How Researchers Could Use Bacteria to Determine Time of Death
When a dead body is discovered, finding out when the person died is just as important as finding out how the person died. Determining the time of death has always involved lots of complicated scientific detective work and less-than-reliable methods. However, a study by Nathan H. Lents, a molecular biologist at the John Jay College of Criminal Justice in New York, is the first of its kind to show how microbes colonize a body's ears and nose after death.
It might sound gross and dark, but these kinds of studies open the door to new ways of determining how long a body has been dead. By carefully studying the microorganisms that invaded a dead body over time, he developed a mathematical algorithm to help determine how much time passed since death.
Bacteria, fungi, viruses, and other microorganisms like to coexist. They form diverse communities, called microbiomes, which can be found in many places on humans. For example, these diverse and unique communities of microbes grow on our skin and in our intestine, among other areas.
Bacteria in the human intestinal microbiome help us digest food. They break down waste left over from digestion into smaller molecules that our human cells can then use for energy. They're also helpful: Special colonies of bacteria produce nutrients, like vitamins B12 and K, that humans can't make on their own.
When a body decomposes, microorganisms from the environment work together to breakdown, biodegrade, and recycle its molecular and cellular components back to nature. This process breaks down valuable nutrients, like minerals and carbon, and returns them to the environment.
The community of bacterial and fungal microorganisms who live on a dead body make up what the researchers have dubbed the "necrobiome" — "necro," from the Greek word nekros, refers to a corpse. Analysis of these necrobiome microbes could provide police and forensics units information about when a person died.
Traditionally, forensic specialists have used several methods to determine time of death. A body's temperature drops at a predictable rate and can be used to back-calculate the time of death, but only if the environmental temperature over the time period is known.
The presence and growth of insects on the body can also give clues to the time of death. Blowflies pass through certain, distinct stages of their life cycle on a corpse. The age of the larvae present and its phase of growth give clues to how long the body has been decomposing. The problem with using insect growth to determine the time of death is that once the insects are fully grown they leave the body, limiting the time window that insects are useful.
Using insects is "very good at producing minimum times, but not in pinpointing exact times," Lents told Invisiverse. For example, if the casings shed from the second growth phase of blowfly larvae are found, the body cannot possibly be less than three days old, he explained, assuming the body was at 70°F the entire three days.
New research has also found that cocaine and heroin in the corpse can speed up the larval development, complicating the picture. Clearly, something that would give more information about a particular individual's time of death was needed. Enter the necrobiome.
Lents and his research team sampled bacteria from the ear and nasal canals of 21 human cadavers as they decomposed over several weeks.
The cadavers were donated to science for research purposes and kept at the Anthropological Research Facility at the University of Tennessee at Knoxville. The facility is in a temperate deciduous forest with fine clay-like soil that is well-drained and provides a relatively stable environment where decomposition can occur. Here, the bodies were placed unclothed on the top of the soil and allowed to decompose naturally.
Ears and nose were chosen because they could be swabbed easily without the body needing to be disturbed — a benefit at a crime scene and a definite advantage over using intestinal bacteria to estimate time of death as earlier studies had done.
A total of 144 sample swabs were taken from the 21 cadavers. Most cadavers were only swabbed once, but four were swabbed every two to three days until the bodies were too decomposed to access the nose or ear. Bacterial DNA was extracted and analyzed from the nose and ear swab samples. These measurements gave the researchers information on what types of bacteria were present, as well as how the type and amount changed in the four cadavers over several weeks.
The predominant types of bacteria found in the ear and nose swabs were from the phyla Actinobacteria, Spirochaetes, and Armatimonadetes.
Bacteria from the Actinobacteria and Spirochaetes groups have been found to infect humans. Propionibacterium acnes, linked to the cause of acne, is a member of the Actinobacteria phylum. Borrelia burgdorferi, the agent that causes Lyme disease, is from the Spirochaetes phylum. Chthonomonas calidirosea is a recently discovered organism of the Armatimonadetes phylum found mostly in moist, warm soil.
As time went on, the number of types of bacteria found at the swab sites decreased. The results were used to create a mathematical formula of how the bacteria that make up the necrobiome changed over time.
Results from just the ear bacteria or nose bacteria alone did not correlate well with time of death. However, when results from both were combined, the mathematical formula could accurately determine the time of death to within two days.
You probably won't see the use of a necrobiome to determine time of death used on your favorite crime show this season.
According to Lents, making three determinations of the necrobiome present on a corpse currently costs less than $10,000, and will continue to decrease. But before the technique is used routinely, the research needs to be tested in a large, multi-site study with potentially hundreds of research subjects and lots of swab sites. It'll also need to be tested in other environments and temperatures to see how those changes impact the microbial community present on a corpse.
And adding the necrobiome results from the ear and nose along with other sites on the body may produce even more reliable formulas to nail the time of death.