Most of us would rather not think about what happens to our bodies after death. But Moheb Costandi finds that this breakdown gives birth to new life in unexpected ways for Mosaic

“It might take a little bit of force to break this up,” says mortician Holly Williams, lifting John’s arm and gently bending it at the fingers, elbow, and wrist. “Usually, the fresher a body is, the easier it is for me to work on.”

Williams speaks softly and has a happy-go-lucky demeanor that belies the nature of her work. Raised and now employed at a family-run funeral home in north Texas, she has seen and handled dead bodies on an almost daily basis since childhood. Now 28 years old, she estimates that she has worked on something like 1,000 bodies.

Her work involves collecting recently deceased bodies from the Dallas–Fort Worth area and preparing them for their funeral.

“Most of the people we pick up die in nursing homes,” says Williams, “but sometimes we get people who died of gunshot wounds or in a car wreck. We might get a call to pick up someone who died alone and wasn’t found for days or weeks, and they’ll already be decomposing, which makes my work much harder.”

John had been dead about four hours before his body was brought into the funeral home. He had been relatively healthy for most of his life. He had worked his whole life on the Texas oil fields, a job that kept him physically active and in pretty good shape. He had stopped smoking decades earlier and drank alcohol moderately. Then, one cold January morning, he suffered a massive heart attack at home (apparently triggered by other, unknown, complications), fell to the floor, and died almost immediately. He was just 57 years old.

Now, John lay on Williams’ metal table, his body wrapped in a white linen sheet, cold and stiff to the touch, his skin purplish-grey—telltale signs that the early stages of decomposition were well under way.

Self-digestion

Far from being ‘dead,' a rotting corpse is teeming with life. A growing number of scientists view a rotting corpse as the cornerstone of a vast and complex ecosystem, which emerges soon after death and flourishes and evolves as decomposition proceeds.

Decomposition begins several minutes after death with a process called autolysis, or self-digestion. Soon after the heart stops beating, cells become deprived of oxygen, and their acidity increases as the toxic by-products of chemical reactions begin to accumulate inside them. Enzymes start to digest cell membranes and then leak out as the cells break down. This usually begins in the liver, which is rich in enzymes, and in the brain, which has a high water content. Eventually, though, all other tissues and organs begin to break down in this way. Damaged blood cells begin to spill out of broken vessels and, aided by gravity, settle in the capillaries and small veins, discoloring the skin.

Body temperature also begins to drop, until it has acclimatized to its surroundings. Then, rigor mortis—“the stiffness of death”—sets in, starting in the eyelids, jaw, and neck muscles, before working its way into the trunk and then the limbs. In life, muscle cells contract and relax due to the actions of two filamentous proteins (actin and myosin), which slide along each other. After death, the cells are depleted of their energy source and the protein filaments become locked in place. This causes the muscles to become rigid and locks the joints.

During these early stages, the cadaveric ecosystem consists mostly of the bacteria that live in and on the living human body. Our bodies host huge numbers of bacteria; every one of the body’s surfaces and corners provides a habitat for a specialized microbial community. By far the largest of these communities resides in the gut, which is home to trillions of bacteria of hundreds or perhaps thousands of different species.

The gut microbiome is one of the hottest research topics in biology; it’s been linked to roles in human health and a plethora of conditions and diseases, from autism and depression to irritable bowel syndrome and obesity. But we still know little about these microbial passengers. We know even less about what happens to them when we die.

In August 2014, forensic scientist Gulnaz Javan of Alabama State University in Montgomery and her colleagues published the very first study of what they have called the thanatomicrobiome (from thanatos, the Greek word for ‘death’).

“Many of our samples come from criminal cases,” says Javan. “Someone dies by suicide, homicide, drug overdose, or traffic accident, and I collect tissue samples from the body. There are ethical issues [because] we need consent.”

Most internal organs are devoid of microbes when we are alive. Soon after death, however, the immune system stops working, leaving them to spread throughout the body freely. This usually begins in the gut, at the junction between the small and large intestines. Left unchecked, our gut bacteria begin to digest the intestines—and then the surrounding tissues—from the inside out, using the chemical cocktail that leaks out of damaged cells as a food source. Then they invade the capillaries of the digestive system and lymph nodes, spreading first to the liver and spleen, then into the heart and brain.

Javan and her team took samples of liver, spleen, brain, heart, and blood from 11 cadavers, at between 20 and 240 hours after death. They used two different state-of-the-art DNA sequencing technologies, combined with bioinformatics, to analyze and compare the bacterial content of each sample.

