September 25, 2013
Forensics May Rely On ‘Microbial Clock’ To Establish Time Of Death
[ Watch the Video: Time Of Death Determined By Microbial Clock ]
April Flowers for redOrbit.com - Your Universe Online
Forensic scientists already have an extensive toolbox of techniques for determining the time of death in cases involving human corpses, but an intriguing new study led by the University of Colorado may just give them a new one.
The findings, published in the new online science and biomedical journal eLIFE, describe a microbial clock that is essentially the lock-step succession of bacterial changes that occur postmortem as bodies move through the decay process. The current study used mice, however previous research into the human microbiome - the estimated 100 trillion or so microbes that live on and inside each of us – suggests that there is solid reason to believe such microbial clocks are ticking away on human corpses.
"While establishing time of death is a crucial piece of information for investigators in cases that involve bodies, existing techniques are not always reliable," said Jessica Metcalf, a postdoctoral researcher at CU-Boulder 's BioFrontiers Institute. "Our results provide a detailed understanding of the bacterial changes that occur as mouse corpses decompose, and we believe this method has the potential to be a complementary forensic tool for estimating time of death."
Forensic scientists currently use tools ranging from the timing of last text messages and corpse temperatures to insect infestations on bodies and "grave soil" analyses, with varying results, according to Metcalf. The longer the time lapse from death until the forensic scientist starts testing, the more difficult it becomes to determine the time of death with any accuracy.
The research team used high-technology gene sequencing techniques on both bacteria and microbial eukaryotic organisms like fungi, nematodes and amoeba postmortem to pinpoint time of mouse death after a 48-day period to within roughly four days. Testing analysis at 34 days postmortem resulted in even more accurate results, correctly estimating the time of death within about three days, said Metcalf.
Over the course of the 48-day study, the research team tracked microbial changes on the heads, torsos, body cavities and associated grave soil of 40 mice at eight different time points.
Chaminade University forensic scientist David Carter said the after-death stages include the "fresh" stage before decomposition, followed by "active decay" that includes bloating and subsequent body cavity rupture, followed by "advanced decay."
"At each time point that we sampled, we saw similar microbiome patterns on the individual mice and similar biochemical changes in the grave soil," said Laura Parfrey, a former CU-Boulder postdoctoral fellow and now a faculty member at the University of British Columbia who is a microbial and eukaryotic expert. "And although there were dramatic changes in the abundance and distribution of bacteria over the course of the study, we saw a surprising amount of consistency between individual mice microbes between the time points -- something we were hoping for."
"Blooms" of a common soil-dwelling nematode well known for consuming bacterial biomass that occurred at roughly the same time on individual mice remains were charted by the researchers during the decay period.
"The nematodes seem to be responding to increases in bacterial biomass during the early decomposition process, an interesting finding from a community ecology standpoint," said Metcalf.
"This work shows that your microbiome is not just important while you're alive," said CU-Boulder Associate Professor Rob Knight, who runs the lab where the experiments took place. "It might also be important after you're dead."
The CU research team worked closely with assistant professors Sibyl Bucheli and Aaron Linne of Sam Houston State University (SHSU). SHSU is the home of the Southeast Texas Applied Forensic Science Facility, an outdoor human decomposition facility known popularly as a "body farm." The body farm research team is testing bacterial signatures of human cadavers over time to learn more about the process of human decomposition and how it is influenced by weather, seasons, animal scavenging and insect infestations.
The current study is the latest in more than a dozen over the last several years from the CU-Boulder research team on human microbiomes. Another study, conducted by Professor Noah Fierer has revealed what could be another potential forensic tool: microbial signatures left on computer keys and computer mice, an idea enthralling enough it was featured on a "CSI: Crime Scene Investigation" television episode.
"This study establishes that a body's collection of microbial genomes provides a store of information about its history," said Knight, also an associate professor of chemistry and biochemistry and a Howard Hughes Medical Institute Early Career Scientist. "Future studies will let us understand how much of this information, both about events before death -- like diet, lifestyle and travel -- and after death can be recovered."
"There is no single forensic tool that is useful in all scenarios, as all have some degree of uncertainty," said Metcalf. "But given our results and our experience with microbiomes, there is reason to believe we can get past some of this uncertainty and look toward this technique as a complementary method to better estimate time of death in humans."