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Could a bacterial ‘fingerprint’ solve a sexual assault case?

(Photo by Alexandra Garcia/The Washington Post)

A new study suggests that the microbes present on pubic hair -- which vary from person to person -- could be used as evidence in sexual assault cases. This particular research is in its early stages, so you probably won't hear about genital microbes in a courtroom anytime soon. But the study is just one example of the effort to turn the incredible diversity of the bacteria that live on human beings into a high-tech forensic toolkit.

Research on the human microbiome -- the bacteria that colonize our bodies and surroundings -- has exploded in recent years. In fact, the microbiome has such a mysterious air, with so many new kinds of bacteria being found in such surprising places, that sometimes the health implications of having one kind of microbiome or another can get pretty overblown. The truth is that we don't know what a human's bacterial population should look like, or even if there's any way all humans should look, bacterially, at all.

But what we do know is that our bacteria are very much a part of ourselves. A recent study  found that humans colonize new living spaces with their own personal bacterial stamp within just a few hours. So if bacteria can be used as a fingerprint, it follows that they can be used to finger perpetrators of crime.

In the new study, published Monday in the journal Investigative Genetics, researchers did some preliminary work to show that these microbial prints might be useful in sexual assault investigations.

The researchers, led by Silvana Tridico from Murdoch University, took scalp and pubic hair samples from seven individuals (three male and four female, with one co-habitating couple in the mix). While hair from the head had around 50 kinds of bacteria a pop, and seemed to be influenced by the environment, pubic hairs had over 70 kinds of bacteria each, which were highly individualized. That's in line with previous studies on the vaginal microbiome, which has shown an unexpected diversity distinguishing one individual from another.

"The advent of DNA profiling has resulted in an increase of sexual offenders using condoms, which they take away, post-assault," Tridico said in a statement. "The implication of this present study is that the transfer of bacteria between victim and offender, in rape cases, may provide a new way of linking the offender to the victim, in instances in which no human DNA is transferred."

But the scientists were most excited about a tiny, very speculative nugget of the study.

At one point in the study, the two people who lived together suddenly had more similarity in their pubic hair bacteria than they had before. It turned out that they'd had sex 18 hours before the sample collection. Ergo, the study authors suggest, a forensic investigator could use microbial shifts to help prove that two people had had intimate contact.

"It's an interesting paper, but of course it's a small study and a very cursory analysis," said Jack Gilbert, an environmental microbiologist at Argonne National Laboratory who wasn't involved in the new study. "It's essentially saying that if two people interact, you can see that exchange."

At such a small scale, Gilbert doesn't think it's safe to tout this as the future of assault investigation -- especially when it would compete with traditional DNA and physical evidence. But maybe one day it could contribute to a case.

But Gilbert is glad to see others working on the forensic applications of bacterial signatures. Gilbert himself is collaborating with police in Hawaii to test the microbial prints left on victims of homicide. Unlike a traditional fingerprint, he said, bacterial signatures slowly decay with time -- so they could help determine who was last with the deceased, but also when their interaction occurred. It would be interesting to explore the use of this bacterial time-stamp for assault victims as well, he said.

"I fully support the idea, but a lot more data is needed to prove its efficacy," Gilbert said of bacterial forensics. "We're probably 5 or 10 years out from using this in even rudimentary investigations."