Investigation of direct and indirect transfer of microbiomes between individuals.

The human microbiome encompasses the fungi, bacteria and viruses that live on, within, and immediately surrounding the body. Microbiomes have potential utility in forensic science as an evidentiary tool to link or exclude persons of interest associated with criminal activities. Research has shown the microbiome is individualised, and that personal microbial signatures can be recovered from surfaces such as phones, shoes and fabrics. Before the human microbiome may be used as an investigative tool, further research is required to investigate the utility and potential limitations surrounding microbial profiling. This includes the detectability of microbial transfer between individuals or items, the associated risks (such as contamination events) and the applicability of microbial profiling for forensic purposes. This research aimed to identify whether an individual's distinguishable microbiome could be transferred to another individual and onto substrates, and vice versa. Paper, cotton, and glass surfaces were chosen to represent a range of substrate matrices. The study involved six participants placed into three pairs; participants took part in two modes of transfer. Transfer Mode 1 involved the pair shaking hands, followed by rubbing a substrate in their right hand. Transfer Mode 2 involved individuals rubbing a substrate in their left hand, swapping substrates with their partner and then rubbing the swapped substrate in their left hand. 16S rRNA sequencing was performed on the extracted microbial DNA from participant and substrate samples. Quantitative Insights into Microbial Ecology 2 (QIIME 2) was used for sequence quality control and beta (between-sample) diversity analyses and taxonomic assignment. Principal Coordinate Analysis (PCoA) based on Jaccard distances was visualised through Emperor software to determine the phylogenetic similarity of bacterial communities between participants and among participant pairs. Statistical testing through PERMANOVA revealed significant differences in the Jaccard distances between each participant pair (P < 0.001), highlighting not only the potential distinguishability of skin microbiomes among individuals, but also the clustering effect observed between participant pairs due to the potential transfer of hand-associated microbiomes between individuals. The study demonstrated that transfer of the human skin microbiome had occurred between all participant pairs, regardless of substrate type or mode of transfer.