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National Institute of Allergy and Infectious Diseases
By Mollie Putzig
Life after death may reveal when someone died, and it might even help catch a killer.
Normally, clues like body temperature and insect larvae help forensic scientist determine time of death. But microbes, bacterial organisms found by the trillions throughout our bodies, may offer a more precise measurement–especially for longer time periods.
Some people donate their bodies to this research. Their corpses lie in the tall grass of a pine forest at the Southeast Texas Applied Forensic Science Facility, commonly called a human body farm. Scientists take samples of their skin and the soil beneath them to study how communities of microbes change as bodies decompose.
Jessica Metcalf, an evolutionary biologist working in the Knight Lab at the University of Colorado Boulder, researches the postmortem microbiome, those tiny organisms that decompose us after we die. By tracking these organisms over time, she’s building a microbial clock to calculate the time since something, or someone, died.
On a normal day, Metcalf studies the microbial communities of dead mice. By scrutinizing the organisms living on their skin, she can calculate the rodents’ time of death within a few days.
“With this experiment what we found is that we could actually build a model and predict with really, really high accuracy how long an unknown sample had been dead,” Metcalf said.
Metcalf recently visited the body farm to determine if humans have a microbial clock like she’d seen in the mice. She said the microbes in the skin and soil samples from the body farm look promising.
“One of the things we’re trying to understand is, ‘how much of the decomposer communities associated with the mice are shared with the decomposer communities associated with the humans out in Texas?’” Metcalf said.
She and Sybil Bucheli, a forensic entomologist with Sam Houston State University who works at the body farm, hope their research of living organisms that grow on the dead can someday assist criminal investigations.
Every person has a unique microbiome, a bacterial signature that can’t be forged, according to research from the Argonne National Lab. Everywhere we go, we leave behind some of the living community that makes up our microbiome, like a time-stamped fingerprint.
This could allow crime investigators to identify the last people to contact a body before or after the person died.
Metcalf is conducting a second study of decomposing mice and their microbes. In her first experiment she placed 40 dead mice on boxes of soil. To make sure the only difference between them was the amount of time since death, she used mice that were all males of the same age and breed that were fed the same food.
“We just want to understand if there’s a pattern there before we get to the question of, alright, how do these various variables interrupt it?” Metcalf said.
Once she found evidence of a microbial clock, she repeated the experiment using various soils to see if soil type affected which microbes grew. She found that placing mice on desert, grassland or alpine soil didn’t significantly alter microbe growth.
She also tested the soils 10, 20 and 30 days after removing the mice, to see if postmortem microbes remain in soil over time.
“They did,” Metcalf said. “So that’s a big part of our second paper.”
If distinct human decomposer microbes remain in the soil, then soil samples might even help locate a buried body.
That’s still a long way off, she said, but Metcalf is confident that studying the life that depends on death could lead scientists to a microbial death clock.