Microbial Marine Decomposition of Human and Animal Remains

The process of human decomposition in marine environments, particularly that of partial human remains, is a significantly understudied area of forensic taphonomic research. Since partial remains lack the enteric microflora – which drive normal putrefactive post-mortem changes – carried by complete cadavers, they may undergo a completely different progression of decompositional change. Furthermore, very little is currently known about the roles of exogenous microorganisms in the marine decomposition process, despite the fact that these organisms are the primary decomposers of organic matter in the sea. A clear understanding of the marine taphonomic processes involved in bringing about specific post-mortem changes of partial and whole cadavers under different sets of known environmental conditions is, however, essential for the accurate reconstruction of post-mortem histories and post-mortem submersion interval (PMSI) estimation of recovered remains. This research aimed to explore the taphonomic and ecological roles of marine bacteria in the degradation of submerged human and animal (pig) remains, to describe progressive changes in marine bacterial community composition, and to assess the potential utility that the successional pattern of bacterial community change has for PMSI estimation of recovered remains. Partial pig (Sus scrofa domesticus) remains were used as models for partial human cadavers. The pigs were submerged in two coastal marine locations, Otago Harbour and Wellington Harbour, during several different seasons. These partial remains exhibited distinct patterns of progressive post-mortem decompositional change, from which several clear decomposition stages could be derived. The decomposition characters pertaining to each of these stages were indicative of the contribution made by colonising marine bacteria to the decay of soft tissues, thus exemplifying the role of exogenous marine bacteria as effective agents of aquatic taphonomic change. Notable on the submerged remains, was the progressive development of a microbial biofilm with increasing submersion time. In order to explore the diversity of the abundant members of this colonising bacterial community, a largely DNA-based, molecular approach was taken. The complementary methods of phylogenetic analysis of cloned 16S rRNA gene sequences and T-RFLP profiling of bacterial community composition were applied. Together, these approaches revealed that diverse marine bacterial groups colonised the submerged remains and rapidly formed complex communities. A successional pattern of colonisation took place over submersion time, which was repeated for decomposition events during different seasons and in the two different geographic locations. Entry of the carcasses into the water forced a significant disruption to the endogenous bacterial community on the skin and these bacteria were quickly displaced by marine phylotypes. Dynamic shifts in bacterial community composition were observed; the stepwise nature of which, indicated that successive colonisation events were taking place over submersion time. Temporally-specific bacterial phylotypes were identified for each of the major successive phases, which constituted potentially useful markers of PMSI. However, many phylotypes were found to be seasonally or geographically specific. This indicates that the derivation of bacterial colonisation data for each season and in distinct geographic areas may be required if accurate PMSI predictions are to be made using bacterial succession patterns. Changes to the population of dominant bacterial groups, each with distinct ecological functions, occurred within the decomposer community after successive submersion intervals. Overall, members of the γ-Proteobacteria, along with Flavobacteriales, were abundant early colonisers, while several phylotypes belonging to the Bacteroidales dominated during the mid-late submersion period. This study is the first to describe in detail the colonisation and taphonomic effects of exogenous bacterial action on submerged mammalian remains for application to forensic investigations involving decomposed remains recovered from coastal marine environments. It is also the first to describe successive changes in bacterial community composition for potential use in PMSI estimation. This novel concept of PMSI estimation has distinct advantages over the current, unreliable method based on visual inspection of the gross morphology of recovered remains. The patterns of bacterial successional colonisation are repeatable and therefore predictable, and can be applied not only to whole bodies, but also to parts of bodies, which can result from dismemberment or taphonomic processes acting on the body post-submersion. Potential application of the method also extends to freshwater environments. Although many aspects of marine bacterial successional colonisation remain unknown, this research has shown that considerable potential exists for its use in forensic cases of PMSI estimation and paves the way for future studies in this exciting new area of forensic taphonomic research