Tuesday, November 16, 2010

On the Horizon: What Are Bacterial Fingerprints?

by Andrea Campbell

“Messieurs, c’est les microbes qui auront le dernier mot.”
Translation: “Gentlemen, it is the microbes who will have the last word.” —Louis Pasteur, (see: photo at left)

Microbes Are Us

You know the National Geographic documentaries you’ve seen that show a rhino with birds sitting on its back? According to the African Wildlife Federation, his feathered friends are called "oxpeckers," also known as tick birds. The oxpeckers are ac
tually nibbling on all sorts of bugs. They also feast on the blood from sores and, even though they obstruct sores from healing, they also help to warn the rhino of danger. This is what is referred to as a symbiotic relationship.

Humans also have their own riders. They are organisms called “microbes.” Microbes live on us by the billions. Scientists have examined six different people’s forearms and found more than 240 distinct microbes hanging on. The skin’s ecosystem, for want of a better term, is as unique as a person's wardrobe. While there is some overlap in microbe types between people, in the study, microbe types are still thought of as being distinct and unique to each individual. (As an aside, what is interesting is that a change in soap, laundry detergent, or even the fabric we wear has an affect on the “skin flora,” which is what scientists really call the microbes.)

Skin Flora and Forensic Science

This past spring, there were some reports about how this phenomenon could be used for identification purposes through a process called bacterial fingerprinting. Scientists in Colorado are surmising that one day, bacteria fingerprinting may find itself in the courtroom. Noah Fierer of the University of Colorado, in Boulder, is one of the scientists who studies the bacteria that live on skin, and he says, "We leave this trail of bacteria everywhere we go, and the idea was could we use this trail to identify who had touched a given object or surface.”

Testing It Out

As a test, Fierer and his colleagues swabbed bacterial DNA from the individual keys of three personal computer keyboards, "and then matched those keys to the bacteria on the fingertips of the owners of the keyboard. And we showed that we could basically identify whose keyboard it was pretty well."

According to U.S. News and World Report, the team used powerful gene sequencing techniques to conduct the analysis. 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 Boulder 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.

For the Future

Right now law enforcement relies heavily on fingerprints, blood, saliva or other tissue evidence. DNA is still difficult to locate and often there is not enough sufficient material for identification. And, let’s face it, without standardization between evidence collection and processing, all identification is an uneven process, and prohibitive both money and time-wise. But still, it is an interesting concept since bacterial cells are always being shed. You can wash your hands, but the microbes, but your "native community" will still come back. And this immature study bore results that seem to be about 70- to 90-percent accurate.

Legal Restrictions

Getting this science to pass a legal hurtle, however, is doubtful to arrive soon. “While there are legal restrictions on th e use of DNA and fingerprints, which are ‘personally-identifying,’ there currently are no restrictions on the use of human-associated bacteria to identify individuals,” Fierer said. “This is an issue we think needs to be considered.”

Nathaniel Burney, a white-collar criminal defense lawyer in New York, is not getting excited. He says, "...the media took this modest finding and blew it way out of proportion. The study’s authors insist that the project 'is still in its preliminary stages.' The media make it sound like we’ll be seeing this stuff in court before we know it. The fact is that using microbial DNA to link a suspect to a crime scene is not going to be a reality any time soon, if ever." Burney says the uniqueness of your bacteria being truly singular is very much an open question. And he points out that bacteria is functionally different on different parts of the body, so coming up with isolated bacterium would be a nightmare for comparison.

Still, it’s an interesting idea considering the possibility; because like that rhino, we all have a symbiotic relationship with the bacteria riding on our own skins.

Next in Forensics: Bacterial 'Fingerprints' US News and World Report

For more information: The findings appear in an issue of Proceedings of the National Academy of Sciences

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