Oral sex can trigger a bacterial vaginosis condition because germs in the mouth can interfere with those that naturally occur in the vagina.
One study found that a common bacterium in the mouth that contributes to gum disease is linked to the embarrassing infection.
Bacterial vaginosis only causes symptoms in about half of the women who get it, but those who show signs complain of a "fishy" odor and unusual discharge.
Scientists say that a woman may be more likely to develop the condition known as BV if she has unprotected oral sex.
The oral bacteria were found to "aid" the growth of vaginal bacteria that cause BV, and the two form "mutually beneficial relationships," researchers said.
Not much was known about how bacteria from other parts of the body contributed to the infection, and scientists now say that studies should be done to see if insects from the penis or anus could trigger them.
Bacterial vaginosis is common, affecting around a third of women at some point in their life, although some may find it recurring. It is generally not considered a sexually transmitted infection (archive image)
Scientists led by the University of California at San Diego conducted the research, examining how the two types of bacteria interact in mice and swabs from human vaginas.
The oral bacterium, called Fusobacterium nucleatum, does not usually cause harm, but it can contribute to gingivitis, also known as gum disease.
And the vaginal bacterium Gardnerella vaginalis is already known to cause BV when the normal balance of the vagina is disturbed.
When the two bacteria come together, they essentially work together to cause the nasty infection, the researchers found.
F. nucleatum was found to increase a certain type of enzyme activity in the vagina – called sialidase activity – which provided the fuel for G. vaginalis to grow.
The researchers said the combination of the two resulted in "colonization".
The researchers, led by Dr. Amanda Lewis wrote, "These results demonstrate that mutually beneficial relationships between vaginal bacteria can aid in the colonization of pathogens and help maintain characteristics of dysbiosis (biological imbalance)."
The study implies that the effect is caused by direct contact between the mouth and vagina, which allows the bacteria in the saliva to move from one to the other.
WHAT IS BACTERIAL VAGINOSIS?
Bacterial vaginosis (BV) is a common cause of unusual vaginal discharge.
At some point in her life, it affects about every third woman.
Although it isn't an STI, it does increase a woman's risk of developing a sexually transmitted infection.
BV is caused by a change in the delicate balance of bacteria in a woman's vagina.
The most common symptom is a discharge that smells like fish, especially after sex.
There may also be a change in the color or texture of the discharge, e.g. B. when it turns gray or watery.
But half of women with BV have no symptoms.
If a woman suspects she may have BV, she should see her family doctor or sexual health clinic to confirm it is not an STI.
Once diagnosed – via a cotton swab – BV is usually treated with prescribed antibiotic tablets or gels or creams.
BV often returns within three months.
Anyone who receives it more than twice in six months must be treated for up to six months.
BV can be prevented by using only water to wash the genital area and using showers over baths.
Perfumed soaps, vaginal deodorants, showers, strong detergents, and even smoking all increase a woman's risk for the disease.
BV is more common in people who are sexually active, have recently changed partners, or have "the reel".
If BV is “caught” during pregnancy, it can lead to premature delivery or miscarriage.
Concerned pregnant women should speak to their GP or midwife.
Source: NHS decisions
Dr. Lewis, who works in the gynecology department, and her colleagues said the finding added another dimension to their understanding of BV.
Previously, they said, it was suspected that there was a lack of a healthy bacterium called lactobacilli.
"These results challenge the simplified dogma that the mere absence of 'healthy' lactobacilli is the only mechanism that creates an acceptable 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 trigger the "colonization" process described, the scientists said, suggesting that oral sex could only be enough once.
They added that knowing that bacteria in the mouth could trigger BV opened the door to bacteria from other parts of the body that were causing it as well.
Although BV is not known as a sexually transmitted infection, sex can cause it.
The team of Dr. Lewis added, "In light of these results, future studies should evaluate whether exposure of the vagina to microbes from other parts of the body (e.g. oral, rectal, penile) can cause the infection."
There was no evidence that women who gave oral sex to men were at risk of bacterial vaginosis.
Professor Claudia Estcourt, UK Association spokesperson for Sexual Health and HIV, told the BBC: "We know that BV is a really complex, multi-faceted entity."
BV is a very common infection, affecting around a third of women at some point. According to Dr. Lewis' study develops it in 29 percent of women in the United States.
It's usually harmless and can be resolved 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 from developing BV during pregnancy, according to the NHS, but it "does not cause problems in most pregnancies."
Possible causes of the infection are activities that allow bacteria to enter the vagina that are not normally present.
These include sex, IUD contraception, and the use of perfumed cosmetics in or around the vagina.
The infection can be transmitted during sex between two women, if either of them has it.
Dr. Lewis' research has been published in the journal PLOS Biology.
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