The samples taken from different organs in the same cadaver were very similar to each other but very different from those taken from the same organs in the other bodies. This may be due partly to differences in the composition of the microbiome of each cadaver, or it might be caused by differences in the time elapsed since death. An earlier study of decomposing mice revealed that although the microbiome changes dramatically after death, it does so in a consistent and measurable way. The researchers were able to estimate time of death to within three days of a nearly two-month period.

Javan’s study suggests that this ‘microbial clock’ may be ticking within the decomposing human body, too. It showed that the bacteria reached the liver about 20 hours after death and that it took them at least 58 hours to spread to all the organs from which samples were taken. Thus, after we die, our bacteria may spread through the body in a systematic way, and the timing with which they infiltrate first one internal organ and then another may provide a new way of estimating the amount of time that has elapsed since death.

“After death the composition of the bacteria changes,” says Javan. “They move into the heart, the brain, and then the reproductive organs last.” In 2014, Javan and her colleagues secured a $200,000 grant from the National Science Foundation to investigate further. “We will do next-generation sequencing and bioinformatics to see which organ is best for estimating [time of death]—that’s still unclear,” she says.

One thing that does seem clear, however, is that a different composition of bacteria is associated with different stages of decomposition.

Putrefaction

Scattered among the pine trees in Huntsville, Texas, lie around half a dozen human cadavers in various stages of decay. The two most recently placed bodies are spread-eagled near the center of the small enclosure with much of their loose, grey-blue mottled skin still intact, their ribcages and pelvic bones visible between slowly putrefying flesh. A few meters away lies another, fully skeletonized, with its black, hardened skin clinging to the bones, as if it were wearing a shiny latex suit and skullcap. Further still, beyond other skeletal remains scattered by vultures, lies a third body within a wood and wire cage. It is nearing the end of the death cycle, partly mummified. Several large, brown mushrooms grow from where an abdomen once was.

For most of us the sight of a rotting corpse is at best unsettling and at worst repulsive and frightening, the stuff of nightmares. But this is everyday for the folks at the Southeast Texas Applied Forensic Science Facility. Opened in 2009, the facility is located within a 247-acre area of National Forest owned by Sam Houston State University (SHSU). Within it, a nine-acre plot of densely wooded land has been sealed off from the wider area and further subdivided, by 10-foot-high green wire fences topped with barbed wire.

In late 2011, SHSU researchers Sibyl Bucheli and Aaron Lynne and their colleagues placed two fresh cadavers here and left them to decay under natural conditions.

Once self-digestion is under way and bacteria have started to escape from the gastrointestinal tract, putrefaction begins. This is molecular death—the breakdown of soft tissues even further, into gases, liquids, and salts. It is already under way at the earlier stages of decomposition but really gets going when anaerobic bacteria get in on the act.

Putrefaction is associated with a marked shift from aerobic bacterial species, which require oxygen to grow, to anaerobic ones, which do not. These then feed on the body’s tissues, fermenting the sugars in them to produce gaseous by-products such as methane, hydrogen sulphide, and ammonia, which accumulate within the body, inflating (or ‘bloating’) the abdomen and sometimes other body parts.

This causes further discoloration of the body. As damaged blood cells continue to leak from disintegrating vessels, anaerobic bacteria convert hemoglobin molecules, which once carried oxygen around the body, into sulfhaemoglobin. The presence of this molecule in settled blood gives skin the marbled, greenish-black appearance characteristic of a body undergoing active decomposition.

As the gas pressure continues to build up inside the body, it causes blisters to appear all over the skin surface. This is followed by loosening, and then "slippage," of large sheets of skin, which remain barely attached to the deteriorating frame underneath. Eventually, the gases and liquefied tissues purge from the body, usually leaking from the anus and other orifices and frequently also leaking from ripped skin in other parts of the body. Sometimes, the pressure is so great that the abdomen bursts open.

Bloating is often used as a marker for the transition between early and later stages of decomposition, and another recent study shows that this transition is characterized by a distinct shift in the composition of cadaveric bacteria.

Bucheli and Lynne took samples of bacteria from various parts of the bodies at the beginning and the end of the bloat stage. They then extracted bacterial DNA from the samples and sequenced it.

As an entomologist, Bucheli is mainly interested in the insects that colonize cadavers. She regards a cadaver as a specialized habitat for various necrophagous (or ‘dead-eating’) insect species, some of which see out their entire life cycle in, on, and around the body.