Receiving oral sex may trigger the condition bacterial vaginosis because germs in the mouth can disrupt those that occur naturally in the vagina.
A study has found that a bacteria that is commonly found in people’s mouths, and can contribute to gum disease, is linked to the embarrassing infection.
Bacterial vaginosis only causes symptoms in around half of women who get it but those who do show signs complain of a ‘fishy’ smell and unusual discharge.
Scientists say a woman may be more likely to develop the condition, known as BV, if she receives unprotected oral sex.
The mouth bacteria have been found to ‘support’ the growth of vaginal bacteria that cause BV and the two form ‘mutually beneficial relationships’, researchers said.
Not much had been known about how bacteria from other parts of the body contributed to the infection and scientists now say studies should be done to see whether bugs from the penis or anus could trigger it.
Bacterial vaginosis is common and affects around a third of women at some point in their lives, although some may find it comes back again and again. It is not generally considered a sexually transmitted infection (stock image)
Scientists led by the University of California, San Diego, did the research and looked at how the two types of bacteria interacted in mice and swab samples from human vaginas.
The mouth bacteria, called fusobacterium nucleatum, is found in the human mouth and does not usually cause any harm but may contribute to gingivitis, also known as gum disease.
And the vaginal bacteria Gardnerella vaginalis is already known to cause BV when the usual balance of the vagina is disrupted.
When the two bacteria come together they essentially work together to cause the unpleasant infection, the researchers found.
The F. nucleatum was found to increase a specific type of enzyme activity in the vagina – called sialidase activity – which provided fuel for the G. vaginalis to grow.
The researchers said the combination of the two led to a ‘colonization’.
The researchers, led by Dr Amanda Lewis, wrote: ‘These results illustrate that mutually beneficial relationships between vaginal bacteria support pathogen colonization and may help maintain features of dysbiosis [biological imbalance]’.
The study implies the effect is produced by direct contact between the mouth and the vagina, which allows the bacteria to move from one to the other in the saliva.
Dr Lewis – who works in the gynaecology department – and her colleagues said that the finding added another dimension to their understanding of BV.
Previously, they said, it had been thought to be caused by a lack of a healthy bacteria called lactobacilli.
‘These findings challenge the simplistic dogma that the mere absence of “healthy” lactobacilli is the sole mechanism that creates a permissive environment for pathogens during vaginal dysbiosis,’ the paper said.
‘Given the ubiquity of fusobacterium nucleatum in the human mouth, these studies also suggest a possible mechanism underlying links between vaginal dysbiosis and oral sex.’
Even small amounts of the bacteria could start off the ‘colonisation’ process described, the scientists said, suggesting having oral sex just once could be enough.
They added that knowing bacteria in the mouth could trigger BV opened the door to bacteria from other parts of the body also causing it.
Although BV is not known as a sexually transmitted infection, it can be caused by sex.
Dr Lewis’s team added: ‘In light of these findings, future studies should evaluate whether exposure of the vagina to microbes from other body sites (for example, oral, rectal, penile) may [cause the infection].’
There was no suggestion that women giving oral sex to men carried any risk of bacterial vaginosis.
Professor Claudia Estcourt, spokesperson for the British Association for Sexual Health and HIV, told the BBC: ‘We know BV is a really complex entity with lots of contributing factors.’
BV is a very common infection and affects around a third of women at some point – Dr Lewis’s study said some 29 per cent of women in the US get it.
It is usually harmless and can be solved with antibiotics, but in some women it can be a recurring problem and require longer-term treatment.
A small risk of miscarriage or complications can arise as a result of developing BV while pregnant, according to the NHS, but it ’causes no problems in the majority of pregnancies’.
Possible causes of the infection include activities that could introduce bacteria into the vagina that aren’t normally there.
These include sex, having IUD contraception, using perfumed cosmetics in or around the vagina.
The infection can be transmitted during sex between two women if one of them has it.
Dr Lewis’s research was published in the journal PLOS Biology.