In every human body, a complex community of trillions of microorganisms thrives. They are essential for health while a person is alive. These microbial symbionts help digest food, produce essential vitamins, protect against infections, and perform many other functions.
Most of these microbes live in the intestines, a relatively stable, warm environment with a steady food supply.
But what happens to these symbiotic allies after death? A team led by Jennifer DeBruyn, a professor of ecological microbiology at the University of Tennessee, studies the necrobiome — the microbes that live inside, on the surface of, and around a decaying body. The team’s work, reported by Science Alert, shows that microbes not only continue living after death but also play a key role in breaking down the body and helping new life thrive.
What happens to microbes after death
You might assume that microbes die when the person dies. People often think that once the body stops functioning, microorganisms simply disperse into the environment and don’t survive. But the reality is more complicated.
When the heart stops, blood stops circulating and oxygen delivery ends. Cells deprived of oxygen begin to digest themselves in a process called autolysis; enzymes that normally break down carbohydrates, proteins, and fats for energy start attacking cell membranes, proteins, DNA, and other cell components.
The products of this cellular breakdown provide rich food for symbiotic bacteria. Freed from the immune system that previously kept them in check and from regular digestive supplies, these bacteria turn to this new source of nourishment. Intestinal bacteria — especially a class called Clostridia — spread through the organs and break down the body from the inside in a process known as putrefaction.
Without oxygen in the body, anaerobic bacteria switch to energy production pathways that don’t require oxygen, such as fermentation. DeBruyn says it makes sense from an evolutionary perspective that microbes have developed ways to adapt to a dying host. Like rats on a sinking ship, these bacteria will need to leave their host and survive in the outside world long enough to find a new host to colonize.
Using the carbon and nutrients released during breakdown allows them to reproduce. The larger the population gets, the greater the chance that at least a few individuals will survive tougher conditions.
Microbial invasion
When a body is buried, its microbes enter a completely different environment and meet an existing soil microbial community. Mixing of two microbial groups happens often in nature — for example when plant roots intertwine or when two people kiss.
The outcome depends on which community is dominant, which microbes are more active, and how much the environment has changed. Microbes adapted to the stable, warm interior of a living body with a steady food supply suddenly face soil, which can be a harsh, highly variable environment with sharp chemical and physical gradients, big temperature swings, and fluctuating moisture.
A diverse soil microbial community is already well adapted to those conditions and is likely to outcompete newcomers. So you might expect the body’s microbes to die once they’re outside the body. But studies have found DNA signatures of host-associated microbes in the soil beneath decomposing bodies, on the soil surface, and in graves for months or even years after soft tissues have broken down.
Scientists asked whether these microbes are alive and active or simply dormant, waiting for a new host. DeBruyn’s team showed that our microbes not only persist in soil but also interact with local soil microbes that help decompose the body. The researchers found these microbes boost steps in the nitrogen cycle and likely help break large nitrogen-containing molecules (such as proteins and nucleic acids) into ammonium, which can then be converted into nitrate.
The researchers concluded that our microbes play an important role in these processes — one of the microscopic ways our life continues after death.
