Microbes may be a forensic tool for time of death

But these puddles of decay aren’t static; the researchers documented two distinct shifts in the soil microbial community within them. One occurs when the body’s integrity is compromised (technically, during the bloat and bloat-active stages), and decomposition fluids gush into the soil. Chemical analyses reveal that during this window, there’s an uptick in extractable carbon and nitrogen (particularly ammonia), and the production of biomass goes up. Then, when most tissue has decomposed (the active to advanced decay stages), biomass production slows, the team reported June 12 in PLOS ONE.

The researchers also found that human-associated Bacteroides species, which thrive in the guts of mammals, persisted in some of the soil samples a whopping 198 days after the cadavers were placed. This suggests that soil microbes beneath a body could help forensic estimates of the “postmortem interval,” or time since death.

Related work by DeBruyn, Kathleen Hauther, a doctoral student in DeBruyn’s lab, and others suggests that microbial sampling of the human gut itself might also serve as a time-since-death stamp. Hauther speculated that changes in the community of gut microbes might offer an internal signal that would remain consistent regardless of where and when death strikes.

So the team regularly swabbed the gut of six bodies donated to the Body Farm over nine to 20 days, duct-taping the incisions to prevent insects from moving in. It was messy work. “We sampled until things turned to mush and we couldn’t find the intestines anymore,” DeBruyn says.

While the research is preliminary, the results suggest that there’s a discernible shift in the community when gut microbes go from digesting our food to digesting our bodies. Quantifying the relative abundances of three ubiquitous genera of gut microbes — Bacteroides, Lactobacillus and Bifidobacterium — revealed that populations of Bacteroides andLactobacillus peter out in a predictable manner, the team reported in June in the Journal of Forensic Sciences. This finding suggests that if at least two samples are taken, allowing calculation of a rate of change, these bacterial genera might be a good way to backtrack to time of death. (The relative abundance of the Bifidobacterium populations didn’t prove useful.)

When the team look at the entire species composition of the decomposing intestines, they found that the kinds of bacteria present shift about halfway through the bloat stage. A few days into bloat, when the body swells with gases, a “decay community” begins to dominate, and bacterial taxa associated with flies, such as Ignatzschineria species, start appearing, the researchers reported in June at the American Society for Microbiology meeting in New Orleans. Previous work focusing on bloat bacteria couldn’t find a consistent signal; the new work offers hope that telltale microbes are out there, DeBruyn says.