How Human Bacteria Affects Forensic Science

Even when it’s 3 a.m. in Las Vegas, the millions of lights and constant traffic of the city make it seem like rush hour. Chief Crime Scene Investigator, Catherine Willows and her second-in-command, Nick Stokes, have just arrived at 27th floor of the opulent Mirage Hotel. Las Vegas police detective Jim Brass is waiting for them and his expression is decidedly dour.

The hotel suite is a horrific crime scene. Supine on the Italian marble bathroom floor lies a 40-something, Caucasian male. He’s lying in a pool of slowly drying blood. His carotid artery is severed. Room service has made a stop at this deadly room and the dinner for two, complete with a chilled bottle of Dom Perignon, will never be eaten.

CSI Willows looks around this scene of mayhem and sighs. “OK, Jim what do you think happened?”

Detective Brass cocks his head as if he hasn’t heard this same question a hundred times. “Well, as best we can tell Catherine this upstanding citizen was about to enjoy a $500 late night snack when someone – maybe his companion or maybe an intruder – decided to call it a night for him. You won’t find any fingerprints, your boy Greg Sanders has already been here and looked for hours. The only thing the killer left behind was a thick, cloth napkin from the hotel dining service. Greg found it by the door, obviously tossed as the killer left the scene.”

Nick Stokes states the obvious, “So, the only forensic evidence that we have is that napkin that the killer handled.”

Catherine just shakes her head and sums up the situation, “Well, there better be something on that napkin… besides crumbs.”

At this point, the opening bars of the song “Who Are You” begins and “CSI Las Vegas” begins another episode. In this fictional account of crime scene investigation, the characters would, no doubt, miraculously obtain the minute particle of DNA from some tiny crevice in the room, trace it to a distant relative of the deceased and with cunning and scientific sleuthing, determine the ultimate perpetrator. That’s how science on television works.

A New Source of Forensic Evidence

Television crime drama is loosely based on scientific fact and there is a good possibility that an upcoming of “CSI” will feature new a new forensic tool that is evaluates bacteria that is found on the human hand. This means that CSI Willows will be able to find evidence on that cloth napkin and ultimately determine the killer’s identity. And she will of course be able to accomplish this feast within a one-hour show.

Research published in March 2010, in the “Proceedings of National Academy of Science” strongly suggests that colonies of bacteria that live on people’s hands are highly personal to each individual. This means that forensic experts could someday use the DNA of those bacteria to prove who touched objects like highly textured materials or fabrics such as cloth napkins!

The lead author of the research, Noah Fierer of the University of Colorado, noted that every human leaves a trail of “bugs” behind as he travels through daily life. In one test, the researchers swabbed computer mice that had not been used in 12 hours and using powerful genetic sequencing compared the bacterial DNA with that collected from the hand of the computer owner along with 270 other people. They found that the closest match was to the computer owner.

Overall, the report noted that this new forensic technique was between 70 and 90 percent accurate. They project that as the technique becomes more sophisticated, the accuracy level will also improve.

New Genetic Sequencing Enables the Analysis

The effort involved isolating and amplifying small bits of microbial DNA, then building complimentary DNA strands with a high-power sequencing machine. This allowed the research team to identify different families, genera and species of bacteria from the sample. This sequencing machine and methodology has only recently become available.

Fierer’s research found that on average, the human hand has about 150 species of bacteria with only 13 percent shared by any two people.  In an effort to test the persistence of the bacteria colonies, the group swabbed the surfaces of the hands of two individuals and froze one sample, while leaving the other sample at room temperature. They found that the bacteria which were left at room temperature were essentially unchanged for two weeks.

Because DNA from saliva, semen, blood or tissue is often hard to obtain, this bacteria research suggests the strong possibility of an excellent forensic tool. It also suggests some ethical questions. There are currently legal restrictions on the use of finger prints and DNA that are “personally identifying” to guard a suspect’s privacy. However, there are no such restrictions placed on human-associated bacteria to identify individuals.

Better understanding of genetic sequencing has dramatically changed medicine and now it is impacting forensic science. As to how the fictional CSI experts use this new bacterial analysis in plot lines – Stay tuned.

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