Over the last couple of decades it’s become almost routine to identify people with DNA. In the past few years, it’s been shown that bacteria on the skin, or in the gut, can be as individual as genetic code. Now a study from the University of Colorado (Boulder, USA) opens the door to forensic (legal – investigative) identification of individuals through the unique types of bacteria they leave behind on objects like doorknobs, computer keyboards, and kitchen utensils.
Published in the Proceedings of the National Academy of Sciences March 15, 2010, this is a preliminary study – meaning that while first experiments have confirmed some hypotheses and results are promising, there is much more work to be done before the results are considered specific and reliable. So, a caveat about the conclusions – they’re not really conclusions.
The research starts with the recently acquired knowledge that each person has a unique mix of bacteria, particularly on the skin, including palms and fingers. In a real sense, these are ‘communities’ of bacteria that given a person’s body chemistry, metabolism, and life-style adapt to various parts of the body in very specific combinations of bacterial types. Even identical twins, whose DNA is the same, have unique bacterial combinations. As people go about their daily activities, they leave a trail of these bacteria on almost anything they touch, although most clearly on heavily used items such as computer keyboards and mice, certain doorknobs, and some kitchen utensils. Picking up these trails, be they ever so miniscule, and identifying the bacteria is where the Colorado study covers new ground.
One test: The team used recently developed gene-sequencing techniques to match bacterial DNA taken from individual keys on three personal computers, and matched them with DNA from bacteria on the fingertips of the keyboard users. (There was also a control group of swabs from keyboards never touched by the users.) The results matched bacteria from the user’s keyboards much more closely than those from keyboards they did not use. (Note that if humans touch something, there are always a number of bacteria left behind. Of course, we don’t see them.)
Second test: The researchers swabbed nine computer mice that had not been touched in more than 12 hours, and collected palm bacteria from the people who used the mice. These samples (of bacteria DNA) were compared to bacteria from 270 randomly selected palms that had never touched the mice. Again, in all nine cases, the bacterial community on each mouse was much more similar to that of the user’s hand.
At this point, the tests are about 70-90% accurate, not close enough for forensic jazz quite yet; but as they say – promising.
Assistant Professor Noah Fierer, chief author of the study, describes the future:
“This is something we couldn’t have done 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. And as the cost of the technology continues to drop, even smaller labs could undertake these types of projects.”
Another reason the new technique may prove valuable to forensic experts is that unless there is blood, tissue, semen or saliva on an object, it’s often difficult to obtain sufficient human DNA for forensic identification, said Fierer. But given the abundance of bacterial cells on the skin surface, it may be easier to recover bacterial DNA than human DNA from touched surfaces, they said. “Our technique could provide another independent line of evidence.”
More research needs to done on how human bacterial signatures adhere to different surfaces like metal, plastic and glass, said Fierer. But the new technique may be useful for linking objects to users in cases where clear fingerprints cannot be obtained – from smudged surfaces, fabrics and highly textured materials, he said. The new technique would even be useful for identifying objects touched by identical twins, since they share identical DNA but they have different bacterial communities on their hands.
The use of DNA in a legal setting took about three decades to be accepted, and is still under review. Certainly the use of bacterial DNA identification will take at least as long, and may in the end prove to be not reliable enough. Before then, however, science has to refine the techniques and demonstrate what levels of bacteria presence (count) are considered reliable, and prove to a relatively high degree that bacterial communities are indeed unique to individuals.
Outside of the strict requirements of legality, this is another avenue of research that is closing in on ways to prove “you were there.” There are ethical issues, the right to privacy for example, that will surface as this technology matures.