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Bacteria may help police identify suspects

Police forces may soon be able to add another forensic tool to their armoury following a study showing that it is possible to identify a person from the bacteria they leave behind after touching an inanimate object such as a computer keyboard or telephone.

Fingerprints can provide an exact match between an object and someone who touches it but the new technique exploits the fact that the microbial community living on someone’s hands is sufficiently unique to a person for it to be used as another form of forensic identification.

Scientists said that with further development it might soon be possible for the police to take a swab from a piece of equipment and be able to decide who was using it days or even weeks ago, even if they are unable to recover any fingerprints.

A preliminary trial has shown that the new technique is between 70 and 90 per cent accurate in identifying someone who used a computer mouse from 270 samples of hand swabs collected at random. Scientists said that the accuracy could be improved with further research.

“Each one of us leaves 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,” said Professor Noah Fierer of the University of Colorado at Boulder, who led the study published in the journal Proceedings of the National Academy of Sciences.

Previous work carried out in 2008 established that the combination of bacteria carried around on someone’s hands – typically about 150 species – is pretty unique to them and it not affected by handwashing. It identified more than 4,700 species of different bacterial species in total across the 102 hands but only five species were shared by all 51 participants and only 13 per cent of the bacterial species found on a single hand were shared by any two people.

“The obvious question then was whether we could identify objects that have been touched by particular individuals,” Professor Fierer said.

In the latest study, the scientists swabbed individual keys on the keyboards of three personal computers and were able to match them in terms of their mix of species to the bacteria living on the fingertips of each computer’s owner.

In another test, the scientists were able to match nine computer mice that had not been touch for 12 hours to their owners from a randomly-selected lineup 270 people who had never touched the mice and whose palms had been swabbed and analysed for their bacterial flora.

In all nine cases the bacterial community found on each mouse was much more similar to those found on the owner’s hand, the scientists said. Further tests showed that the bacterial community found on a computer mouse remained essentially unchanged after two weeks kept at room temperature.

“That finding was a real surprise to us. We didn’t know just how hearty these creatures were,” Professor Fierer said.

The technique relies on a revolutionary way of analysing an entire collection of genomes in one go, called “metagenomics”. At present it is possible to sequence bacterial DNA from 450 samples at once but this number is expected to go up to 1,000 by next year, Professor Fierer said.