How Insects and Bacteria Can Solve Murder Mysteries

Methods

Summary

Data Collection

1. Establish patterns of taphonomic changes for decomposition for indoor versus outdoor decomposition.

2. Establish species identity for bacterial microbiome community of southeast Texas specific to stage of cadaver decomposition for indoor versus outdoor decomposition. One of the biggest challenges, historically, with identifying bacterial species from environmental samples is that identification has been a long and difficult process. We will use high-throughput sequencing techniques to test our hypotheses related to the microbial community structure on and in the decomposing cadaver. Identification of the bacteria present on the decomposing cadavers will be examined using methods consistent with those that have been developed for the NIH Human Microbiome Project (HMP).

3. Establish species identity of carrion insect of southeast Texas specific to stage of cadaver decomposition for indoor versus outdoor decomposition. Cadavers will be visited four times at approximately the same time each day. Cadavers will be handled as little as possible to reduce disturbance to the invertebrate community and insects will be collected in numbers that will not affect the decomposition process. Cadavers will be photographed at each collection point to produce a timeline of decomposition. Insect specimens will be first photographed and then collected in a variety of ways. Adult flies will be collected via sweep-netting and hand sampling. Adult insects will be killed with acetone and stored in the freezer until curation. Specimens will either be pinned or mounted and then labeled. Specimens will be identified in-house. All insects from this study will be added to the database and deposited in the Sam Houston Entomology Collection for future reference.

4. Establish correlation between bacterial succession and insect recruitment and abiotic factors. For each cadaver studied, a timeline of state of decomposition, abiotic factors (i.e, temperature, relative humidity, UV radiation, and rainfall, all available from the weather monitoring device), bacterial (i.e., intrinsic, soil, fly) and insect species will be created.

Challenges

Though the strength of this proposal lies in its ability to test human cadavers decomposing under experimental conditions, this approach is not without challenges. It is difficult to generate replicates as a means to decrease sampling error as cadavers are often a limited resource. We propose using three cadavers for each experimental condition per trial to in effort determine if variable community structure is due to individual cadaver variation.

Pre Analysis Plan

Understanding the identity of bacteria and insects will provide insight into how a cadaver decomposes. By accounting for the influence of abiotic factors, such as decomposition indoors, these data will have later significance when incorporated into models that will aid in the determination of the postmortem interval by providing a more precise method of analysis. These data will be used as markers for comparison when evaluating change in the microbiome as decomposition progresses. In the future, scientists will likely be able to distinguish the successional pattern of bacteria and insects based upon abiotic factors. The novelty of this research will allow for the development of a predictive biodiversity program.

Currently at SHSU, we are developing a pilot computer program with a built-in insect species analysis that uses environmental factors surrounding a cadaver, as well as cadaver state (if known) to predict insect fauna that should be present. The proposed research will allow us to incorporate bacteria and gasses, as well as solar irradiance measurements, into this model. After substantial time has passed during decomposition, physical and chemical analyses are not as accurate as insect data to predict the TSD. No current studies within the field of forensics address the statistical predictive capability of environmental variables for decomposition. Ultimately, the ability to predict species diversity based on ecological parameters could some day enable investigators to estimate the time of death of cadavers more accurately by accounting for bacteria, insects, and environmental factors.

This proposed study would be the first set of controlled experiments comparing the rates of decomposition of human cadavers as well as the succession of bacteria in indoor and outdoor settings. This ecosystem approach to the study of decomposition is consistent with the call by the National Research Council’s Committee on Identifying the Needs of the Forensic Sciences (2009) to strengthen basic research in forensic science.

All data will be presented at national meetings and in peer-reviewed scientific journals. Funding from this proposal will be used to attend The American Academy of Forensic Sciences. Additional national meetings, not funded by this grant, where data will be presented will include The American Society for Microbiology General Meeting, The Entomological Society of America Annual Meeting, and The North American Forensic Entomology Association. Results from this funded proposal will be published in The Journal of Forensic Science and Forensic Science International. Additional smaller publications may be published in Annals of the Entomological Society of America, The Journal of Medical Entomology, Journal of Bacteriology, and Applied and Environmental Microbiology, as deemed appropriate.