Ready to trade bacteria Deux/Corbis/getty

Having sex with a male partner can disrupt the balance of bacteria in the vagina – which might put women at risk of infections.

The bacterial communities of healthy vaginas tend to be dominated by one type of bacteria. Women with higher levels of other bacteria are more likely to get urinary tract infections, or even give birth prematurely. So far, though, it has been unclear if the bacteria are a cause of these problems or a result of them.

To find out if sexual activities could shape the vaginal microbiome and, potentially, women’s health, Lenka Vodstrcil at Melbourne Sexual Health Centre in Australia and her colleagues tracked the bacteria living in 52 volunteers’ vaginas.


Each woman was asked to swab their vaginas every three months for a year, and to keep a record of any sexual activities. The women were all young students, and 19 had not yet experienced penetrative sex at the start of the study. “It was a very diverse group.”

Sexually transmitted bacteria

The volunteers were also asked to record the type of sex they had over the course of that year and whether they used condoms. Vodstrcil’s team compared this record with the types of bacteria collected on each swab.

The team found that women who had unprotected penile-vaginal sex were much more likely to have a vaginal microbiome dominated by the species Gardnerella vaginalis and Lactobacillus iners. Those that started having penile-vaginal sex for the first time during the study tended to go on to have a vaginal microbiome dominated by these two strains.

This suggests that as well as STIs like chlamydia, non-pathogenic bacteria are sexually transmitted too, says Vodstrcil.

This is probably a bad sign for vaginal health, as both species have been linked to bacterial vaginosis, a poorly understood condition that causes abnormal discharge and bad odour.

It is very likely that sex changes the vaginal microbiome, says Janneke van de Wijgert at the University of Liverpool, UK – although she warns that studies like this can be unreliable, because people often lie about sex. Her own research has also found a link between unprotected sex and imbalanced vaginal bacteria – and this has been linked to an increased risk of contracting an STI or developing pelvic inflammatory disease.

Immune response

But penile-vaginal sex isn’t necessarily always bad for the vagina. Van de Wijgert thinks that a woman’s vaginal microbiome probably adapts to the bacteria present on the penis of a long-term partner. “The risk lies in having sex with a new partner, which is a microbial assault on the vagina,” she says. “The vagina will mount an immune response against the bacteria, causing inflammation.”

Larry Forney at the University of Idaho agrees. “There are about 100 million bacteria per millilitre of vaginal secretion, and 10 million bacteria per millilitre of ejaculate,” he says. Each time bacteria are added to the system, you might expect it to move away from its equilibrium point, he says.

Researchers are trying to develop probiotics that can restore a healthy community of vaginal bacteria. In the meantime, there are ways to protect the vagina’s microbiome. Hormonal contraceptives seem to promote a healthy community of bacteria, says van de Wijgert, and using condoms will protect the vagina not only from STIs, but from other bacteria too.

Male partners can help by maintaining a penile hygiene regimen, van de Wijgert adds. The area under the foreskin is a bacterial haven, she says, and keeping this clean could minimise the spreading of bacteria during sex.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0171856

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