SSOs Succession As A Potential Indicator For PMI Estimation

Background:The estimation of postmortem interval (PMI) of putrescent corpses has long been a hotspot and difficulty in forensic practice. It’s well known that sarcosaphagous insects are closely related to the progress of postmortem putrefaction, while the act of bacterial communities has been overlooked and few studies can be referred during the past thirty years. Actually, there are bacteria called specific spoilage organisms (SSOs) acted on the whole progress of postmortem putrefaction are major components of the decomposition processes, which has played an important role on the study of postmortem bacteriology.Purpose:This study aimed to investigate the microbes involved in decomposition in different surroundings, to determine the types of SSOs with inferred important value, and to evaluate the potential use of bacterial succession for postmortem interval (PMI) deduction.Methods:Using Miseq high-throughput sequencing the bacterial succession of rat corpses was investigated preliminarily. In this study, the comparative sequence analysis of the Ml-sample (PMI=0h), the M2-sample (PMI=24h) and the M3-sample (PMI=216h) was conducted. Based on the results, Bacterial culture method and16S rDNA amplification were conducted to thoroughly study the SSOs succession. SD rats were placed in an isolated environment and open environment. Samples were collected at fixed positions for bacterial culture regularly. The complexity and identity of microbe inhabiting the specific sites for particular PMI was determined by using culture-independent biochemical methods and16S rDNA amplification. After collecting adult bodies with clear PMI and identifying the distribution of SSOs, the analogy between SD rats and adults was studied. Results:81different types of bacteria were obtained in taxonomic levels by Miseq high-throughput sequencing, and Streptococcus_sp and Pasteurella_pneumotropica have dominanted the oral site, respectively accounted for more than25%of the sequence number. After death, most bacteria decreased and disappeared, while only Sporosarcina and Staphylococcus showed increasing trend and become the dominant bacteria, and then the diversity of bacteria significantly decreased. SSOs succession on mouth cavity, conjunctival, and vaginal of SD carrions, respectively under isolated and un-isolated situation, was separately studied thoroughly. Overall, seven orders (Fungi, Staphylococcus, Eubacteriales, Actinomycetaceae, Pseudomonadaceae, Lactobacillus, Enterobacteriaceae),15families and38species of bacteria were identified by using culture-independent biochemical methods and16S rDNA amplification.8families and30species of bacteriawere found in adult cadavers, and the species and quantities of SSOs were much higher than the healthy adult.Conclusion:By Miseq high-throughput sequencing, comparative analysis of the samples found that the oral bacterial diversity of rat body decreased significantly after24hours, and the site was predominated by a few bacterial. Staphylococcus is always the common dominant bacteria among different sites during the SSOs succession, which is promising to the key of studying forensic microbiology. It is full of regularity among Streptococcus suis I, Neisseria lactamica and Enterococcus faecalis. Escherichia coli and Proteus mirabilis have showed significant succession regularity and specificity in the corpse decay process, building a relationship between the bacteria succession and the actual PMI deduction. At last, lots of similarities between the cadavers of human and rat have been confirmed to further prove the universality of conclusion reached.