Scientists identify a 'global network' of microbes to decompose meat

Establishing an exact time of death (post-mortem interval, or PMI) when a body is discovered is notoriously difficult, although to fictional medical examiners it may seem easy. Some forensic scientists use the life cycle of flies that search for bodies and lay eggs on them. But there is a lot of variation between fly species and seasonal influences, so it would be useful to develop new methods.

It turns out that studying the microbes that thrive in decomposing corpses could provide useful clues. Forensic scientists have now identified about 20 microbes that they believe form a kind of global network that drives the decomposition of dead animal flesh, according to a new study. New paper Published in Nature Microbiology.

“One of the key questions in any death investigation is 'When did this person die?' Nancy Lavin said, director of the National Institute of Justice, which funded the research. “This continuing line of NIJ-funded research shows promising results for predicting the time of death of human remains, assisting in identifying the deceased, identifying potential suspects, and confirming or refuting alibis.”

The work builds on nearly a decade of previous research. For example, in 2015, scientists managed to do this Accurately estimate the time of death of mice and human cadavers within two to four days, even after the bodies have decomposed for weeks. Previous experiments have shown that regardless of the season, surroundings and species of dead, communities of flesh-eating microbes seem to have a predictable schedule when they eat a corpse. As Beth Mol reported for Ars at the time, “Eating times are linked to the stages of decomposition that the body goes through — from fresh meat to swollen carcass, to the tearing and leaking of nitrogen-rich fluids to active decomposition, and then to the eventual state of dehydration.” Each stage attracts “Specific microbes feed on the body, many of which taste good for amino acids.”

But the researchers weren't sure whether timed microbial transmission would look similar in different scenarios, such as corpses in the desert versus the forest, in summer versus winter, or in peacefully rotting corpses versus those recovered by humans. Scavengers. Experimental results from 2015 showed that temperature determines the pace of the microbial eating schedule. But the timeline was remarkably similar between the four human subjects used in the experiment, as well as the mice, even with outdoor exposure. Therefore, the microbial chewing pattern could serve as a global clock to determine the time of death.

This latest paper expands the scope of the investigation to take a closer look at specific types of microbes that tend to thrive in decomposing corpses. “When you talk about investigating death scenes, there are very few physical pieces of evidence that you can guarantee are present at every scene.” said co-author David Carter, a forensic scientist in forensic science at Chaminade University in Honolulu, Hawaii. “You never know if there will be fingerprints, bloodstains or camera footage. But microbes will always be there.” He added that in the case of this latest study “we are talking about outdoor death scenes.” “It can be difficult to collect information in this type of investigation.”

This time, the team conducted outdoor experiments between 2016 and 2017 on 36 human cadavers in three different facilities (Body farms): Colorado Mesa University Forensic Investigation Research Station (FIRS), the Southeast Texas Sam Houston State University Applied Forensic Science Facility (STAFS), and the University of Tennessee Anthropology Research Facility (ARF). Cadavers were studied over a period of 21 days during different seasons (spring, summer, fall, and winter). Samples were taken daily from both grave soil associated with decomposition and skin from the head and torso of the bodies, in addition to samples from control soil. Daily temperature, humidity and other environmental factors were also recorded.

The three facilities are located in two different climate types (temperate forest and semi-arid steppe), yet the team identified the same 20 decomposing microbes in all the cadavers, which again showed up in predictable chewing patterns regardless of external variables. “It's really cool that there are these microbes that always show up to decompose animal remains.” said co-author Jessica Metcalfe From Colorado State University. “We hope we can open up this whole new field of environmental research.”

Zoom in / Agent Booth (David Boreanaz) and Temperance Brennan (Emily Deschanel) visit a body farm in a 2011 episode of Bones.

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These specific microbes are not found in databases of microbes commonly found in soil, human skin, and gut microbes, so how do they find their way into delicious decaying meat? The authors suggest that insects likely play a key role, since these global decomposer microbes are commonly found in insects such as carrion beetles and blow flies.

The authors also used their new data combined with machine learning to develop a predictive model for time of death based on microbial activity. The model performed remarkably well, predicting time of death within three calendar days in independent tests. The team attributes the remaining errors to intrinsic factors such as BMI/total body mass and extrinsic factors such as scavengers and rainfall. These issues will be studied in future research to further improve predictive models.

Natural Microbiology, 2024. DOI: 10.1038/s41564-023-01580-y (About digital IDs).