Having too few "friendly" vaginal bacteria can increase the risk of ovarian cancer in a woman, and swabs can be used to spot it, according to the researchers.
The team, led by University College London, hopes this discovery can be used to identify women at high risk of cancer, who do not have a screening test.
However, they say that more work is needed to explore this.
It is too early to recommend that women take protective doses of the good bacteria, they say.
The work, which is published in the Lancet Oncology, was funded by the government tax stamps, as well as grants from the EU and the charity Eve Appeal.
About ovarian cancer
Every year, in the UK, ovarian cancer is diagnosed in more than 7,300 women.
Early diagnosis improves the chances of successful treatment, but symptoms – bloating and discomfort – can be confused with more common and less serious conditions, such as menstrual cramps or irritable bowel syndrome.
Many women are not diagnosed until the cancer begins to spread.
If your doctor thinks your symptoms might be due to ovarian cancer, he will recommend blood tests and tests.
The exact cause of ovarian cancer is unknown, but some factors increase a woman's risk: age, family history of ovarian or breast cancer, and overweight .
Researchers now believe that microorganisms living in our bodies can also play a role.
More and more scientific evidence is showing that the community of bacteria and other microbes that reside in us – our microbiome – influences our well-being and our health.
A beneficial bacterial species considered particularly important in the vagina is called Lactobacillus.
Experts believe that this prevents other unnecessary or bad microbes from settling and causing damage.
The study involved 176 women with ovarian cancer, 109 on genes at high risk of inherited ovarian cancer (BRCA1 genes) and 295 women without known genetic risk .
Women were examined and samples taken using the same collection method used for screening for cervical cancer.
Lactobacilli levels were significantly lower in women under 50 years old with ovarian cancer or high risk cancer genes.
What do the results mean?
It is not clear if this link is causal or if other factors could explain it, or what impact it has on risk.
Helen Callard of Cancer Research UK said: "The microbiome is a really interesting area of research and we are gathering evidence of how our natural bacteria could affect our health, but when you interpret such research, the association does not mean causality.
"Several factors can affect the risk of ovarian cancer, and different factors can affect the composition of vaginal bacteria – and it's not always easy to separate these elements, so we need to know how Vaginal bacteria could directly affect the risk of developing ovarian cancer, or if it's a totally different factor. "
Alexandra Holden, of Target Ovarian Cancer, said, "Before women worry about the bacteria in their vaginas, further research is needed to better understand how the vaginal microbiome can contribute to ovarian cancer and find Better ways to detect the disease In the meantime, it is essential that women are aware of the symptoms and go to the general practitioner without worrying. "
Investigators believe that good insects provide a protective barrier against other infections, preventing them from moving up the gynecological tract to the fallopian tubes and ovaries.
Professor Martin Widschwendter, researcher, said: "We still do not know for sure if low levels of beneficial bacteria lead to an increased risk of ovarian cancer, but that is what we suspect.
"This is consistent with other research.It has been shown that women who use too much vaginal hygiene products have lower levels of this bacteria and that they are at an increased risk of developing cancer." l & # 39; ovary. "