After sex, we leave a unique genital microbial 'fingerprint' on our partners

Media release

After sexual intercourse, both partners leave traces of their own unique genital microbiome

Criminal investigations of heterosexual sexual assault often include a DNA analysis of the woman’s genitals with the aim of identifying the presence of the perpetrator’s sperm for proof of intercourse. However, in cases where no sperm is detected, including in assaults where the perpetrator uses a condom, these exams are often ineffective. In research publishing in the Cell Press journal iScience on February 12, 2025, researchers show that bacterial species are transferred between both individuals during sexual intercourse, and these species can be traced to a sexual partner’s unique genital microbiome. The authors say that analyses of these genital microorganisms—which they called the “sexome”—may be useful in identifying perpetrators of sexual assault.

“This research is based on the forensic concept that every contact leaves a trace,” says chief investigator Brendan Chapman of Murdoch University in Australia. “Until now, few studies have explored the vaginal and penile microbiomes within a forensic context. This research demonstrates that we can observe microbial traces from heterosexual couples’ genital microbiomes following sex.”

In this study, the researchers confirmed that both men and women have unique populations of bacteria in their genital areas. They then recruited 12 monogamous, heterosexual couples to investigate whether these sexomes are transferred during sexual intercourse, including when a condom is used. At the beginning of the study, each participant collected samples of their genital microbiome using swabs. The investigators used RNA gene sequencing to determine which bacteria strains were present—down to the sub-species level—and identified microbial signatures for each participant.

Couples were then asked to abstain from sex for varying lengths of time (from 2 to 14 days) and then to participate in intercourse. Afterwards, samples were collected again from each individual’s genital microbiome. Analysis showed that a participant’s unique bacterial signature could be identified in their sexual partner’s sample following intercourse.

Three of the couples reported using a condom. The analysis found that although this did have some impact on the transfer of microbial content, it did not inhibit it entirely. “When a condom was used, the majority of transfer occurred from the female to the male,” says Ruby Dixon of Murdoch University. “This shows promise for a means of testing a perpetrator post-assault and means there may be microbial markers that detect sexual contact even when a condom was used.”

The investigators also looked at whether males were circumcised and whether the participants had pubic hair, but found that neither factor seemed to affect the transfer of bacterial species between partners. However, they did find that the makeup of the vaginal microbiome changed during menstruation, which they note could affect results.

“The application of the sexome in sexual assault casework is still in its infancy,” Dixon says. “It’s important to completely understand the external factors that may have an impact on the microbial diversity of both males and females, and this is something we plan to continue studying.”