Next in Forensics: Bacterial 'Fingerprints'

A new discovery may allow scientists to identify bacterial "signatures" left at crime scenes.


By Marlene Cimons, National Science Foundation

While its widespread application in law enforcement is still years away, scientists at the University of Colorado, Boulder have developed a technique that can match the “personal” bacteria on an individual’s hands and fingers with bacteria deposited on computer keyboards and mice.  Once perfected, the process eventually could become another tool in the forensics arsenal, along with fingerprint and DNA analysis.

“It could be used as part of a line of evidence, and there even may be situations where it might work better than standard fingerprinting,” said Noah Fierer, an assistant professor in the department of ecology and evolutionary biology. “There are a lot of circumstances where you can’t get clear fingerprints.”

All individuals carry bacteria on their bodies, the vast majority of it harmless. On average, any two people share only about 13 percent of their bacterial species, which also can vary in quantity, Fierer said.  “We really are pretty distinct.”   Moreover, everyone “leaves a unique trail of bugs behind as we travel through our daily lives,” he said. 

“People are so distinct, and that was the impetus for our study: Can you take advantage of that distinctiveness?” he added.  “The next question is: why are people so different? We don’t know. It could be related to any number of things: our physiology, the environment, diet, or all of the above.” 

The team used powerful gene sequencing techniques to conduct the analyses. The researchers swabbed bacterial DNA from all of the individual keys on three personal computers and also swabbed all ten fingers of the keyboard owners, comparing the results to swabs taken from other keyboards never touched by the subjects. The bacterial DNA from the keys matched much more closely to bacteria of keyboard owners than to bacterial samples taken from random fingertips and from other keyboards. 

The process involved examining a specific bacterial gene from each sample. The gene, called the 16S ribosomal RNA gene, is a useful tool for identifying bacterial species. All cells carry a 16S gene, but the gene changes just enough over time to distinguish one species from another. Each bacterial sample is capable of generating a unique “signature” of all the bacteria that are present. Comparing those signatures, which derived from algorithms developed by Rob Knight, another member of the team, can identify two microbial communities as being closely related.  In this case, the 16S profiles from the fingers of the keyboard users closely matched the 16S profiles from each user’s keyboard.

Put another way, the CU-Boulder team used a “metagenomic” survey to simultaneously analyze all of the bacteria on the fingers, palms and computer equipment, Knight said. The effort involved isolating and amplifying tiny bits of microbial DNA, then building complementary DNA strands with a high-powered sequencing machine that allowed the team to identify different families, genera and species of bacteria from the sample.

“This is something we couldn’t have done even two years ago,” Fierer said. “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.”

In a second test, the researchers swabbed nine keyboard mice that had not been touched in more than 12 hours and collected palm bacteria from the mouse owners. The team compared the similarity between the owner’s palm bacteria and owner’s mouse with 270 randomly selected bacterial samples from palms that had never touched the mouse. In all nine cases, the bacterial community on each mouse was much more similar to the owner’s hand.

Knight said when the team began its study, he expected the computer devices to carry one distinct community of bacteria, while a person’s hands would support a different community. “You’d think the conditions for bacterial survival on a keyboard--a hard, dry surface--would differ enough from the conditions on the skin that the bacteria would differ,” he said.