Microbial communities could be new tool for forensic science
Dec. 10, 2015 Jessica Metcalf
There’s a new and versatile tool for forensic scientists to help estimate time of death of humans. In a joint study, researchers from CU-Boulder and UC-San Diego discovered that unique microbial communities present on the human body during decomposition look to be a reliable “clock,” says CU-Boulder’s Jessica Metcalf, coleader of the study.
CUT 1 “When a mammal dies, such as a human, there’s a very predictable sort of march of microbes that occur during decomposition. We call this a ‘colony succession.’ (:12) What is really powerful about this knowledge is that it means we can build a model or make a clock that uses the microbes that are present to help estimate how long a body has been decomposing.” (:25)
Metcalf is a senior research associate with CU-Boulder’s Department of Ecology and
Evolutionary Biology. She says this new method is as accurate as standard tools currently used by forensic scientist to determine the time of death in humans.
CUT 2 “Right now with the experiments that we run what we’re finding is that this method, for example, over 25 days of decomposition, is accurate to within two to four days of death using the skin-soil microbes.” (:17)
And, she says, unlike other methods that rely on insects to help determine time of death, this method is consistent throughout the seasons.
CUT 3 “One thing we found is that our microbial clock was still accurate in the winter where as in the winter many of the insects such as blowflies are absent and so you can’t use that tool. (:10) You know we’re not looking to replace other tools but this is a complimentary tool and it may be the most robust tool in some scenarios.” (:19)
Anther plus, says Metcalf, is that this method could not only be used to estimate time of death in different seasons, but as a way to determine the original location of moved corpses and even help in locating buried corpses.
CUT 4 “What we did is that we looked at the soil microbial communities and what we found is that the decomposition community is very distinct. The change in the microbes in the soil during decomposition is very dramatic. (:15) And so it’s very easy to say this is a soil where something has been decomposing or is decomposing.
And so in cases where a body might be buried or a body might be moved we may be able to use these techniques.” (:30)
In addition to studying humans, the team studied the decomposition of mice on three different soil types: desert, short-grass prairie and high alpine forest.
Surprisingly, the “decomposer” microbial communities under mice were similar in

all three soils, much like the predictable succession in soils beneath the human, says
Metcalf.
CUT 5 “And these are really different soil types with very different soil microbial communities. And what we wanted to know is that even when we put these different communities under decomposing mice do we still get the same microbial decomposer community? And the answer was yes. It was extremely similar regardless of the different soil types. (:19)
Each human harbors up to an estimated 100 trillion microbes – as many as 10 times the number of cells in the body – that undertake functions ranging from food digestion to strengthening of the immune system.
-CU-