Microbes that lurk in decomposing human corpses could help forensic detectives establish a person's time of death, a new study finds.
The research, published Monday (Feb. 12) in the journal Nature Microbiology, involved placing 36 human cadavers in three locations during spring, summer, fall and winter. The scientists chose locations that were geographically distant from each other — in Tennessee, Texas and Colorado — and that had either a temperate, humid climate or a semiarid climate.
DNA samples were taken from the surface of the cadavers' skin and from the surrounding soil during the first 21 days of decomposition. This is when decomposition rates are generally fast and dynamic, as the tissues quickly begin to break down.
Related: After you die, your microbiome cooperates with soil microbes to 'recycle' your body
In decoding all of the DNA, the scientists revealed that the bodies bore the same profile of microbes regardless of the location, climate or season. Previous studies had identified key members of this profile but were limited to laboratory experiments or to single locations.
"What we ended up finding was a lot of microbes that showed up in every single data set. These were the key decomposers at every environment," Zachary Burcham, first study author and a research assistant professor at the University of Tennessee, told Live Science.
This network of around 20 microbes was made up of a mix of bacteria and fungi that are typically absent from human bodies — that is, until they start decomposing. During the study, the researchers found that these microorganisms appeared in the cadavers like clockwork at specific points during the 21-day decomposition period. This led them to suspect that insects involved in the decomposition cycle, such as blow flies (Calliphora) and carrion beetles, were responsible for introducing these microbes, possibly bringing them over from a previous corpse they visited.
Burcham said they found that, in addition to seeing the same microbes crop up across corpses, they also discovered that the microbes were increasing and decreasing in abundance as time went by, in a wave-like fashion. By tracking these waves of the different microbes, combining all of the data and using a machine learning model, they found that they could calculate how long a body had been decomposing in a particular area.
"We've given the model the microbial abundances over time, the season, the location," Burcham said. "But pretty consistently, it's always been the actual microbial members themselves that the model deems to be the most important. Essentially it will look for these top organisms first and it can get most of the information or predictions from those."
Looking at the microbial community as a whole, the researchers did find variations between the different locations and seasons. However, the microbes that were responding consistently to decomposition always appeared the same, regardless of the external factors. These are the microbes the machine learning model zooms in on, while ignoring the rest.
Related: How long does it take for a body to decompose?
Because their machine learning model could help determine a person's time of death, also referred to as the postmortem interval (PMI), the researchers believe their findings may be useful in forensic investigations across various locations and climates. The results they've obtained so far were found to be highly accurate, give or take three days on each end.
The key members of the newfound microbial decomposer network have been associated with swine, cattle and mouse carcasses in the past, meaning they likely aren't human-specific.
Frederike Quaak, a microbiologist at the Netherlands Forensic Institute (NFI) who was not involved in the study, told Live Science that the findings could become a useful addition to the PMI estimation toolbox. However, she said more research needs to be done for the techniques to actually be used out in the field.
"In real casework, the scenarios would be very different than in their research setting," Quaak said of the new paper. "While they put the bodies on top of the soil, often bodies are being buried, while wrapped in carpets or plastic bags, making them less accessible for insects. Sometimes they are even submerged underwater. So, in these scenarios, the degradation of the body will be very different."
Burcham confirmed that they are now working on new studies investigating the decomposition of organisms within enclosed systems and of buried remains, so they'll be able to see if similar patterns pop up.
"We're working on the basic science, and hopefully someday, we'll see this be useful in a court case," Burcham said.
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