Couples Exchange Bacteria During Intercourse, Known as the ‘Sexome.’ It Might Help Track Down Sexual Assault Perpetrators
In a study of 12 heterosexual couples, researchers noted the presence of a partner’s genital bacteria after sex, even when the pair used a condom
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Researchers in Australia have found that heterosexual couples exchange genital bacteria during sex, according to a study published last week in the journal iScience. These bacteria could potentially be traced back to an individual’s sexual microbiome, dubbed the “sexome,” which may hold important implications for forensic investigations involving sexual assault.
“In forensic science, we work on the concept that all contact leaves a trace behind. With fingerprints, that’s obviously the residues from our skin, and in the case of the sexome, we’re using the healthy bacterial communities living on, and within, our bodies as a means of detecting that transfer,” study co-author Brendan Chapman, a forensic scientist at Murdoch University in Australia, tells Gizmodo’s Ed Cara.
The researchers collected and genetically sequenced baseline samples of genital microbiomes from 12 monogamous heterosexual couples. They then asked the couples to abstain from sexual intercourse for a period of time lasting between 2 and 14 days. Once the period was over, the couples could have intercourse, after which the team collected and sequenced a new batch of genital microbiome samples.
“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,” Chapman explains to Live Science’s Emily Cooke. “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.”
Though there might be some homogenization of the sexomes of long-term monogamous couples over time, Chapman tells New Scientist’s James Woodford that the microorganisms inhabiting male and female genitals are markedly different.
“The great benefit we have with the penile and vaginal microbiomes is that because of the vast difference in the two environments, we observe very different bacteria types on each,” he says to New Scientist. “It’s a bit like comparing land and sea animals—there are some that exclusively live in one or the other location and would die if removed, but also some that happily move between and persist.”
Standard investigations into cases of male-on-female sexual assault normally include a sexual assault forensic exam, also referred to as a “rape kit,” to search for evidence of intercourse in the woman’s genitals, such as semen. As such, the exam is mostly useless in cases of sexual assault where the man does not ejaculate or uses a condom.
In the recent study, three of the couples reported using condoms during intercourse, and their sexome analysis still revealed that the microbial transfer was not completely blocked.
“When a condom was used, the majority of transfer occurred from the female to the male,” forensic scientist Ruby Dixon of Murdoch University, who was also a co-author of the study, says in a statement. “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.”
Researchers also took into account the impact of circumcision and pubic hair, but did not find these features to impact bacterial exchange. They did note a change, however, in the vaginal sexome during menstruation, which could impact the analysis.
For the individuals involved in the study, each of their sexual microbiomes was unique. With more research, the team hopes to determine whether this uniqueness will hold up to the high standards of forensics. “We’ve only scratched the surface in demonstrating this as a technique for use in real cases,” Chapman says to Live Science. “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.”
However, the research so far provides hope that “bacterial genetic profiles may one day complement DNA evidence or may even help point to a perpetrator of a sexual assault in the rare cases where DNA profiles are not available,” says Dennis McNevin, a forensic scientist at the University of Technology Sydney who did not participate in the study, to New Scientist.
