Establishing an accurate time of death (the postmortem interval, or PMI) when a corpse is discovered is notoriously challenging, no matter how easy fictional medical examiners may make it seem. Some forensic scientists use the life cycle of flies, which search for and lay eggs in corpses. But there is a lot of variation between fly species and weather effects, so it helps to develop new methods.
It appears that studying the microbes that thrive on decaying corpses can provide helpful clues. Forensic scientists have now identified about 20 microbes that they believe form a kind of universal network that drives the decomposition of dead animal flesh, according to a new paper published in the journal Nature Microbiology .
“One of the main questions in any death investigation is ‘when did this person die?'” said Nancy La Vigne, director of the National Institute of Justice, which funded the research. “This ongoing line of NIJ-funded research is showing promising results for predicting time of death in human remains, helping to identify the deceased, identify potential suspects, and confirm or disprove alibis.”
The work builds on nearly a decade’s worth of prior research. For example, in 2015, scientists were able to accurately estimate the time of death of rats and human cadavers within a two- to four-day window, even though the bodies had been decomposing for weeks. Previous experiments have shown that, regardless of the time, environment, and species of the dead, communities of flesh-eating microbes seem to have a predictable timetable for when they will eat the corpses. . As Beth Mole reported for Ars at the time, “Those mealtimes have to do with the stages of decay a body goes through — from fresh meat to rotting corpse, to decay and decay.” -flow of nitrogen-rich liquids to active decomposition, then to a dry state. . Each stage attracts specific microbes that digest the body, many of which have a taste for amino acids.”
But researchers aren’t sure if the microbes in the good-weather transition are the same in different scenarios—such as desert vs. forest, summer vs. winter, or peaceful vs. which rots against what people have taken. scavengers. The 2015 results of the experiment show that the temperature determines the course of the microbial dining schedule. But the schedule is remarkably similar between the four human bodies used in the experiment, as well as the rats, although given external exposure. Therefore, the microbial munching pattern may be a universal clock for calling the time of death.
This latest paper expands the scope of the investigation to take a closer look at the specific types of microbes that tend to thrive in decomposing corpses. “When you’re talking about investigating death scenes, there are very few types of physical evidence that you can guarantee are at every scene,” said co-author David Carter, a forensic scientist. in forensic sciences at Chaminade University of Honolulu in Hawaii. . “You don’t know if there are fingerprints or blood stains or camera footage. But germs are always there.” In this recent case study, “We’re talking about outdoor death scenes, ” he added. “Collecting information in these types of investigations can be difficult.”
This time, the team conducted outdoor experiments between 2016 and 2017 on 36 human corpses in three different facilities (body farms): the Colorado Mesa University Forensic Investigation Research Station (FIRS), the Sam Houston State University Southeast Texas Applied Forensic Science (STAFS). ) Facility, and the University of Tennessee Anthropology Research Facility (ARF). The bodies were studied for 21 days in different seasons (spring, summer, autumn, and winter). Daily samples were taken of burial soil related to decomposition and skin from the head and body of the bodies, as well as control soil samples. Daily temperature, humidity, and other environmental factors are also recorded.
The three facilities were located in two different types of climate (temperate forest and semi-arid steppe), but the team identified the same 20 decomposing microbes in all the bodies, which once again showed each- eating patterns regardless of external variables. “It’s very interesting that these microbes are often found to decompose animal remains,” said co-author Jessica Metcalf of Colorado State University. “Hopefully, we are opening a new area of ecological research.”
These particular microbes were not found in the databases of microbes commonly found in soil, human skin, and gut microbiomes, so how did they find their way into the delicious rotting flesh? The authors suggest that insects likely play an important role because these universal decomposer microbes are often found in insects such as beetles and flies.
The authors also used their new data together with machine learning to develop a predictive model for time of death based on microbial activity. The model performed very well, predicting the time of death within three calendar days in independent trials. The team identified persistent errors in intrinsic factors such as BMI/total body mass and extrinsic factors such as scavengers and precipitation. This will be studied in future research to further improve predictive models.
Nature Microbiology, 2024. DOI: 10.1038/s41564-023-01580-y (Part of DOIs).