Sexual assault crimes are some of the hardest to prove. They rely heavily on DNA evidence, typically sperm cells recovered from the victim’s body or clothing. But DNA isn’t always present — perpetrators may use condoms, victims may shower, or too much time may pass before forensic sampling. In cases where no DNA is found, convictions can be all but impossible.

It gets even more difficult. Traditional forensic DNA profiling has severe limitations and attempts to isolate DNA are not always successful. This has driven scientists to explore alternative forensic markers — such as bacterial communities.

In a new study, researchers explored the “sexome”: the unique bacterial species that are transferred during intercourse. They looked at how these bacteria could be used to confirm sexual contact, even when human DNA is unavailable.

Biological fingerprints

Whether you like it or not, your body is teeming with bacteria. The composition of the human microbiome is unique in each individual, a sort of “biological” fingerprint we carry along. Researchers have now tried to use a part of this microbiome as a sexual detector.

“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.

The study involved 24 participants, comprising 12 heterosexual couples (12 males and 12 females). Each couple engaged in a controlled sexual encounter as part of the study, with the goal of examining bacterial transfer during intercourse.

Your unique sexome

Before engaging in intercourse, both partners provided microbiome samples from their genital regions. Three couples used condoms while the others did not. After intercourse, participants again provided microbiome samples from the same body sites (penile skin and vagina). This allowed researchers to track microbial transfer between partners.

Researchers found that each person has a distinct bacterial community on their genital skin and in the vaginal environment. During sex, some of these bacteria migrate between partners. In particular, Lactobacillus species — dominant in the vaginal microbiome — were consistently transferred to male partners. Even in cases where a condom was used, bacterial transfer still occurred, though at a lower rate.

“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 study also examined whether circumcision, pubic hair, or oral sex influenced bacterial transfer, but these factors didn’t seem to make much of a difference.

Not ready for court yet

However, while researchers were able to detect microbial transfer during intercourse and identify some unique bacterial signatures that were transferred between partners, but they were not able to definitively identify individuals based on their unique “sexome” to a standard what would be admissible in a court of law.

The persistence of bacterial traces over time, the uniqueness of individual microbiomes, and the influence of external factors like hygiene or environment must be further studied. Additionally, microbial evidence does not yet meet the rigorous standards required for forensic admissibility. Bacterial profiles do not provide a unique identifier like DNA — at least not yet. More research, larger sample sizes, and validation studies in real-world forensic settings are necessary before this method can be reliably used in criminal investigations and legal proceedings. But the method is promising and it could soon help authorities crack some difficult cases.

“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.

Bacterial transfer during sexual intercourse as a tool for forensic detection, iScience (2025). DOI: 10.1016/j.isci.2025.111861. www.cell.com/iscience/fulltext … 2589-0042(25)00121-X