Cold Case Experiment Demonstrates the Potential Utility of Aquatic Microbial Community Assembly in Estimating a Postmortem Submersion Interval

Abstract

Microbial community assembly (MCA) of both human and nonhuman animal carcasses provides indicators useful for estimating the postmortem interval (PMI) in terrestrial settings. However, there are fewer studies estimating postmortem submersion intervals (PMSIs) in aquatic habitats. No aquatic studies to date assessed MCA in the context of a death investigation, with all previous studies focusing on important basic ecological questions. Within the context of a cold case investigation, we performed an experiment using replicate adult swine carcasses to describe postmortem MCA variability within a nonflowing aquatic habitat. Using high‐throughput sequencing of carcass postmortem microbiomes, we described MCA variability and identified key taxa associated with decomposition in an aquatic habitat similar to the cold case body recovery site. We also modeled key taxa for estimating PMSIs, modeling within ±3 days (mean square error) postmortem using random forest regression. Our findings show significant changes in microbial communities as decomposition progressed, and several taxa were identified as important indicator taxa which may be useful for future estimates of PMSI. While descriptive, this study provides initial findings quantifying MCA variability within a nonflowing aquatic habitat. Within the context of the cold case investigation, we discuss how postmortem microbial samples collected at the time of body recovery could have been an important piece of evidence for understanding the PMSI of recovered remains. Additional experimental studies are needed to explicitly test and identify mechanisms associated with postmortem MCA variability in other habitats and under different temperature (e.g., seasons) conditions.

Microbial community succession on submerged vertebrate carcasses in a tidal river habitat: Implications for aquatic forensic investigations

“…These findings corroborate those by Cartozzo et al (16) but also augment them since we examined entire carcasses over a short period and where they examined two types of bone over a longer period. Our findings suggest that habitat‐specific taxa, i.e., along a fresh to brackish water continuum commonly observed in a tidal‐influenced river system (56) as well as how they change in relative abundance during decomposition (e.g., describing habitat‐specific MCA) can be utilized to predict a PMSI through the use of machine learning algorithms as demonstrated by Kaszubinski et al (15) and Cartozzo et al (16). The increased use of machine learning modeling like we have done here illustrates how powerful this tool can be for elucidating complex patterns in microbial community datasets and underscores how a diverse microbial community, as well as the reproducibility of microbial community succession, can be used to predict PMI in terrestrial environments and PMSI in aquatic ecosystems (57).…”

“…Compared to studies conducted in terrestrial habitats, only a few have investigated microbial community succession using high throughput sequencing on carcasses in aquatic habitats (13)(14)(15)17), and none from intertidal or estuarine ecosystems. The few aquatic studies using carcass microbiome assembly for predicting PMSI have been in freshwater small streams (13,14) and an isolated freshwater pond (15,16). Thus, there is limited knowledge on how specific, and important, abiotic factors such as salinity have on the endogenous microbial communities representing a certain type of aquatic habitat (e.g., freshwater stream compared to a pond or the estuary of a river).…”

“…What is less understood is how different aquatic habitats affect microbial succession on decomposing organic matter, and this is especially true for carrion. Indeed, this is the first study to characterize microbial taxonomic succession of vertebrate carcasses in tidal river habitat, with only four in freshwater (13)(14)(15)(16)(17) and one from a purely marine system (17) The interplay between riverine discharge and tide amplitude may influence the bacterial populations in tidal influenced rivers (50). We In some instances (e.g., advanced FD and sunken remains stages) there was overlap in day and ADH range; this was due to variability in decomposition among the carcasses over the course of decomposition found that salinity fluctuated daily and may have influenced, which bacterial taxa colonized the carcass over the decomposition process.…”

“…Historically, research suggests that microbes may be a more suitable metric than insect activity for predicting PMSIs based on their critical role in biofilm development associated with decomposition of plant organic matter (20)(21)(22)(23)(24), and influence on vertebrate carrion decomposition, energy flow, and nutrient cycling in both freshwater and marine ecosystems (25)(26)(27)(28). More recently, microbial research interests have shifted to address both human and wildlife forensic questions such as the postmortem interval (PMI) for terrestrial systems (29)(30)(31) and the PMSI for freshwater and marine systems (13)(14)(15)(16)(17)32). However, there are no studies to our knowledge of using microbial communities in PMSI estimates for estuarine or other tidally influenced ecosystems.…”

The applicability of forensic time since death estimation methods for buried bodies in advanced decomposition stages

“…Nevertheless, this initial work with a limited PLOS ONE sample set demonstrated that both community structure and some individual bacterial taxa could be used to classify sample time point categories (e.g., PMI) with reasonable accuracy; however, there was not sufficient statistical power to perform regression models to better represent a continuous timeline. While we were limited to modeling individual time points and not along a continuum, the descriptive characterizations (e.g., community diversity and taxa relative composition) did show qualitative successional changes over time that could be potentially useful in identifying individual taxa as important to indicators of large differences in time since burial, much like that done recently in an aquatic habitat [75]. Such information has potential forensic utility especially in cases when other forms of evidence (e.g., insects) are not available.…”