Body Mass Index (BMI) Impacts Soil Chemical and Microbial Response to Human Decomposition.

Microorganisms are key decomposers of vertebrate mortalities, breaking down body tissues and impacting decomposition progress. During human decomposition, both extrinsic environmental factors and intrinsic cadaver-related factors have the potential to impact microbial decomposers either directly or indirectly via altered physical or chemical conditions. While extrinsic factors (e.g., temperature, humidity) explain some variation in microbial response during human decomposition in terrestrial settings, recent work has noted that even under the same environmental conditions, individuals can have different decomposition patterns, highlighting the potential for intrinsic factors to impact microbial decomposers. The goal of this study was to investigate the effects of several intrinsic factors (age, sex, diseases at time of death, and body mass index [BMI]) on chemical and microbial changes in decomposition-impacted soils. In a field study conducted at the University of Tennessee Anthropology Research Facility, soils were collected from the decomposition-impacted area surrounding 19 deceased human individuals through the end of active decomposition. Soil physicochemical parameters were measured, and microbial (bacterial and fungal) communities were assessed via amplicon sequencing. BMI was shown to explain some variation in soil pH and microbial response to human decomposition. Hierarchical linear mixed (HLM) effects models revealed that BMI category significantly explained variation in pH response within decomposition-impacted soils over time (HLM F = 9.647; P < 0.001). Additionally, the relative abundance of soil Saccharomycetes in decomposition soils under underweight donors displayed little to no changes (mean maximum change in relative abundance, +6.6%), while all other BMI categories displayed an increased relative abundance of these organisms over time (normal, +50.6%; overweight, +64.4%; and obese, +64.6%) (HLM F = 3.441; P = 0.11). Together, these results reveal intrinsic factors influencing decomposition patterns, especially within the soil environment, and suggest BMI is an important factor for controlling decomposition processes. IMPORTANCE This work begins to address questions about interindividual variation in vertebrate decomposition attributed to intrinsic factors, that is, properties of the carcass or cadaver itself. Most research on factors affecting decomposition has focused on the extrinsic environment, such as temperature or humidity. While these extrinsic factors do explain some variation in decomposition patterns, interindividual variability is still observed. Understanding how intrinsic factors influence microbial decomposers will help reveal the ecological impacts of decomposition. This work also has forensic applications, as soil chemical and biological changes have been suggested as indicators of postmortem interval. We reveal factors that explain variation in the decomposition environment that should be considered in these estimates. This is particularly important as we consider the implications of variations in human populations due to diet, age, BMI, disease, toxicological loading, etc. on forensic investigations dealing with decomposing remains.

Postmortem Skeletal Microbial Community Composition and Function in Buried Human Remains.

Bones and teeth can provide a lasting resource to identify human remains following decomposition. Bone can support dynamic communities of micro- and macroscopic scavengers and incidental taxa, which influence the preservation of bone over time. Previously we identified key microbial taxa associated with survivability of DNA in bones of surface-decomposed human remains, observing high intra- and interindividual variation. Here we characterized the postmortem bone microbiome of skeletal remains in a multi-individual burial to better understand subsurface bone colonization and preservation. To understand microbial community origins and assembly, 16S rRNA amplicon sequences from 256 bone and 27 soil samples were compared to bone from individuals who decomposed on the ground surface, and human gut sequences from the American Gut Project. Untargeted metabolomics was applied to a subset of 41 bone samples from buried remains to examine potential microbe-metabolite interactions and infer differences related to community functionality. Results show that postmortem bone microbial communities are distinct from those of the oxic surface soils and the human gut. Microbial communities from surface-deposited bone and shallow buried bone were more similar to those from soils, while bones recovered from saturated areas deeper in the grave showed increased similarity with human gut samples with higher representation of anaerobic taxa, suggesting that the depositional environment affected the established bone microbiome. Correlations between metabolites and microbes indicate that phosphate solubilization is likely an important mechanism of microbially mediated skeletal degradation. This research expands our knowledge of microbial bone colonizers, including colonizers important in a burial environment. IMPORTANCE Understanding the microbes that colonize and degrade bone has important implications for preservation of skeletal elements and identification of unknown human remains. Current research on the postmortem bone microbiome is limited and largely focuses on archaeological or marine contexts. Our research expands our understanding of bone microbiomes in buried remains by characterizing the taxonomic and metabolic diversity of microbes that are colonizing bone after a 4-year postmortem burial interval and examines the potential impact of microbial colonization on human skeletal DNA preservation. Our results indicate that the postmortem bone microbiome is distinct from the human gut and soil. Evidence from combined metabolomic and amplicon sequencing analysis suggests that Pseudomonas and phosphate solubilization likely play a role in skeletal degradation. This work provides important insight into the types and activities of microbes controlling the preservation of buried skeletal remains.

Comparative Decomposition of Humans and Pigs: Soil Biogeochemistry, Microbial Activity and Metabolomic Profiles.

Vertebrate decomposition processes have important ecological implications and, in the case of human decomposition, forensic applications. Animals, especially domestic pigs (Sus scrofa), are frequently used as human analogs in forensic decomposition studies. However, recent research shows that humans and pigs do not necessarily decompose in the same manner, with differences in decomposition rates, patterns, and scavenging. The objective of our study was to extend these observations and determine if human and pig decomposition in terrestrial settings have different local impacts on soil biogeochemistry and microbial activity. In two seasonal trials (summer and winter), we simultaneously placed replicate human donors and pig carcasses on the soil surface and allowed them to decompose. In both human and pig decomposition-impacted soils, we observed elevated microbial respiration, protease activity, and ammonium, indicative of enhanced microbial ammonification and limited nitrification in soil during soft tissue decomposition. Soil respiration was comparable between summer and winter, indicating similar microbial activity; however, the magnitude of the pulse of decomposition products was greater in the summer. Using untargeted metabolomics and lipidomics approaches, we identified 38 metabolites and 54 lipids that were elevated in both human and pig decomposition-impacted soils. The most frequently detected metabolites were anthranilate, creatine, 5-hydroxyindoleacetic acid, taurine, xanthine, N-acetylglutamine, acetyllysine, and sedoheptulose 1/7-phosphate; the most frequently detected lipids were phosphatidylethanolamine and monogalactosyldiacylglycerol. Decomposition soils were also significantly enriched in metabolites belonging to amino acid metabolic pathways and the TCA cycle. Comparing humans and pigs, we noted several differences in soil biogeochemical responses. Soils under humans decreased in pH as decomposition progressed, while under pigs, soil pH increased. Additionally, under pigs we observed significantly higher ammonium and protease activities compared to humans. We identified several metabolites that were elevated in human decomposition soil compared to pig decomposition soil, including 2-oxo-4-methylthiobutanoate, sn-glycerol 3-phosphate, and tryptophan, suggesting different decomposition chemistries and timing between the two species. Together, our work shows that human and pig decomposition differ in terms of their impacts on soil biogeochemistry and microbial decomposer activities, adding to our understanding of decomposition ecology and informing the use of non-human models in forensic research.