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The Search for a Microbial Death Clock

National Institute of Justice

Criminal investigators use physiological changes and insect development to determine how long a body has been dead, but scientists are using the trillions of microbes involved in human decomposition to find more accurate postmortem intervals.

Except for the dead bodies, the landscape is idyllic. There is a forest of shortleaf pines, with boxelder and white ash trees nearby. There is restored prairie land, bottomland hardwood, and an area called Palmetto Flat. The overall ecology is not only inviting but important, as the soil conditions, temperature, rainfall and heat all affect how the human bodies scattered across the land decompose.

The cadavers are purposefully placed there to help scientists, medical examiners, and criminal investigators. The donated bodies at the Southeast Texas Applied Forensic Science Facility (STAFS), near Sam Houston State University in Huntsville, answer questions about the biology after death, that time when a human body goes from being the home of a living person to, as researchers put it, a "rich nutrient source" for insects and microbes.

Scientists have long known about the important progression of insect populations during the first two weeks of decomposition, and how that can serve as a clock to determine how long a body has been dead. But in recent years, several researchers, supported by National Institute of Justice grants, have tried to extend the clock to a month or more by focusing on the microbes that consume a body after death.

"Microbes are everywhere and everyone has them," said Jessica Metcalf, an evolutionary biologist at Colorado State University, who is one of the leading scientists in developing the microbiome as postmortem interval clock. "Unlike insects, which may not be present in the winter or in other circumstances, there are always microbes," she said. "We have them in our bodies and they are in the environment."

Although individual humans live in mostly symbiotic relationships with the trillions of bacteria, fungi, and viruses in and on the body, at the moment of death the immune system stops and the relationship with the microbes changes dramatically. Waves of microbes, first those inside the body and then those from the surrounding environment, start breaking down organs and other tissue. Knowing which microbes appear and when, as decomposition progresses, is the basis for the postmortem microbial clock. Microbial clocks may cover longer timescales and also be a more precise predictor of the postmortem interval than insects.

Using microbe progression as a clock to determine the postmortem interval has been made possible in the last decade due to advances in DNA sequencing and increasingly sophisticated data analysis, scientists note. Those advances have allowed Metcalf and other researchers to understand the progression of the microbes and to realize that, like the stages of insect development but on a much grander scale, the stages of microbial progression on a cadaver can be reliably tracked.

In a 2018 paper in Forensic Science International: Genetics, based on several of her NIJ grants, Metcalf described the current knowledge gaps in using microbes as a postmortem interval indicator and noted that "developing and transitioning new forensic science technologies into the justice system requires overcoming scientific, investigative, and legal hurdles." To do that, scientists must gather mountains of data on the progression of microbes in cadavers that are decomposing in all seasons in an array of environments.

The STAFS facility and six other "body farms" in the United States provide the fundamental data on the microbial progression of decomposition, but analyzing and categorizing the trillions of microbes presents a problem of scale. Metcalf and her colleagues are gathering data from STAFS and two of the other facilities for their current postmortem interval research.

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The Biological Evidence Preservation Handbook:
Best Practices for Evidence Handlers

Technical Working Group on Biological Evidence Preservation

The Biological Evidence Preservation Handbook offers guidance for individuals involved in the collection, examination, tracking, packaging, storing, and disposition of biological evidence. This may include crime scene technicians, law enforcement officers, healthcare professionals, forensic scientists, forensic laboratory managers, evidence supervisors, property managers, storage facility personnel, lawyers, testifying experts, court staff members, and anyone else who may come in contact with biological evidence. While many of the recommendations relate to the physical storage, preservation, and tracking of evidence at the storage facility, this handbook also covers the transfer of the material between the storage facility and other locations and discusses how the evidence should be handled at these other locations.

This report is divided into five main sections that detail issues and make recommendations related to biological evidence storage, tracking, preservation, and disposition. A glossary, which provides standard definitions of the technical terms used in this report, follows these sections.

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Receive, analyze and maintain secure custody of items of evidence submitted for DNA analysis. Perform DNA examinations including the collection of biological samples using approved procedures. Generate STR profiles utilizing analytical DNA equipment and instrumentation. Analyze and record DNA test data. Interpret the data of complex DNA mixtures using probabilistic genotyping computer software. Enter DNA data into CODIS.
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Charleston Police Department, Charleston, South Carolina, USA

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Evaluate, compare, and identify latent prints from crime scenes and physical evidence. Search latent prints on local, state, and federal fingerprint databases. Testify in court concerning the results of analyses and the methods used to draw the conclusions.
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