Hand-borne bugs could offer crime scene clues
March 15, 2010
Joseph Hall
HEALTH REPORTER
Horatio Caine, shake hands with a new clue.
In a study that will surely spawn a story line for the smugly cool CSI Miami character, played by David Caruso, U.S. scientists have discovered that our hands are smeared with signature colonies of germs that are unique to each individual.
When people touch any surface, they can lay down a load of these germs that might be traced back — through DNA analysis of the bugs — to their original owner.
“We’re coated in bacteria, and it’s unavoidable,” says Noah Fierer, a University of Colorado at Boulder bacteriologist and lead study author.
“And potentially, this could be another tool in the (crime scene) tool kit,” Fierer says.
Fierer says there are likely 100 different varieties of bacteria that can live comfortably and benignly on a human hand. Of these species, which are not pathogenic and are often helpful to our health, each individual carries a unique number and concentration.
“There is some overlap, there are certain species...of bacteria that we typically see on lots of individuals,’ says Fierer. “But then there’s also lots of variability both in the types and in the relative abundances of those bacteria.”
Indeed, Fierer says, individuals would be expected to share an average of only 13 per cent of their skin-borne bacteria with any other person, Fierer says.
The study appeared Monday in the journal Proceedings of the National Academy of Sciences.
Fierer says the types and concentrations of bugs we carry appear to remain constant over time, and that they quickly recolonize in those signature configurations after handwashing.
“The communities seem to recolonize pretty quickly after handwashing, within an hour or two,” Fierer says. “And they seem to be pretty similar to what they were prewashing.”
They can also live comfortably on surfaces they’ve been transferred to for about two weeks, giving police ample time to collect them after a crime.
Fierer says high throughput DNA sequencers – advanced by the Human Genome Project – are now able to identify the genetic makeup of a person’s bacterial loads. Those unique germ genetics might one day be matched to samples collected from murder weapons or other crime scene objects.
Fierer says there are many surfaces that do not take real fingerprints well, and that the bacterial DNA variety might prove useful in those situations.
“We may be able to get bacteria because we know that we have thousands and thousands per square centimetre on our skin,” he says.
“There are also places where we can’t get human DNA off of objects and perhaps we may be able to get bacterial DNA.”
Fierer cautions that the study, which matched individuals to the bacteria they left on their computer mouses, merely provides a proof of concept for the method.
“There is still a lot of work we need to do to figure out how robust the method is and what its limitations are,” he says.
“But the techniques are becoming more and more widespread, led by this revolution in (DNA) sequencing.”
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