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Researchers Propose Forensic Applications for Skin Bacterial Communities

By Andrea Anderson

NEW YORK (GenomeWeb News) – The microbial communities living on human skin differ enough from one individual to the next to potentially serve as a useful forensic tool, according to a paper scheduled to appear online this week in the Proceedings of the National Academy of Sciences.

"We knew from previous work that people have pretty unique skin bacterial communities," lead author Noah Fierer, an ecology and evolutionary biology researcher at the University of Colorado at Boulder, told GenomeWeb Daily News.

Based on such findings, he and his colleagues suspected it might be possible to take advantage of this individuality to link individuals to objects they had handled.

In a series of experiments, the team tested the individuality, persistence, and stability of the bacterial communities left behind by human hands on computer keys and mice. Their findings suggest bacterial communities found on individuals hands correspond well to those found on objects they have touched, leading the researchers to propose that these bacterial fingerprints may eventually complement other forensic methods.

Past studies have identified overlapping core bacterial taxa on skin samples from different individuals, Fierer noted. But, as he and his colleagues reported in PNAS in 2008, just 13 percent or so of the bacterial phylotypes found on the palm of one individual's hand overlap with those on another person's palm.

"Recent work has demonstrated that our skin-associated bacterial communities are surprisingly diverse, with a high degree of inter-individual variability in the composition of bacterial communities at a particular skin location," the researchers wrote. "Given that individuals appear to harbor personally unique, temporally stable, and transferable skin-associated bacterial communities, we hypothesized that we could use these bacteria as 'fingerprints' for forensic identification."

For their first set of experiments, the team swabbed between 25 and 30 keys from each of three personal computer keyboards that had not been touched for an hour or two. The keyboards belonged to three healthy 20- to 35-year old individuals, whose fingertips were also swabbed. The researchers then sequenced 16S DNA from keyboard and fingertip bacterial communities using the Roche 454 Genome Sequencer FLX.

They found that bacterial communities on each individual's keyboard corresponded well to communities found on his or her fingertips.

These communities were also more similar to one another than they were to the other individuals tested or to communities found on keys from other personal and public computers on the university campus.

"Together these results demonstrate that bacterial DNA can be removed from relatively small surfaces, that the composition of the keyboard-associated communities are distinct across the three keyboards, and that individuals leave unique bacterial 'fingerprints' on their keyboards," Fierer and his co-workers wrote.

Next, the team took skin swabs from two individuals and either froze these swabs immediately or left them at room temperature for two weeks. Results from that experiment indicated that bacterial communities remain fairly stable over time under typical indoor conditions.

"Storage under typical indoor conditions had little to no influence on bacterial community composition, or the ability to resolve differences between the bacterial communities on the skin of the two individuals, even after two weeks," they noted.

The researchers then tried to determine if they could detect individual samples within a broader set of bacterial community samples. To test this, they sampled nine computer mice belonging to four women and five men between the ages of 20 and 35 years old and compared the bacterial communities on the mice not only to those found on the individuals' palms but also to a database of microbial communities found on the hands of 270 other individuals.

Again, bacterial communities found on the computer equipment more closely resembled communities found on the skin of those using the equipment than bacterial communities from other individuals.

While they concede that more research is needed to assess the bacterial fingerprinting approach more thoroughly, the team says the findings so far are encouraging and indicate that bacterial communities could be useful for linking an individual to an object that he or she has touched.

"Although additional work is needed to further establish the utility of this approach, this series of studies introduces a forensics approach that could eventually be used to independently evaluate results obtained using more traditional forensic practices," they wrote.

As such, Fierer said the current study is just the tip of the iceberg. "It's going to take a lot of time and a lot of research," he said, noting that the researchers are interested in exploring everything from the method's accuracy to its applicability in situations where more than one individual has touched an object.

"This is just sort of proof-of-concept," Fierer explained. "There are obviously lots of things we need to check."

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