The clock is essentially the lock-step succession of bacterial changes that occur postmortem as bodies move through the decay process, researchers said.
Previous studies on the human microbiome - the estimated 100 trillion or so microbes that live in each of us – indicate there is good reason to believe similar microbial clocks are ticking away on human corpses, said Jessica Metcalf, first author of the study from University of Colorado Boulder.
"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 Metcalf.
"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," said Metcalf.
Currently, investigators 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, she said.
The more days that elapse following a person's demise, the more difficult it becomes to determine the time of death with significant accuracy.
Using high-technology gene sequencing techniques on both bacteria and microbial eukaryotic organisms like fungi, nematodes and amoeba postmortem, the researchers were able to pinpoint time of mouse death after a 48-day period to within roughly four days.
The results were even more accurate following an analysis at 34 days, correctly estimating the time of death within about three days, said Metcalf.
The researchers tracked microbial changes on the heads, torsos, body cavities and associated grave soil of 40 mice at eight different time points over the 48-day study.
The stages after death include the "fresh" stage before decomposition, followed by "active decay" that includes bloating and subsequent body cavity rupture followed by "advanced decay," said Chaminade University forensic scientist David Carter, a co-author on the study.
"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 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," said Parfrey.
The study was published in the journal eLIFE.


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