When you die, a new life begins for the billions of microbes you carry with you. Unchecked by your immune system, waves of species start multiplying and breaking down your body. Microbes from the environment join in as well. Geneticist Jessica Metcalf of the University of Colorado, Boulder, hopes this macabre procession can provide a microbial clock that can help investigators tell the time of death more precisely than they can with current methods, which rely on body temperature, rigor mortis, and insects.
Early in the decay, for instance, bacteria from the Moraxellaceae family and the genus Acinetobacter begin gorging on dying human cells. Soon after, the Rhizobiaceae family, often involved in breaking down nitrogen sources, takes over. The gases produced by these bacteria cause the body to bloat and eventually rupture, allowing oxygen in and giving aerobic species the upper hand. Microscopic worms also start to multiply, probably feasting on the bacterial biomass now covering the corpse.
Metcalf first showed that she could use microbes, combined with a statistical model, to pinpoint the time of death of mice to within 3 days, even weeks after death. Then her team took samples from four human bodies at a so-called body farm, where cadavers are placed outside so that forensic scientists can study how they decompose. In a paper published in Science (8 January, p. 158), they reported that, again, the microbial dance was predictable enough to set a clock. “Over 25 days our error rate is about 2 to 4 days,” says Rob Knight of the University of California, San Diego, who is collaborating with Metcalf. In a large new project, the researchers will expose 36 bodies, three at each of three different body farms, in all four seasons. That will help them further calibrate their clock further and tell them how it is affected by the environment.