Sex leaves 'microbial traces' on genitalia, even when a condom is used — scientists call it the 'sexome'

Male and female sexual partners leave traces of their "genital microbiomes" during intercourse, even when they use a condom, a new study suggests.

In the study, researchers had 12 monogamous heterosexual couples collect swab samples after a period of abstinence and then shortly after having sex. The subsequent analyses led the scientists to confirm that, at baseline, each of the male and female participants had a unique collection of microbes in their nether regions.

After penetrative sex, though, these distinct microbial communities, or microbiomes, were transferred to their partners in a reciprocal way. These microbiome shifts occurred even in couples that used condoms, and they could be detected using simple lab tests.

The researchers who conducted the study, which was published Wednesday (Feb. 12) in the journal iScience, have dubbed this sexual microbiome the "sexome."

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With further research, the scientists hope that sexome analysis could somehow be used to help pinpoint perpetrators of sexual assault. Particularly, such analyses could be applied to assaults against women, who are significantly more likely to be victims of such attacks than men. In these instances, investigators would typically analyze samples of DNA from sperm found in a woman's genital area. But sometimes no sperm is detected in these swab tests, which can affect the results of the exams.

In their study, Chapman and colleagues asked a dozen heterosexual couples to abstain from having sex for at least two to four days. After this period of abstinence, they had the participants take swabs of their genital regions to be sent to the lab for analysis.

Overall, the female participants had a greater volume of bacteria in their genital microbiomes than the male participants did — equating to an average of 8,038 bacterial genetic sequences in females, compared with 6,661 in males. However, males showed a greater diversity of bacterial species, with approximately twice the number of species represented compared with females.

In the second round of the study, the couples were asked to wait between two and 14 days before having sexual intercourse. Then, within three to 12 hours of doing so, the participants were asked to take a second series of swabs for analysis. This subsequently revealed that a participant's unique genital microbiome could be identified in their partner's swab.

"When we compared the before and after samples we could see bacterial DNA signatures from the female on the male and the male on the female," said study co-author Brendan Chapman, a forensic scientist at Murdoch University in Perth, Australia.

"In forensic science this is what we call a 'trace' or 'transfer' and that's the sort of thing that we ultimately use to show that there was contact," he told Live Science in an email.

What's more, three of the 12 couples reported using a condom during intercourse, which impacted how many, and which, microbes were transferred between partners, but did not completely prevent the formation of the sexome.

Other things that didn't seem to affect microbial transfer included whether males were circumcised or if either partner had pubic hair. However, the researchers did note that the composition of the genital microbiome in females changed at different points in their menstrual cycle, which could affect the results of future swab tests.

More experiments are now needed to replicate these findings in larger groups, given that this initial study included only 24 people, the researchers said. Future studies could also aim to answer questions such as how long the sexome lingers after intercourse.

"We’ve only scratched the surface in demonstrating this as a technique for use in real cases," Chapman said. "We still need more participants to make sure that we can reliably develop a test that's suitable for the robust validation that forensic science requires."