CSI: Bacteria, Thanks to Unique 'Germ Signatures'

(March 15) -- The colonies of bacteria that inhabit your own skin are unique, which means everything you touch carries a telltale trace of the germs that thrive on your fingertips.

That discovery could one day help forensic investigators track down criminals, without relying on fingerprints or sufficient blood, hair or bodily fluid to yield an accurate DNA sample.

Researchers at the University of Colorado, whose findings are published in this week's issue of Proceedings of the National Academy of Sciences, said their testing method uses powerful DNA analysis techniques that were impossible even two years ago.

They used swabs to collect samples from three computer keyboards, then compared the germ colonies found on each swab to those on the fingers of the three computer users.

The team also tested nine computer mice, 12 hours after they had been used, to see if the unique bacterial communities could persist over time.

To determine just how unique the bacterial groupings were, the team cross-tested the samples with those found on dozens of public and private computers, and evaluated them against bacteria found on the hands of 270 other people who had never touched the computer keyboards or the mice.

They concluded that their ability to pinpoint somebody based on a bacterial trail ranged from 70 percent to 90 percent, and expect the degree of certainty to increase with subsequent research.

"We think the technique could eventually become a valuable new item in the toolbox of forensic scientists," the study's lead author, Noah Fierer, said in a statement.

A human hand contains around 150 species of bacteria. Only 13 percent of those species are shared between an individual and a random second person, according to earlier studies by the research team.

Unlike fingerprints, which can easily smudge, the bacterial trails are resilient: The researchers left them exposed for two weeks, subjected them to sunlight, frozen temperatures and humidity, and found that each bacterial "ecosystem" remained largely unaffected. The communities also repopulated within a few hours of hand-washing.

"We didn't know just how hardy these creatures were," Fierer said.

But they need to prove more than just hardiness to cut it in the courtroom, according to Jacques Ravel, a microbiologist at the University of Maryland.

"When we do a human genotyping for forensics, we can tell you this is the person and there is one chance in X billion that it is someone else," Ravel told Wired Science. "Here, they don't have that power. They can't tell you that. The statistics support is still very weak. You can't bring that in the courtroom."

The team's research remains preliminary, but it's part of a larger effort to "map" the entire microbiome: a vast collection of every native bacteria found on, and in, the human body.

That's a lofty goal: In a 2008 study, the University of Colorado team identified more than 4,000 bacterial strains on a mere 102 sample hands.