Metabarcoding to Investigate Changes in Soil Microbial Communities within Forensic Burial Contexts

The estimation of the time elapsed since death (post-mortem interval, or PMI) is one of the key themes that forensic scientists have to address frequently. However, the estimation of PMI still suffers from poor accuracy and biases especially when decomposition stages are prolonged, so further improvements in methods for PMI estimation are desirable. Soil microbial communities associated with decomposing bodies have been shown to be good candidates for the estimation of the PMI of exposed bodies. However, further research is required to better understand the bacterial succession associated with decomposition of buried carcasses, to test its reliability and applicability for the estimation of PMI and to better understand the dynamics involved with decomposition within this particular scenario. Therefore, we explored the succession of soil microbial communities associated with four decomposing pig carcasses (from one to six months PMI) using a metabarcoding approach. The sequencing of the bacterial 16S rRNA variable region 4 (V4) revealed trends linking specific microbial taxa with specific PMIs, and in particular an increase in Proteobacteria, Firmicutes and Bacteroidetes at specific PMIs as well as a decrease in Acidobacteria after the introduction of the carcass in the soil. Our results, in accordance with previous studies conducted on exposed bodies of different mammalian species (including humans), showed also a general reduction of the soil taxonomic richness from two months PMI onwards, as well as an incomplete re-establishment of the starting soil microbial conditions after six months PMI, with great relevance for forensic investigations. We also found specific mammals-derived taxa, such as Bacteroides spp., being still present in the soil after six months PMI. This study serves as a baseline for additional research to allow the characterisation of specific biomarkers associated with specific PMIs. Due to the similarity between the results presented here and those reported in other types of decomposition studies we believe that the metabarcoding approach might have considerable potential in the estimation of the PMI, particularly to clarify cases involving heavily skeletonised bodies or for the investigation of clandestine graves in which the carcass has been moved from its original place of deposition.