The microbiome of fly organs and fly-human microbial transfer during decomposition

During decomposition, flies interact with the remains to lay eggs and acquire nutrients, and in the process, they bring their microbes with them. While it is known that flies have their own unique core microbiome, it is not known if flies associated with human cadavers have a different core microbiome. Differences in the fly microbiome may influence the types of microbes transmitted from the flies to the cadaver, therefore potentially affecting assembly of the human decomposer microbiome. The first purpose of this study was to characterize the microbiome of flies associated with human cadavers by fly organ and season. This is because fly interactions with cadavers vary by season, and because it is likely that external fly organs [i.e., the labellum and tarsi] make more direct contact and are likely involved in increased mechanical transmission with the cadaver than internal organs such as the oocyte. The second purpose of this study was to determine if the fly microbes contribute to the human decomposer microbiome. To accomplish these aims, 10 human cadavers were placed outdoors across three seasons and allowed to decompose. A total of 40 flies that landed on the cadaver were collected and dissected by the labellum, tarsi, and oocyte. In addition to fly collections, samples from the cadavers were collected using a sterile swab at sites including the cheek of the face, inner cheek, bicep, torso, and anus. Overall, it was shown that flies associated with human cadavers have a similar microbiome to flies from previous studies that were not associated with human cadavers. However, there are differences in the microbiome between seasons and fly parts. We also show evidence that flies act as a microbial source to the human decomposer microbiome, which is important for understanding the ecological mechanisms of human cadaver microbial community assembly.