Death and DNA

The Soil DNA profiling project (Dirt and DNA) is also exploring a new dimension, using soil DNA profiling for "time since death" estimation.

After death, the human body decays in a succession of stages.
The four stages are:

• fresh
• bloat
• decay and
• dry/skeletonisation.

For a body on top of soil, the products of decomposition seep into the ground. This input of substrates and microorganisms from the body causes the profiles of the microbial community in the soil to change significantly over the period of decomposition. Understanding these changes could help estimate the time elapsed since death in a way similar to that used by forensic entomologists.

The timing and succession of insect colonisation of a body can be used to estimate the stages of decomposition, based on knowledge of the lifecycle of these species. Like insect colonisation of a body, the bacterial community associated with a corpse is dynamic and heterogenous, and may also display characteristics specific to decompositional stages.

This project is an international collaboration between ESR scientists and Dr Arpad Vass of Oak Ridge National Laboratory in Tennessee (USA). Dr Vass is also attached to the Forensic Anthropology Centre (nicknamed The Body Farm), and this has provided ESR with the opportunity to work with samples collected from underneath human bodies. Microbial community DNA profiles are generated using the same profiling technique as for the soil-matching project.

This exploratory work has shown there are marked and persistent changes in the bacterial profiles over time. The major microbial community shift during decomposition occurs at the beginning of the bloat stage, and is likely to be in response to purging of body fluids that occurs at this stage. It is possible that the body microflora released during decomposition enter the soil bacterial community and out-compete or simply overwhelm the soil bacteria.

Rachel Parkinson is undertaking a PhD at Victoria University, co-supervised by Dr Jacqui Horswell (ESR) and Arpad Vass, trying to determine the bacterial species present at each stage of decomposition. She is working in New Zealand on pigs (which are as close to humans as we can get) before she goes to the Body Farm in 2006.

The pig experiment specifically aims to determine the contribution of body and insect bacteria on microbial community profiles. This should allow the identification of key microbes that are responding to specific decomposition products and could eventually lead to development of an indicator species system for time since death estimations.