ABOVE: The bacteria that decompose corpses are diverse but shared across bodies in different environments. © iStock, Design Cells
Decaying corpses can be completely unrecognizable with peeling skin, bloated abdomens, seeping liquids, and a foul smell. Microbes are one of the key architects of this process. A body’s organic matter provides a buffet of nutrients for savvy microbes that break down these molecules to transform a corpse. To study these corpse communities, some scientists are repurposing microbiology techniques typically used to profile complex microbial communities in the gut or skin.
Jessica Metcalf, a microbiologist at Colorado State University, has studied the microbial world of decomposing bodies for more than a decade. In a 2013 study, she reported that as mice decomposed in a lab setting, the microbes colonizing their corpses changed, but the changes were consistent across mice.1 “That was our first clue that these predictable changes in microbial communities exist,” she said.
In that experiment, Metcalf and her team tightly controlled the mice and their environments. In another study where the researchers exposed decomposing human bodies to different temperatures and weather conditions, they similarly observed constant sets of microbes.2 But the study was small, and to confirm its findings, Metcalf needed more data.
In a new study published in Nature Microbiology, Metcalf and her team presented some of their most ambitious work so far: tracking the microbes on 36 decomposing human bodies across multiple locations, seasons, and durations.3 They found that a core network of microbes regulates corpse decomposition across dramatically different environments. Metcalf thinks that her work could help forensic scientists predict how much time has passed since a person died.
“From our previous research, we sort of suspected that there were some key microbial decomposers, but the question was, how universal are they?” Metcalf said. “It was cool that there really are these microbes that are, at least at the continental level, universal.”
Studying the microbes on a human body in real world conditions requires a complicated experimental set up. Metcalf partnered with three forensic research centers in Texas, Colorado, and Tennessee, and over the span of two years and all four seasons, her team placed donated cadavers on the ground. The bodies were left to slowly decay as they faced the elements.
“There is a tremendous amount of work that goes into collecting, processing, and analyzing these types of samples, so it's no easy task to accomplish the experimental design that was presented in this paper,” said Jennifer Pechal, an entomologist at Michigan State University who was not involved in the study.
Every day for three weeks, researchers swabbed the bodies and surrounding soil to collect microbes. They sequenced the microbial DNA to identify the species and track their levels over time. Although geography and climate affected the assortment of microbes, certain groups of microbes appeared in bodies everywhere, from the Colorado snow to the searing Texas heat. These bacterial and fungal species specialize in decomposition, and Metcalf quickly realized that they weren’t microbes typically found in the human gut or in soil.4,5
Instead, the identified microbes matched species found in insects such as blowflies, which lay eggs on carcasses. This made sense to Metcalf. “If you're a microbe looking to decay flesh, riding along on insects that lay their eggs on decaying flesh is probably a pretty good plan,” she said.
Pechal is interested to see future work in partnership with medical examiners that explores how the microbes on decomposing bodies compare to those on intact bodies from routine death investigations.
Given that the microbial species also changed over time, they could be good indicators of time since death, a key piece of information for forensic investigators. Using their data, Metcalf’s team built a machine learning model to predict the number of days since death based on the levels of each microbial species.
To make the model a more reliable tool for forensic scientists, Metcalf’s team is currently collecting decomposition microbiome data from additional sites in colder climates.
Metcalf thinks that models like this will be part of the future of forensics.6 “We're getting fairly close to it,” she said. “What we need is for some brave lawyer to bring it to a judge to decide whether the science is robust enough for this to be used as evidence.”
References
1. Metcalf JL, et al. A microbial clock provides an accurate estimate of the postmortem interval in a mouse model system. eLife. 2013;2:e01104.
2. Metcalf JL, et al. Microbial community assembly and metabolic function during mammalian corpse decomposition. Science. 2016;351(6269):158-62.
3. Burcham ZM, et al. A conserved interdomain microbial network underpins cadaver decomposition despite environmental variables. Nat Microbiol. 2024;9(3):595-613.
4. McDonald D, et al. American gut: an open platform for citizen science microbiome research. mSystems. 2018;3(3):e00031-18.
5. Thompson LR, et al. A communal catalogue reveals Earth's multiscale microbial diversity. Nature. 2017;551(7681):457-463.
6. Metcalf JL. Estimating the postmortem interval using microbes: Knowledge gaps and a path to technology adoption. Forensic Sci Int Genet. 2019;38:211-218.