by Every human body contains a complex community of trillions of microorganisms that are important to your health during your lifetime. These microbial symbionts help you digest food, produce important vitamins, protect you from infections, and perform many other important functions. In turn, the microbes concentrated primarily in your gut live in a relatively stable, warm environment with a steady supply of food.
But what happens to these symbiotic allies after you die?
As an environmental microbiologist who studies the necrobiome—the microbes that live in, on, and around a decaying body—I was curious about our postmortem microbial legacy. You may assume that your microbes will die with you – once your body breaks down and your microbes are flushed into the environment, they will no longer survive in the real world.
In our September 2023 study, my research team and I share evidence that your microbes not only live on after you die, but actually play an important role in recycling your body so new life can thrive.
Microbial life after death
When you die, your heart stops circulating the blood that carried oxygen throughout your body. Cells deprived of oxygen begin to digest themselves, which is called autolysis. Enzymes in these cells—which normally digest carbohydrates, proteins, and fats for energy or growth in a controlled manner—begin work on the membranes, proteins, DNA, and other components that make up the cells.
The products of this cell breakdown are excellent food for your symbiotic bacteria, and without your immune system to keep them in check and your digestive system without a constant supply of nourishment, they will resort to this new source of nutrition.
Intestinal bacteria, specifically a class of microbes called Clostridiaspread throughout your organs and digest you from the inside out in a process called putrefaction. Without oxygen in the body, your anaerobic bacteria rely on energy-producing processes that don’t require oxygen, such as fermentation. These produce the distinct smelling gas signature of decomposition.
From an evolutionary perspective, it makes sense that your microbes have evolved ways to adapt to a dying body. Like rats on a sinking ship, your bacteria must soon leave their host and survive out in the world long enough to find a new host to colonize. Using your body’s carbon and nutrients can increase their numbers. A larger population means a higher chance that at least a few will survive in the harsher environment and successfully find a new body.
A microbial invasion
When you’re buried in the earth, your microbes are washed into the ground along with a soup of decomposition fluids as your body breaks down. You enter a completely new environment and encounter a completely new microbial community in the soil.
The mixing or merging of two different microbial communities occurs frequently in nature. Fusion occurs when the roots of two plants grow together, when wastewater is discharged into a river, or when two people kiss.
The outcome of mixing—which community dominates and which microbes are active—depends on several factors, such as how much environmental change the microbes experience and who got there first. Your microbes are adapted to the stable, warm environment in your body, where they receive a constant supply of nutrition. In contrast, soil is a particularly harsh habitat – it is a highly variable environment with strong chemical and physical gradients and large fluctuations in temperature, moisture and nutrients. Furthermore, the soil is already home to an exceptionally diverse microbial community full of decomposers that are well adapted to this environment and would probably outcompete any newcomer.
It’s easy to assume that your microbes will die once they’re outside your body. However, previous studies by my research team have shown that the DNA signatures of host-associated microbes can be detected in the soil beneath a decomposing body, on the soil surface, and in graves months or years after the body’s soft tissue has decomposed. This raised the question of whether these microbes are still alive and active or whether they are simply in a dormant state waiting for the next host.
Our latest study suggests that your microbes not only live in the soil, but also work with native soil microbes to help decompose your body. In the laboratory, we showed that mixing soil and decomposition fluids filled with host-associated microbes increased decomposition rates beyond those of soil communities alone.
We also found that host-associated microbes increased nitrogen cycling. Nitrogen is an essential nutrient, but most of the nitrogen on Earth is bound as atmospheric gas that cannot be used by organisms. Decomposers play a critical role in recycling organic forms of nitrogen, such as proteins, into inorganic forms such as ammonium and nitrate that microbes and plants can use.
Our new findings suggest that our microbes are likely involved in this recycling process by converting large nitrogen-containing molecules such as proteins and nucleic acids into ammonium. Nitrifying microbes in the soil can then convert the ammonium into nitrate.
Next generation of life
The recycling of nutrients from detritus or non-living organic matter is a central process in all ecosystems. In terrestrial ecosystems, the decomposition of dead animals or carrion promotes biodiversity and is an important link in food webs.
Living animals are a bottleneck for an ecosystem’s carbon and nutrient cycles. They slowly accumulate nutrients and carbon from large areas of the landscape over their lifetime and then deposit them all at once in a small, localized area when they die. A dead animal can support an entire food web of microbes, soil animals and arthropods that feed on carcasses.
Insect and animal eaters contribute to the further redistribution of nutrients in the ecosystem. Decomposer microbes convert the concentrated pools of nutrient-rich organic molecules from our bodies into smaller, more bioavailable forms that other organisms can use to support new life. It is not uncommon for plants to thrive near a decaying animal – visible evidence that nutrients in the body are being recycled back into the ecosystem.
The fact that our own microbes play an important role in this cycle is a microscopic way in which we continue to live after death.
This article was republished from The Conversation, a nonprofit, independent news organization that brings you facts and trusted analysis to help you understand our complex world. It was written by: Jennifer DeBruyn, University of Tennessee
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Jennifer DeBruyn receives funding from the U.S. Department of Agriculture, the National Science Foundation, the Department of Justice, and the Defense Advanced Research Projects Agency.
