developing STIs, bacterial vaginosis, and pelvic inflammatory disease," Dr Huston says.

There is also evidence to show that once a woman is pregnant, certain pathogens can cause a loss of pregnancy or adverse outcomes for the baby, she says.

"What we don't know yet is how much your vaginal microbiome or your upper reproductive tract microbiota can influence whether or not you get pregnant."

Research into IVF has found the type of microorganisms present in the uterus at the time of embryo transfer interplay into the outcome of pregnancy.

"So we do know from preliminary evidence that the composition of the microflora in your whole reproductive tract is likely to be really important for your ability to have a successful pregnancy, or even to conceive.

"But we don't know enough yet about that … And there's a lot more work to be done."

What researchers do know is that women with pelvic inflammatory disease — which usually occurs when sexually transmitted bacteria spread from the vagina to the uterus, fallopian tubes or ovaries — are more at risk of becoming infertile.

"So [the microbiome] certainly interplays into developing infertility which might be tubal damage, but whether it interplays exactly into fertility outcome at the time is less understood," she says.

"I think that's the next frontier of microbiome research … a better understanding of the microbiome in the upper reproductive tract and how that impacts on pregnancy and fertility."

Avoid douching

The best thing you can do for your vaginal microbiome is to do nothing at all, Dr Huston says.

"You don't need to go and do too much interfering with your vagina. It's a pretty efficient self-cleaning system," she says.

She warns against douching and using other vaginal cleaning products, which tend to upset the balance of bacteria.

"Unless something's wrong and you've got symptoms that are uncomfortable, it's best to leave your vagina microflora to look after itself," she says.

If you do have symptoms, it's worth making an appointment at the GP.

"You know your body the best. If something's not right or it just feels a bit different, don't have any hesitation in going to the doctor. And don't have any hesitation having a frank conversation with your doctor about your vagina," Dr Huston says.