When we die, what actually happens to our body?

At any given time, your body contains as many microbes as human cells. Many of these tiny parasites are waiting for you to die so they can start decomposing you.

What these microbes do after death may provide clues to some of forensic science’s most challenging questions, such as determining the time of death or the location of a hidden grave.

Here’s how decomposition works: About four minutes after you die, your body’s enzymes begin breaking down your cells, which open like champagne bottles to the hordes of hungry bacteria waiting in your gut. Meanwhile, the bacteria on your skin begin to attack you from the outside in.

This is the first phase of decomposition, with all the familiar signs of death that we have all learned from detective series: rigor mortiswhich strains the muscles as proteins that lack energy bind, and cyanosis, which creates a purple color on the skin as blood settles under the force of gravity. At this stage, bacteria in the gut begin to multiply out of control.

Then, as decomposition progresses, the gut bacteria escape their confines and begin to consume the rest of the body, depleting oxygen from the tissues as they go. Over time, microbes that can tolerate low-oxygen conditions begin to proliferate (especially bacteria known as Clostridium) produce gases that cause the body to swell. After about two days, these bacteria become so numerous that they are called ” Clostridia postmortem“.

Diverse community Clostridium and other microbes that take over when you die are called the necrobiome, a dying version of the microbiome that inhabited you while you were alive. By tracking the patterns of its replacement in a corpse, much like the replacement of new trees in a devastated forest, scientists are already beginning to unlock some of the mysteries of death.

It’s all a bit disturbing when you start thinking about the ultimate fate of our bodies. And the question arises: what was stopping all those bacteria from destroying us alive?

You can thank your immune system, mostly. As long as you’re alive, it defends itself against a constant onslaught of bacteria, fungi, and viruses that would like to get inside you, devour your tasty organic matter, and reproduce like crazy. We call the microbes that do these things germs, and when they cause us harm (by eating us alive, hijacking our cells to reproduce, or producing toxic waste), we call it an infection.

Our immune system is on standby 24/7 to fight these invaders, while letting other, more beneficial bacteria through, like those that help us digest food. When harmful microbes evade our immune system, we turn to Plan B and try to flush the invaders out of the wound or poison them with antibiotics.

But we ultimately lose the battle with bacteria when we die, as the benign microbial groups give way to those that the immune system could otherwise fight.

While most of us prefer not to think about our eventual decomposition in detail, forensic scientists have no such qualms. Instead, they find it very useful to document exactly what bacteria are feeding on our corpses and when they do so.

Until recently, scientists knew very little about the microbes that eat people and the little dramas that occur as they compete for dominance over a corpse. The effect Clostridia postmortemfor example, was not described until 2017. The necrobiome was assumed to be too random and variable to be of much importance.

That began to change in 2013, when biologist Jessica Metcalf of Colorado State University and her colleagues were the first to report that microbes growing in dead mice changed consistently over time. This meant that these microbes could actually be a useful tool for investigating death.

If the types of microbes that grow on a corpse, and when they grow, follow a certain pattern, scientists might be able to figure out when a person died by looking at how the microbes on a particular dead body compare to that general pattern. It’s the same idea used by forensic entomologists, who can roughly estimate the time of death by looking at the growth of maggots in a corpse.

The first to test the necrobiome idea on human cadavers were researchers at the Southeast Texas Center for Applied Forensics, one of a handful of U.S. research centers, often called body farms, where scientists study the decomposition of human remains.

In 2013, researchers took samples from two cadavers before and at the end of the bloat stage, when anaerobic bacteria produce huge amounts of gas that causes the body to swell. Indeed, they found that, as in mice, there were patterns in the bacterial groups in the cadavers over time.

Scientists will then have to identify these patterns in many more corpses and in different settings. That’s what Jennifer Pechal, a coroner at Michigan State University in the US, set out to do. Working with the Detroit coroner’s office, he has collected samples from more than 2,000 bodies. So far, he has found patterns in the microbial community that allow us to determine whether a corpse has been dead for more or less than 48 hours.

Meanwhile, Metcalfe is working on computer models that can crunch massive amounts of data on postmortem microbes and their byproducts to calculate an even more precise postmortem interval, or time since death. One version of their model could narrow the time of death to about three days in a three-week window. That may not sound precise, but it would be a big improvement for forensic scientists in cases where a body has been dead too long to use metrics like body temperature or blood pressure. rigor mortis.

Determining time of death isn’t the only potential use for the necrobiome. Researchers like Pechal hope that one day forensic scientists will routinely sample corpses to better understand deceased people through their microbial communities.

Finally, some researchers hope that the necrobiome can lead us directly to where bodies are buried. At the University of Tennessee, the first and most famous body farm in the United States is being researched by Jennifer DeBruyne, a microbial ecologist, and Neil Stewart, a plant ecologist. Together, they are studying what happens in the soil as a corpse decomposes, and how the nutrients and chemicals released affect nearby plants.

Farmers know that nutrients in the soil can cause noticeable changes in the color of plant leaves, or more subtle differences in how they reflect different wavelengths of light. It’s now common to use drone imaging to determine the nutritional status of crops. What if the same idea could be applied to covert burial, with drones flying over potential burial sites looking for telltale signs of a buried corpse?

Meanwhile, Pechal is optimistic: “I’m optimistic that this (using the necrobiome to determine time of death) will be something that forensic scientists can use in the future.” As DNA analysis and genomic sequencing advance, she and other forensic scientists say it’s an exciting time for the field.

Perhaps one day, scanning a corpse’s microbial and chemical fingerprints will become as commonplace as inking its fingerprints.