Fingertip Bacteria: A Promising Forensic Tool

The genetic makeup of microbes on a person’s skin could provide crime scene evidence.

It’s not just our genomes that make us unique. The genomic profile of bacteria that rub off our fingertips and onto objects we touch–a computer keyboard, for instance–also provides a “fingerprint” that could be used for forensic purposes, according to researchers at the University of Colorado at Boulder.

Noah Fierer, Rob Knight, and colleagues recovered bacteria from keyboards of three individuals and sequenced large numbers of bacterial genomes at once.

The researchers extracted bacterial DNA from numerous samples taken from the three keyboards and sequenced more than 1,400 copies of bacterial ribosomal gene from each sample to identify the individual species of bacteria each sample contained, finding they could match the three individuals with the keyboards they used. They then took swabs from computer mouses of nine different people. When they compared the bacteria found in the samples to a database of microbial communities found on 270 hands of people who had never touched any of the computer mouses, the researchers found that the bacteria on each person’s mouse was more similar to that on their hand than to samples in the database. So far, Fierer notes, the technique is extremely preliminary, but it could one day be as accurate as techniques like DNA or fingerprint analysis, he says.

The idea of using a microbial “signature” to identify individuals is not new, says David Relman, a professor of medicine, and of microbiology and immunology at Stanford University. For decades, researchers have wondered whether it may be possible to identify individuals based on, say, the unique strains of Escherichia coli harbored in their gut. Until recently, though, “all the ideas that were floating around couldn’t really be explored in a really detailed and methodical way,” Relman says.

In the past few years, faster and cheaper sequencing technology has paved the way for extensive studies of the various microbial communities harbored in the human body, and researchers have devised DNA “bar codes”–short strands of characteristic DNA–that allow them to easily identify species of bacteria.

“You could not do this literally three years ago,” says Lance Price, director of the Center for Metagenomics and Human Health at the Translational Genomics Research Institute in Arizona.

A handful of recent studies within the Human Microbiome Project, including one from the same researchers, have shown that the makeup of microbes in the skin of different individuals, and even those found on different parts of the same person’s body, varies consistently. The current study shows that “even the residue of microbiota that are left behind retains the features of individuality,” says Relman. As a forensics technique, he notes, “this is way too early for application, but one day this could become robust.”

The approach, when developed more fully, could potentially provide information where existing forensic techniques fall short, says Martin Blaser, a professor of medicine and microbiology at New York University. “When you just swab the skin, you get at least 100 times more microbial DNA than human DNA,” he says, so less material could give investigators a stronger signal.

Blaser also notes that fingerprints can’t be accurately read if they’re smudged, while a smudged print could still contain enough microbes to analyze. “The microbiome is us–it’s just another form of fingerprint, just like genomic DNA is us,” says Blaser, who wrote an accompanying commentary to the study, both published in The Proceedings of the National Academy of Sciences.

So far, though, a lot of questions remain about how accurate the technique can become. “We did these studies as a proof of concept,” says Fierer. “Now we need to do the hard work.”

For one thing, it is unclear whether an individual’s microbial signature could be retrieved if another person has touched the object being sampled. Another open question is just how stable an individual’s microbiome truly is. Antibiotics, for example, change an individual’s bacterial profile, although nobody knows for how long. A key step to developing the needed level of confidence, says Fierer, will be to expand the database of microbial communities found on individuals’ hands. Being able to compare a profile to a large number of other profiles will provide a baseline for extracting the truly individual elements.

However, says Relman, “the very reason that makes it more complex gives it all kinds of value that DNA will never have.” For example, he says, a person’s microbiota can reveal not just his or her identity–it can also give clues as to what that individual tends to eat, for example, or where he or she works or lives, so researchers could determine how the types of microbes carried on the body are dependent on such factors.

“I think this is the beginning of the process,” Relman says. “But we are going to need a lot more sources of the variation before we can still see the individual shining through.”

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