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Bacteria transferred between people during sexual intercourse could be used in forensic testing to help identify sexual assault perpetrators, an Australian study suggests.
Genital bacteria, similar to the microorganisms that make up the gut microbiome, vary between individuals. They are transferred to sexual partners during intercourse and leave specific signatures that can subsequently be detected, researchers found.
Study lead supervisor Dr Brendan Chapman, of Murdoch University, said the technique of tracing an individual’s sexual microbiome – or sexome, as the researchers have termed it – could eventually be used in sexual assault cases where no sperm is detected.
In Australia, 97% of sexual assault perpetrators are male, while one in five women over the age of 15 have experienced sexual assault.
“If there’s either no ejaculation, barrier contraceptive or a vasectomised male … that’s where this becomes really important as a potential second approach,” Chapman said.
The researchers homed in on a bacterial gene known as 16S rRNA, not present in humans. Its genetic sequence differs in bacteria found on different people.
Genital swabs from 12 monogamous, heterosexual couples before and after intercourse showed that a person’s bacterial signature could be identified on their partner after sex.
The signature was still transferred even when a condom was used during intercourse, though in these cases most of the transfer was from the female to male partner. Chapman noted, however, unknown factors such as the timing of condom introduction during intimacy.
Pubic hair, oral intercourse, circumcision or lubricant use did not appear to affect the transfer of bacteria. Male personal hygiene following intercourse was a factor that might affect tracing, the study noted.
In one couple, a bacterial signature appeared to persist for five days after it was transferred between partners, Chapman said.
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The researchers suggested the technique could increase the window for testing after a sexual assault “beyond what is currently possible” with traditional DNA analysis, in which the likelihood of sperm detection is highest in the first 24 hours after an assault.
Chapman, however, said the technique was “still some way off being used in the courtroom”, citing the need to finesse the uniqueness of bacterial signatures able to be detected. The research team also hoped to better understand the sexome in the absence of intercourse, as well as how it changed through the female menstrual cycle.
Dennis McNevin, a professor of forensic genetics at the University of Technology Sydney, who was not involved in the study, said bacterial genetic profiling could be used to corroborate or oppose testimony in alleged sexual assault cases where other DNA evidence was lacking or insufficient.
“DNA is always going to be the first port of call,” he said. He described bacterial tracing as a last resort because it “is going to take more time, it is more expensive – it’s a boutique analysis”.
The paper was published in the journal iScience.
