Each human body contains a complex community of trillions of microorganisms that are important to your health while you live. These microbial symbionts help you digest food, produce essential vitamins, protect you from infections, and perform many other crucial functions. In turn, the microbes, which are mainly found in your intestines, live in a relatively stable, warm environment with a constant 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 decomposing body—I am curious about our postmortem microbial legacy. You might assume that your microbes are dying along with you. Once your body breaks down and your microbes are released 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 not only do your microbes live on after you die, but they even play an important role in recycling your body so that new life can flourish.
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 in a process called autolysis. Enzymes in those cells – which normally digest carbohydrates, proteins and fats in a controlled manner for energy or growth – begin to act on the membranes, proteins, DNA and other components that make up the cells.
The products of this cellular breakdown make excellent food for your symbiotic bacteria, and without your immune system to keep them in check and without a steady supply of food from your digestive system, they turn to this new source of nutrition.
Intestinal bacteria, especially called a class of microbes Clostridiaspreads through your organs and consumes 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 create the distinctly odorous gases characteristic of decomposition.
From an evolutionary perspective, it makes sense that your microbes have developed ways to adapt to a dying body. Like rats on a sinking ship, your bacteria will soon have to abandon their host and survive in the world long enough to find a new host to colonize. Using your body’s carbon and nutrients, they can increase their numbers. A larger population means a greater chance that at least a few will survive in the harsher conditions and successfully find a new body.
A microbial invasion
When you are buried in the ground, your microbes are washed into the ground along with a soup of decomposition fluids as your body breaks down. They enter a completely new environment and encounter a completely new microbial community in the soil.
The mixing or merging of two different microbial communities often occurs in nature. Coalescence occurs when the roots of two plants grow together, when wastewater is emptied into a river, or even 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 steady supply of food. In contrast, soil is a particularly difficult place to live; it is a highly variable environment with steep chemical and physical gradients and wide swings in temperature, moisture and nutrients. Furthermore, soil already harbors an exceptionally diverse microbial community full of decomposers that are well adapted to that environment and would likely outcompete any newcomers.
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, for months or years after the body’s soft tissues have decomposed . This raised the question of whether these microbes are still alive and active, or if they are merely in a dormant state, waiting for the next host.
Our latest research shows that your microbes not only live in the soil, but also work with native soil microbes to help your body decompose. In the laboratory, we showed that mixing soil and decomposition fluids filled with host-associated microbes increased decomposition rates beyond those of the soil communities alone.
We also found that host-associated microbes improved nitrogen cycling. Nitrogen is an essential nutrient for life, but most of Earth’s nitrogen is trapped as atmospheric gas that organisms cannot use. Decomposers play a crucial 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 likely play a role 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
Recycling nutrients from waste, or non-living organic matter, is a core process in all ecosystems. In terrestrial ecosystems, the decomposition of dead animals, or carrion, fuels biodiversity and is an important link in food webs.
Living animals create a bottleneck for an ecosystem’s carbon and nutrient cycles. They slowly collect nutrients and carbon from large areas of the landscape throughout their lives and deposit it all at once in a small, local spot when they die. One dead animal can support an entire pop-up food web of microbes, soil fauna and arthropods that live on carcasses.
Insect and animal scavengers help further redistribute nutrients in the ecosystem. Breakdown microbes convert the concentrated amounts 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 unusual to see plant life flourishing near a decomposing animal, visible evidence that nutrients in bodies are being recycled back into the ecosystem.
That our own microbes play an important role in this cycle is a microscopic way in which we live on after death.
This article is republished from The Conversation, an independent nonprofit organization providing 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.