Microbes might help crime scene investigators pinpoint a person’s time of death.
In a step toward using bacteria and other microbes as forensic evidence, Jessica Metcalf of the University of Colorado Boulder and colleagues tracked how microbial populations changed as mouse carcasses decomposed. By determining the type and abundance of certain bacteria on the body, the researchers could determine when the mice died to within about three days, the team reports September 23 in a paper to be published in eLife. Populations of microscopic worms, called nematodes, also bloomed at predictable times as the mice decayed.
It makes sense to use microbes to investigate when a person or animal died, says Jeffery Tomberlin, a decomposition ecologist at Texas A&M University in College Station. “Microbes play a huge role in how we live.”
“And how we decompose,” chimed in Eric Benbow, a community and disease ecologist at the University of Dayton in Ohio, during a joint Skype interview.
Tomberlin and Benbow have used insects to investigate how bodies break down. With insects and other clues, CSI teams can at best narrow time of death to a several-day window, but the microbes could potentially pinpoint death to within hours, Benbow says.
And microbes have other advantages. “Sometimes insects aren’t around, so you can’t use them,” says forensic scientist and study coauthor David Carter of Chaminade University of Honolulu. “But the microbes are already there.”
For the study, Metcalf, Carter and colleagues created graves for five mice. Each mouse corpse rested in a plastic container on top of soil from a local creek bed. As the bodies broke down over the next 48 days, the researchers swabbed microbes from skin on the mice’s heads and torsos, and took samples of the animals’ intestinal microbes and the soil around the bodies. The team examined DNA from each sample to determine which microbes were present and the abundance of each type.
About six to nine days into decomposition, the number of oxygen-hating microbes in the gut increased, causing the body to bloat. Around day nine, the abdominal cavity ruptured, allowing oxygen-loving bacteria to take over. When the corpses broke open, ammonia-rich fluids spilled into the surrounding dirt, making the soil more alkaline and killing off microbes, such as Acidobacteria, that grow better in more acidic soil. Alphaproteobacteria flourished in the fluid-soaked soil after rupture. The abundance of these bacteria did not change in mock graves with no corpses.
Once the corpses were in an advanced stage of decomposition at about day 20, a type of soil nematode called Oscheius tipulae dominated the soil. The worms eat bacteria, so their numbers increased along with bacterial growth.
To the researchers’ surprise, the dramatic shifts in the microbial makeup of the gut weren’t the best signs of time of death, because each corpse broke open at a different time. Instead, bacteria on the skin of the mice’s heads were the most reliable. In the early stages of decay, the abundance of Pseudomonadaceae bacteria on the head increased, peaking and then declining shortly before the body cavity ruptured. During advanced stages of decomposition, soil microbes such as Sphingobacteriaceae became prominent on both the skin and in the surrounding dirt. That finding could mean skin bacteria, which are found in low numbers to begin with, are easily overwhelmed by more abundant microbes from the surroundings, the researchers say.
Demonstrating the reliability of microbes as death timers is necessary if investigators ever hope to use the techniques in criminal cases, Tomberlin says. That’s something Metcalf and her colleagues did well, he says. “They created the foundation; now it’s time to take it out and implement it.”
The researchers next hope to learn how changes in temperature and humidity influence decay-inducing microbes. The team is also collaborating with other groups to track microbes in the decomposition of human corpses and pig carcasses.
“I’ve been studying DNA from dead things for a while,” says Metcalf. She got interested in microbes because she thought studying ancient poop might give clues to how people lived thousands of years ago. “The problem is, we don’t really understand what happens when things decompose,” she says. That question led to the mouse study, and then to the human and pig studies. “That’s how it works sometimes in science,” Metcalf says. “You start out with one question and then you end up sampling human corpses.