Sexual assault is a widespread issue, with women facing a higher risk than men. Sexual assault investigations often depend on identifying a perpetrator’s DNA, but this method has limitations. Now, researchers have found that specific bacterial species are transferred between partners during sexual intercourse.

The findings, published in iScience, suggest that microbiota analysis could help sexual assault investigations by detecting microbial markers of contact, even in cases where traditional DNA evidence is lacking.

“This research is based on the forensic concept that every contact leaves a trace,” says study senior author Brendan Chapman at 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.”   

Previous research suggests that the human microbiota could serve as forensic evidence by identifying unique bacterial signatures. While the approach is promising, more work is needed to refine it for forensic use.

To investigate whether microbial signatures are exchanged during sex, Chapman and his team recruited 12 monogamous, heterosexual couples.

Bacterial signature

At the start of the study, each participant collected genital microbial samples using swabs. The researchers then analyzed the samples to identify bacterial strains unique to each person.

Participants abstained from sex for varying periods, ranging from 2 to 14 days, before engaging in intercourse, after which new microbial samples were collected. Analyses revealed that a person’s bacterial signature could be detected in their partner’s microbiota after sex.

Bacterial transfer occurred in all couples, with Lactobacillus species being the most commonly transmitted bacteria from women to men.

Microbial markers

Condom use reduced but did not prevent the transfer of microbiota, the researchers found. “When a condom was used, the majority of transfer occurred from the female to the male,” says study lead author Ruby Dixon. “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.”

Other factors, such as circumcision and pubic hair, did not affect the transfer of bacterial species between partners. However, menstruation altered the makeup of the vaginal microbiota, which could influence forensic analysis.

“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,” Dixon says.