The microbiome in the vagina has received less attention than its cousin in the gut – but a well-balanced vaginal microbiome can help protect overall health.

Inside the vagina is a delicate ecosystem, made up of thousands of different types of bacteria, fungi and viruses jostling alongside one another, competing for nutrients and space.

Some of the bugs that live in the vagina can play a surprisingly positive role in wider wellbeing – from preventing disease to improving pregnancy outcomes. When bacteria known as Lactobacillus are abundant, the risk of infertility, miscarriage, premature birth and even cervical cancer are all reduced.

Scientists are now racing to develop diagnostic tests and treatments to better manage these vital microorganisms. If realised, this new era of personalised medicine could revolutionise reproductive healthcare.

So how exactly does the vaginal microbiome predict health – and what can be done to protect it?

During childhood, the vaginal microbiome tends to be dominated by bacteria that thrive in low-oxygen conditions, known as anaerobic microbes. However, the rise in oestrogen that occurs during puberty creates the perfect environment for another family of bacteria called Lactobacillus to take up residence.

Lactobacillus appear to be especially beneficial to health. In sufficient numbers, their presence protects against infections by stopping harmful pathogens from getting a foothold in the vagina.

"They crowd out pathogens, competing with them for nutrients and space," says Chrysi Sergaki, head of microbiome at the UK's Medicines and Healthcare products Regulatory Agency (MHRA).

Lactobacillus also produce lactic acid, which makes the vagina a less welcoming habitat for pathogens.

"It makes that environment hostile for harmful microbes, so the microbes don't dare to come there," says Sergaki. "They [Lactobacillus] can also regulate local immune responses to defend against infection, and produce natural antibiotics – so they can basically kill other bacteria that try to come in."

The flip side of the coin is that if the vaginal microbiome becomes disturbed and Lactobacillus is lost – in an imbalance known as vaginal dysbiosis – then opportunistic infections such as bacterial vaginosis (BV), thrush and urinary tract infections (UTI) become more likely.

What's more, if a woman is sexually active, vaginal dysbiosis makes her more at risk of contracting sexually transmitted infections (STIs). For instance women with lower levels of Lactobacillus are more at risk of HIV.

"In Cape Town in South Africa, the rate of HIV is between 20-30%, and yet some sex workers seem to get lucky and avoid infection," says Laura Goodfellow, a clinical lecturer in women's and children's health at the University of Liverpool, UK.

"It turns out that the chance of acquiring HIV as a sex worker in Africa is related to what else is going on with the vagina microbiota, and if you have a lower proportion of Lactobacillus, you have a higher chance of catching HIV."

Women with lower levels of Lactobacillus are also at greater risk of catching human papilloma virus (HPV), the leading cause of cervical, vulval and vaginal cancers.

Women with vaginal dysbiosis take longer to clear HPV infections and have a higher chance of an infection turning into cervical cancer.

"The microbiome is a bit like a forest," says Goodfellow. "With a healthy forest, if you throw some seeds onto the forest floor, they're not going to grow, because there's already a healthy ecosystem living there. But if the microbiome that's there isn't as well set up, then the bad bacteria or viruses can grow more easily."

Intriguingly, the community of bacteria living in a woman's nether........

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