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Demystifying death with BMI And Other Intrinsic Factors
The concept of postmortem interval (PMI), which enables crime scene investigators to estimate the time since a person’s death, is an indispensable tool in forensic microbiology. When determining the PMI, studying bacterial succession patterns triumphs over other methods (example insect behaviour), as explained in a previous article in the blog. In addition to succession patterns during decomposition (change of species in their community structure), estimating the time of death is also eased by the predictable microbial ecology that are involved.
It is known that external (environmental) factors influence microbes during corpse decomposition, however, recent research suggests that the intrinsic factors of the body which have barely been explored, play a major role too. Such intrinsic factors (e.g. properties of the cadaver) vary from person- to person, and include age, sex, diseases at the time of death, and even BMI. Together, they influence the soil beneath the decaying body as well as the decomposition process itself.
Influence of carcass decomposition in soil ecology
The intrinsic factors mentioned above attribute to differences in the body physiology and microbiome among individuals at the time of death. For example, a study found that among those with high BMI (obese individuals- especially obese women), decomposition occurs faster as fat maintains the inherent higher body core temperatures for longer periods of time and encourages putrefaction quicker.
In the study by Dr Mason and her team, the scientists explain that changes in decomposition patterns have been observed in various cadavers despite decomposing at the same time and place(!). Physiological differences among cadavers also affect the behaviour of insects/ scavengers and the decomposition products. This is why it is vital to understand the role of the intrinsic factors during the decomposition progression.
In their work, the researchers used 19 individuals (body-donors) to study the influence of the intrinsic factors as indicators of PMI by looking at biochemical changes in the soil.
Changes in soil chemistry influenced by varying intrinsic factors in the cadavers
Within the soil, chemistry and microbial activity alter. Microbial abundance and diversity reduce throughout the decomposition process. In addition, electrical conductivity (48 times more!), organic content and ammonia increase. The pH is another story, though.
The pH in decomposition-impacted soils increases in underweight people (<18.5 BMI) and decreases in normal, overweight, and obese people (>18.5 BMI). The reason is that during decomposition, fat degrades into organic acids such as stearic, oleic and palmitic, which decrease the pH, while muscle decomposition results in ammonium release into the soil increasing soil pH. Thus, those with a high BMI (more fat) release more acid, decreasing the soil pH, while those with a low BMI increase the pH by releasing more ammonia.
Shifting microbial dynamics in soil with increasing body decomposition
Generally, the microbial diversity in the soil below underweight and normal individuals decreases over time, while the diversity below overweight individuals remains constant. The fungal composition eventually stabilises and becomes similar during active decomposition among the individuals. On the other hand, bacterial diversity varies considerably per individual throughout the process.
This finding is important to us as differences in microbial diversity can influence their resistance – ability to withstand change, and/or resilience – rate of community recovery, to stressors. The bacterial composition was diverse from the start (pre- decomposition) until the end. However, the low fungal diversity from the start led to a selection process, weeding out the unfavourable species, making the fungal community quite identical over time between the individuals. Therefore, the bacterial community showed higher resistance and resilience over its fungal peers.
Changes amongst individual cadavers varying in BMI
The BMI, which relates to body height and weight, is used as the best representative in the study for its (body) composition in regards to fat : muscle tissue ratio.
Scientists claim that variations in body composition (fat, muscle, and bone) among the individuals alter resource pools in the soil, and thus influence the activity and the abundance (or presence) of decomposers. For example, in a high BMI individual having more fat than muscle, lipid metabolising microorganisms like the fungus Yarrowia lipolytica have a dominant competitive edge over other microbes. The fungi Yarrowia and Dipodascus, which adapt to variable pH, thrive over other fungi like Saccharomyces cerevisiae (bakers yeast) which stick to low pH, making their presence more noticeable.
Interestingly, although decomposition fluids varied in their bacterial and fungal composition among the body donors, none of the intrinsic factors tested managed to explain this trend.
Generally, diseased individuals are not linked to a profile with a diverse community, particularly fungi. For instance, donors with cancer have a rather uniform microbial composition and different decomposition patterns due to the disease itself, or associated therapeutic interventions (therapeutics intervene with the body physiology of the patients and influence their microflora), implying a negative impact on the necrotic community of decomposing microbes. Here, Firmicutes are more abundant than Bacteroides.
To understand the trend better, the scientists suggest to increase sample sizes, include toxicological screening, characterize the initial microbiome of the body at the time of death and perform time-series analysis of fluid communities.
Limitations and further research
BMI is an indirect measure of body composition and disregards differences like age, sex, etc during the analysis. Hence, a direct approach such as dual-energy X-ray absorptiometry (for muscle, fat and bone analysis) could be more beneficial for future decomposition studies.
An area that could be further explored is the effects of intrinsic factors on diseased individuals at their time of death, especially those that suffered from cancer. After all, the study did find that cancer (and/or its therapeutic interventions) could potentially inhibit microbial decomposers.
Other intrinsic factors like diet, ancestry and the presence of drugs were not considered during the decomposition-pattern analysis. To mitigate this, the researchers recommend conducting further studies with a larger study population and including BMI-influenced decomposition rates and patterns for taphonomy studies (study of how organic remains pass from the biosphere to the lithosphere).
Since the necrobiome (decomposing microbes) showed different dynamic changes within the small sample size of only 19 individuals, it is proposed that a larger and more diverse sample size would better explore the succession patterns of the microbes that play a big part in determining the postmortem interval estimation. In the end, including intrinsic and environmental data for larger samples would doubtlessly improve postmortem interval models.
Link to the original post:
Mason, A. R., McKee-Zech, H. S., Hoeland, K. M., Davis, M. C., Campagna, S. R., Steadman, D. W., & DeBruyn, J. M. (2022). Body Mass Index (BMI) Impacts Soil Chemical and Microbial Response to Human Decomposition. Msphere, e00325-22. DOI: https://doi.org/10.1128/msphere.00325-22
Featured image: Original image by author using craiyon.com