UC Davis

Understanding human decomposition has the potential to significantly improve estimations of postmortem intervals (PMI), a key component of forensic investigations (1). The post-mortem interval is one of the most challenging pieces of evidence to obtain in the field of forensic science. In recent years, a novel approach for PMI calculations has emerged through the use of a microbial clock, an estimation tool based on microbial community data (2). This method, based on predictable patterns in microbial community progression, demonstrates that microbes provide an accurate clock starting at death and relies on the ecological changes in the microbial communities in the body and surrounding environment. However, there is not much known about how interactions between soil and the host microbiome influence PMI estimation. In the current study, we utilized specific pathogen free (SPF) and germ-free (GF) mice buried in non- sterile and sterile graves to identify sources of variability in microbial community progression. Intestinal and soil contents were collected over the course of a 21-day decomposition period and bacterial communities were identified by 16S sequencing. We found that GF mice remained sterile over the study period, regardless of soil sterility. In contrast, soil sterility did have an impact on microbial community dynamics in SPF mice. The data demonstrates that microbial communities at the time of death influence the entry of environmental microbes and microbial progression. Together, these results suggest that differences in the host microbiome at the time of death can significantly impact the predictive power of microbial succession in calculating PMI and should be taken into consideration when developing future models.