The bacterial communities that live on human skin may form a bacterial fingerprint on the items that you touch.

In a new study led by microbiologists Rob Knight and Noah Fierer of the University of Colorado, Boulder, researchers swabbed three different keyboards and nine mice for bacteria, then compared the genomic variation between the communities to deduce whose hands had been touching what. The people were clearly identifiable from the bacterial communities they'd transferred to their computer input devices.

"The results demonstrate that bacterial DNA can be recovered 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," wrote Knight and his colleagues at the University of Colorado, Boulder in a new paper in the Proceedings of the National Academy of Sciences.

The results are the latest to show the variety and complexity of the bacterial communities living in a variety of different human ecosystems like the gut, saliva and skin. The Human Microbiome Project at the Institute for Genome Sciences is out to catalog and understand the relationships between our bacteria and ourselves. Early results suggest "our microbial partners may be essential for our survival as a species."

Microbiome science is just a few years old. It was only in the middle of the decade that sequencing and computational technology became available to do this kind of work. Already, the work is beginning to rewrite what it means to be a human.

"If humans are thought of as a composite of microbial and human cells, the human genetic landscape as an aggregate of the genes in the human genome and the microbiome, and human metabolic features as a blend of human and microbial traits, then the picture that emerges is one of a human ‘supra-organism'," argued a 2007 Nature paper lead-authored by Peter Turnbaugh, a Harvard microbiologist.

Despite the excitement, we're just beginning to understand the bacterial community variations within a single body and between individuals. The new study adds a helpful plank of knowledge in pointing out that human skin microbiomes are diverse enough to allow researchers to distinguish between people. That's a bit surprising as a 2008 article had found "a low level of interpersonal variation" in skin microbiomes relative to studies performed on gut bacteria.

While the most obvious implication of the work might seem to be that the skin microbiome technique could be used in forensic settings, this technique is not coming to a courtroom near you, said Jacques Ravel, a microbiologist at the University of Maryland who works on both the human microbiome and more general forensic science.

"It's a nice piece of work but the forensic aspect as far as I'm concerned is the weakest," Ravel concluded.

The researchers will need a lot more evidence that human microbiomes don't change rapidly in time — and that bacterial communities transferred to keyboards endure with few changes.

Without that proof, Ravel said, the technique is unlikely to be used in real-world forensics, where evidence is often collected long after contact with a keyboard or other surface would have ceased.

Forensic scientists will also just need more data on variations in skin microbiomes to reduce the uncertainty associated with identifications.

"When we do a human genotyping for forensics, we can tell you this is the person and there is one chance in X billion that it is someone else," Ravel said. "Here, they don't have that power. They can't tell you that. The statistics support is still very weak. You can't bring that in the courtroom."

There is one forensic niche, though, where the microbiome could eventually come in handy: identical twins. A 2008 study found that identical twins showed substantial gut bacteria variation. Skin microbiomes could be similar.

"Even identical twins harbor substantially different microbial communities, suggesting that the collective genomes of our microbial symbionts may be more personally identifying than our own human genomes," the Colorado researchers conclude.

Citation: "Forensic identification using skin bacterial communities" in PNAS by Noah Fierer et al.

Image: flickr/6x7

See Also:

WiSci 2.0: Alexis Madrigal's Twitter, Tumblr, and green tech history research site; Wired Science on Twitter and Facebook.