Necrobiome

The necrobiome has been defined as the community of species associated with decaying remains after the death of an organism. The process of decomposition is complex. Microbes decompose cadavers, but other organisms including fungi, nematodes, insects, and larger scavenger animals also contribute. Once the immune system is no longer active, microbes colonizing the intestines and lungs decompose their respective tissues and then travel throughout the body via the circulatory and lymphatic systems to break down other tissue and bone. During this process, gases are released as a by-product and accumulate, causing bloating. Eventually, the gases seep through the body's wounds and natural openings, providing a way for some microbes to exit from the inside of the cadaver and inhabit the outside. The microbial communities colonizing the internal organs of a cadaver are referred to as the thanatomicrobiome. The region outside of the cadaver that is exposed to the external environment is referred to as the epinecrotic microbial communities of the necrobiome, and is especially important when determining the time and location of death for an individual. Different microbes play specific roles during each stage of the decomposition process. The microbes that colonize the cadaver and the rate of their activity are determined by the cadaver itself and the cadaver's surrounding environmental conditions.

There is textual evidence that human cadavers were first studied around the third century BC to gain an understanding of human anatomy. Many of the first human cadaver studies took place in Italy, where the earliest record of determining the cause of death from a human corpse dates back to 1286. However, understanding of the human body progressed slowly, in part because the spread of Christianity and other religious beliefs resulted in human dissection becoming illegal.

Non-human animals only were dissected for anatomical understanding until the 13th century when officials realized human cadavers were necessary for a better understanding of the human body. It was not until 1676 that Antonie van Leeuwenhoek designed a lens that made it possible to visualize microbes, and not until the late 18th century when microbes were considered useful in understanding the body after death.

In modern times, human cadavers are used for research, but other animal models can provide larger sample sizes and produce more controlled studies. Microbial colonization between humans and some non-human animals is so similar that those models can be used to understand the decomposition process for humans. Swine have been used repeatedly to understand the human decomposition process in terrestrial environments. Pigs are suitable for studying human decomposition because of their size, sparse hairs, and similar bacteria found in their GI tracts. Using nonhuman carcasses as study subjects also offers the benefit of minimizing variation in the sample population.

Sophisticed molecular techniques have made it possible to identify the microbial communities that inhabit and decompose cadavers; however, this research is fairly new. Studying the necrobiome has become increasingly useful in determining the time and cause of death, which is useful in crime scene investigations.

As the necrobiome deals with the various communities of bacteria and other organisms that catalyze the decomposition of plants and animals, this particular biome is an increasingly vital part of forensic science. The microbes occupying the space underneath and around a decomposing body are unique to it—similar to how fingerprints are exclusively unique to only one person. Using this differentiation, forensic investigators at a crime scene are able to distinguish between burial sites, as well as gain concrete factual information about how long the body has been there and the predicted area in which the death possibly occurred.

Forensic microbiologists investigate ways to determine time and place of death by analyzing the microbes present on the corpse. The microbial timeline of how a body decays is known as the microbial clock. It estimates how long a body has been in a certain place based on microbes present or missing. The succession of bacterial species populating the body after a period of four days is an indicator of minimum time since death. Recent studies have taken place to determine if bacteria alone can inform the post-mortem interval. Bacteria responsible for decomposing cadavers can be difficult to study because the bacteria found on a cadaver vary and change quickly. Bacteria can be brought to a cadaver by scavengers, air, or water. Other environmental factors like temperature and soil can impact the microbes found on a cadaver.

The time of death can be estimated not only by the type and amount of bacteria on a cadaver, but also by the chemical compounds produced by those bacteria. Forensic anthropologist Arpad Vass determined, from research he undertook in the 1990s, that three types of fatty acids, produced when bacteria break down fat tissues, muscles, and food remnants in the stomach are useful in predicting the time since death during forensic investigations.