Partners transfer microbial 'sexome' during intercourse

Researchers in Australia have found that sexual partners transfer components of their distinctive genital microbiomes to each other during intercourse. This sexome - as it has been dubbed - is unique, and it’s thought that it might prove pivotal in sexual assault investigations in future. Ruby Dixon and Brendan Chapman are researchers at Murdoch University, in Perth…

Brendan - We have had challenges for a long time in forensic science with our ability to identify perpetrators of sexual assault. What we end up with are usually samples from a sexual assault victim, which in most cases is a female victim, and we are looking for cellular material or biological material that's been deposited by the male offender in most cases. We're largely looking for sperm cells. They can be quite difficult to isolate. We find that we have the most opportunity as soon as possible following the event.

Chris - So, what do you think it might be possible to do about this then?

Brendan - So, given that window, any opportunity for finding biological material - or a trace beyond that window - is incredibly important.  So, hence the research that we are looking at. The idea came more so from the ability to identify something that wasn't human material. And that's where we came to approach it from the perspective of a transfer of bacterial material or the healthy bacteria that lives on us.

Chris - Ruby, tell me more. How does this work?

Ruby - So, in this project, we kind of worked off the principle, which is quite well known in forensics, that every contact leaves a trace. And in this circumstance, we looked at the bacteria that naturally exists on the genital region of both males and females. And we wanted to see whether there was any bacteria that was unique to a person, and if that potentially unique bacteria could transfer during sexual contact.

Chris - Well, they do say that your microbiome is more unique to you than your fingerprint even. I mean, I think that's a slight exaggeration there. But that's sort of what you're saying then, isn't it? That the microbial makeup on one party will be conveyed to the other party during any kind of sexual contact, and it might be possible to pick vestiges of that up.

Ruby - Yeah, that's exactly right.

Chris - So how did you explore it? What did you actually do to test whether that might be the case?

Ruby - What we did was we recruited volunteers who were in monogamous, long-term relationships to collect penile and vaginal swabs both before and after intercourse. We then took those swabs, extracted the bacterial DNA from them, and run a bunch of sequencing to show us what kind of bacteria was present in each sample. And then we wanted to compare them between the individual themselves. So, whether we looked at the same individual both before and after intercourse to see how their bacterial makeup had changed.
And then we also compared them between couples and then across all of the volunteers in our cohort.

Chris - What did you find, Brendan? Did this lead to a shift in the genetic fingerprint of the microbiome after there had been sexual contact, indicating that there had been things conveyed from one to the other that you could detect?

Brendan - One of the first things that we really noticed was that there was a disruption to the microbiome or a change. So, then what we had to do is really kind of hone in on that and look at what that change was. And that's where, I suppose to use the term fingerprint, we were really looking for those unique signatures, those genetic traces that were unique to one individual that we saw before intercourse in that individual, and then to see the same trace in their partner after intercourse. And that was really, I suppose, the eureka moment where we were able to observe that there was this very, very specific signature that was transmitted through sex that we could detect.

Chris - Was it on both parties, Ruby? Did you get the same sort of exchange both ways? And for how long could you pick that up?

Ruby - So, to answer your first question, it did go both ways. But we found that there was more transfer that occurred from the female to the male. We also weren't able to determine how long after intercourse that those bacterial signatures stayed present, as we had only taken a single swab.

Chris - Do you think, Brendan, this is relevant to a one-off event, though? Because as you've said, you're talking to monogamous couples. So, won’t there have already potentially been some mixing of the microbiomes, and therefore they might be in a slightly different situation because they'll have selected out or changed the microbiomes between the two parties already because of the influence of each other? And were these two completely unknown formally to each other? And I know that's not exclusively the case and that people are raped and assaulted by people they know. But in cases where you don't know the person, would that not be something interesting to look at to see if the same thing occurs or you get an even stronger signal under those circumstances?

Brendan - You're absolutely correct. And my response has really gone back to the point that we've had to start somewhere - this is the starting point. We suspect that because of the long-term nature that these couples have been together, then there may be some sort of sharing of some bacterial types. But really at the moment, we've kind of scratched the surface of understanding how this works and what happens. And as you would know, Chris, with any research, a lot of the time the results just open up more questions. And those questions are great because they drive the next phase of this research.

Chris - What's the next step, Ruby? Where are you going next?

Ruby - We want to go back and potentially target those bacterial species that we know are common in the female vaginal microbiome. And we want to be able to figure out a methodology that we can use to target those specifically rather than having to sort through an entire puzzle, I guess, of different bacterial species that might not be relevant to sexual assault casework.