Hand Bacteria May One Day Aid Forensic Identification
Next time you leave your computer station or close the lid of your laptop think about this: your mouse and keyboard are covered in hand bacteria that could be traced back to you, according to a new US study that suggests the unique bacterial communities we leave behind on objects we have handled may one day sit alongside DNA and fingerprints as part of the forensic toolkit for identifying individuals.
You can read about the study conducted by researchers at the University of Colorado at Boulder (CU-Boulder) and the Howard Hughes Medical Institute (HHMI) in Chevy Chase, Maryland, in the 15 March online before print issue of PNAS, Proceedings of the National Academy of Sciences.
Lead author Professor Noah Fierer, who is an assistant professor in the ecology and evolutionary biology department at CU-Boulder, told the press that we each leave a “unique trail of bugs behind as we travel through our daily lives”.
“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 added.
Fierer and colleagues showed that individual computer users leave DNA signatures of bacteria on mice and keyboards that more closely match the colonies that inhabit the fingers and hands of those individuals than other randomly selected people.
While it is still too early to tell how useful such a technique might be, it could one day be used as an independent way of confirming the accuracy of DNA and fingerprint tests, said Fierer.
Recent studies have revealed that the diversity of bacteria living on our skin is much higher than previously thought, and it varies widely from individual to individual.
For instance, in an earlier study Fierer and colleagues found that a typical hand carries about 150 species of bacteria, and only about 13 per cent of them are shared between any two people.
“The obvious question then was whether we could identify objects that have been touched by particular individuals,” said Fierer.
For this study, they did a test where they swabbed bacterial DNA from keyboards on three personal computers and matched them to the bacteria on the fingertips of their owners. The bacteria colonies on those keyboard were much more like the ones on the owners than bacteria on other keyboards they had not touched or bacteria taken from randomly selected people.
In a second test, Fierer and colleagues swabbed nine computer mice that had not been touched for more than 12 hours and also collected palm bacteria from their owners. They compared these samples with samples taken from 270 palms of randomly selected people who had never touched those mice.
In all nine cases they found that the bacteria on the mice were a much closer match to those on the hands of the owners than those on the hands of the randomly selected people.
With further tests involving private and public computers at CU-Boulder, and taking samples of hand bacteria from campus volunteers, the researchers were able to estimate that the method is between 70 to 90 per cent accurate. Fierer said this will probably improve as the technology develops.
To find out how persistent the bacterial signature might be, the researchers did a further test where they swabbed the skin of two people, froze one set of samples to minus 4 deg F (-20 deg C) and left the other at room temperature. After two weeks there was essentially no difference in the samples, thus demonstrating that:
“…the structure of these [bacterial] communities can be used to differentiate objects handled by different individuals, even if those objects have been left untouched for up to 2 weeks at room temperature,” wrote the researchers.
Fierer said they were really surprised about this:
“We didn’t know just how hearty these creatures were.”
Fierer and colleagues simultaneously analyzed the DNA of bacteria on the fingers, palms and computer objects using a “metagenomic” survey. They isolated and amplified tiny bits of microbial DNA, then reassembled them in a sequencing machine, compared them to DNA databases, and identified families, genera and species of bacteria in each sample.
This high-throughput method is a huge advance in gene sequencing technology. Something that would not have been possible even two years ago, said Fierer.
“Right now we can sequence bacterial DNA from 450 samples at once, and we think the number will be up to 1,000 by next year.”
He said as the cost of such technology drops, smaller labs will be able to afford to do this too.
The technique, if proven, will be invaluable for forensic scientists, especially in cases where there isn’t enough human DNA: it may be easier to recover bacterial DNA than human DNA from touched surfaces.
“Our technique could provide another independent line of evidence,” said Fierer.
However, Fierer said there are also ethical points to consider. For instance, while the law currently restricts the use of human DNA and fingerprints because they are “personally identifying”, there are no such restrictions on bacterial DNA. No doubt a significant part of that debate lies in deciding whether the bacteria that we carry around are part of our identity or not.
Also, more research is needed on how the human-associated bacteria stick to other surfaces like glass, metal and plastic. However, this method could be a useful option where clear fingerprints are not available, such as smudged surfaces, highly textured materials and fabrics, said Fierer.
Another application could be to differentiate between identical twins: they may have the same DNA but the chances are they only share about 13 per cent of finger and palm bacteria.