Traces of human DNA are not the only thing left behind after sexual intercourse. A new study has found that bacterial DNA is also transferred between males and females during penetrative vaginal sex, and that these microbial signatures could be invaluable to forensic scientists working to catch sex criminals.

PhD student Ruby Dixon, working with senior lecturer Brendan Chapman at Murdoch University in Perth, Western Australia, discovered that sex organs have their own bacterial colonies – they’ve called it the “sexome”. Their new research suggests that after a sexual assault is committed, traces of the offender’s sexome could be used to identify them, or at least to rule out potential suspects.

“This research shows that we can detect that a heterosexual couple has had intercourse based on the bacteria we find after sex,” Dixon said in a statement sent to IFLScience. “Some ‘male bacteria’ stays on the female and some ‘female bacteria’ stays on the male. The end goal is that we’ll be able to take a swab, analyse the bacteria, and link it back to an individual, or at the very least eliminate suspects.”

Investigating sex crimes is a difficult task, but it can be made all the more so if there are no human DNA traces to be found. This can happen, for example, if semen is not present. "Human male DNA cells aren’t always present following an assault due to non-ejaculation or condom use," Dixon explained to IFLScience. "This is quite a common occurrence and the lack of perpetrator DNA can, but not always, have a negative impact on the police investigation and whether the case results in a conviction". 

"If/when this technique is used in forensic casework, the collection of biological samples following an assault will not change. This is very important because we won’t be introducing further trauma on the [...] survivor post assault," she said.

And learning more about the sexome of the penis is useful in and of itself, as the male genital microbiome has been considerably less well studied to date than its female counterpart.

The team collected penile skin samples and vaginal swabs from six heterosexual couples before and after penetrative sex. The couples were aged between 22 and 30 years, and a variety of different contraception methods were being used.

Bacterial genetic sequencing was performed, and the team was able to demonstrate the transfer of microbial DNA between both partners after intercourse. This was most evident in one couple who were not using a barrier contraceptive method. Bacterial transfer from the female partner to the male partner, predominantly Lactobacillus species, was most significant.

Even if the bacterial signature detected was not completely unique to an individual, Dixon described how this information might still be used by investigators to rule out other suspects.

“It’s like when you buy a packet of M&M’s, you know that there are six colours in every packet, so you might say they’re identical from the outside. However, each packet has a different number of each colour - one might have five blue, while another only three - and it goes the same for each colour. It’s similar when it comes to bacteria, although male bacteria may look relatively similar at a glance, we found the composition of each person’s bacterial makeup is probably different enough to use for identification.”

The proof-of-concept study demonstrates that the genital flora of both participants are disrupted during sexual intercourse, and that it is possible to use forensic samples to detect bacterial taxa that are unique to each individual. This is just the first step, though, and there are lots of other factors that the researchers are keen to explore next.

“We need to look at the natural variation of bacterial communities over time, as a response to the menstrual cycle, hygiene habits, co-inhabitation and probably some factors we don't even know yet," Chapman told IFLScience. “Because sexual intercourse is such a complex interaction, we need to know more about [...] influences of skin and oral microbiomes too.”

“We've just cracked the surface of getting to a technique that can be used in casework but hopefully, from our publication, it spurs on others to take up this research alongside us too,” he said.

The study is published in Forensic Science International.

This article was amended to include statements from Dixon and Chapman about this research.