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Life After Death: Using the Human Microbiome as a Forensics Tool
Death is a natural part of life on Earth. Once an organism dies, the body begins to decompose with the help of microbes on the skin, in the gut, and those that are normally present in the environment. The process of decomposition often makes it challenging to determine the time passed since death. When forensic scientists arrive at a crime scene, they often collect physical evidence, like fingerprints, hair, and DNA to investigate when and how a person died.
Unfortunately, our understanding of the decomposition process is fairly limited, and the environment can greatly impact how a body decays over time. Warmer weather often speeds up the rate of decomposition, which makes it more challenging to determine the time of death. Additionally, natural events or scavenger animals can easily destroy what little evidence is left behind. Current forensic tools have been helpful in estimating the time of death, also known as the postmortem interval (PMI), but more precise methods are needed to improve the field of forensics. Microbiologists have recently developed an approach that uses a person’s microbiome to accurately calculate the time of death.
A collaborative group of experts tracked the composition of bacteria, fungi, and other microbes in and around a corpse following death. They discovered a biological mechanism they called a “microbial clock”, in which changes in the corpse’s microbiome over time can be used to accurately estimate the time passed since death. This newfound technique of calculating PMI can be easily applied to forensics when determining the time of death of a human body.
Described in an article by Metcalf and others, the research team sampled microbial communities from corpses throughout the process of decomposition. They used laboratory mice in the study to make the experiments more controlled and easier to replicate. Over the course of 48 days, researchers swabbed different regions of the corpse as well as the soil around it (Figure 1). The team then sequenced the samples of microbes and used advanced computational methods to assess any changes in the microbial communities at distinct stages of decomposition.
Overall, their experimental findings revealed significant and consistent changes in the microbiomes during decomposition across all 40 corpses. The level of consistency in the changes of microbial communities suggested that microbiome composition over the course of decomposition can be used to calculate the time lapsed since the death of the organism (give or take about 3 days, due to the natural variability of microbiome composition over time).
Interestingly, they found that samples taken from the skin of the corpse and the soil around the corpse were more informative than those from the abdominal cavity. The authors observed that the corpses had their body cavities ruptured at different times, which could have caused more variation in microbial communities. Sequencing also revealed that pathogenic bacteria belonging to the Escherichia and Klebsiella genera rapidly grew on the corpses following body cavity rupture. Additionally, the research team found worms apart from bacteria and viruses. Nematodes, in particular, appeared to be abundant across all corpses after 20 days of decomposition. The nematodes likely fed on the bacteria and other smaller microbes that accumulated on the corpses during early decomposition. Other eukaryotes that increased in abundance during decomposition included species of fungi within the Zygosaccharomyces genus and amoebae known as Nucleariidae Fonticula.
The “microbial clock” demonstrated in this study holds great potential as a new and advanced tool for forensic scientists to investigate an individual’s death. While more studies are critical to better understand how corpse microbiomes shift due to larger environmental factors, advances in sequencing technologies and bioinformatics strengthen the feasibility of this methodology within the criminal justice system. Who knew that there was so much life after death?
Link to the original post: Metcalf JL, et al. (2013). A Microbial Clock Provides an Accurate Estimate of the Postmortem Interval in a Mouse Model System. ELife Microbiology and Infectious Disease; 2:e01104
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