People leave more than fingerprints when they touch objects: They also deposit a tell-tale trail of germs that could help investigators solve crimes, according to US researchers.
They were able to map a unique bacterial genetic signature left by nine different people, and say this germy DNA lasted through day-to-day temperature changes, humidity and sunlight.
"Each one of us leaves a unique trail of bugs behind as we travel through our daily lives," says Assistant Professor Noah Fierer, a researcher at the University of Colorado at Boulder who led the study, which appears this week in the Proceedings of the National Academy of Science.
"While this project is still in its preliminary stages, we think the technique could eventually become a valuable new item in the toolbox of forensic scientists," he says.
Researchers have long known that people are colonised with billions of microbes, both inside and on the body. And studies have shown that these colonies are unique to the individual and even their location on the body.
Fierer's team wanted to see how much of a trail might be left by these mostly benign bacteria. So they swabbed the computer keyboards of volunteers to show that each person left not only a trail of unique bacteria, but one that lasted.
In each case, they could show the DNA from the keyboards and computer mice more closely matched DNA from germs on the hands of the owners than they did anybody else's hands.
According to Fierer's team, the technique was about 70% to 90% accurate.
The University of Colorado team had previously found that a typical person carries about 150 bacterial species on the hands, and that any two given people only share about 13% of these different species.
"The obvious question then was whether we could identify objects that have been touched by particular individuals," says Fierer.
They also left the bacteria out in the open for two weeks to see if they would break down. "That finding was a real surprise to us," said Fierer. "We didn't know just how hardy these creatures were."
The researchers were able to do the study because of rapid advances in techniques and equipment for sequencing DNA. A larger project is under way to sequence all the DNA in the human 'microbiome' - the collection of bugs that live on the skin, in the nose, hair, ears and digestive tract.
These organisms help digest and metabolise food and may affect skin conditions.
And unlike non-native disease-causing germs, they are not dislodged by standard hygiene.
According to the researchers, "Palm surface bacterial communities recover within hours after hand washing."