Exploring the forensic skin and 'touched' bacterial community through full-length and variable region sequencing of the 16S rRNA gene.

The human skin microbiome is unique to each individual, and the microbial exchange between human skin and their touched objects could aid in tracing individuals. However, the microbial transfer and deposition on transiently touched items, and their forensic potential application remain insufficiently understood. Therefore, we simulate a scenario where a suspect holds a knife for a brief time to investigate the transfer of skin-associated microbiota from the hand to the touched object's surface using sequencing of the 16S rRNA gene full length and variable region. The results showed the full-length 16S rRNA gene sequencing (V1-V9) achieved significantly higher taxonomic resolution (species-level) and enriched α-diversity in both skin and contact-objects microbiomes compared to the variable region (V3-V4) sequencing approach. The skin bacterial community exhibited greater biodiversity than the "touched" bacterial communities, and there was no significant similarity between the skin and "touched" bacterial communities for each individual. The intra-individual β diversity of "touched" bacterial communities was close to 1, while the β diversity within individuals (ranging from 0.7 to 0.9) was significantly lower than that observed between individuals in the skin bacterial community. Skin bacterial communities were individual-specific, and the efficacy in discriminating individuals through individual-specific amplicon sequence variants was 100 % using the support vector machine model, in both of the two sequencing datasets. Individual-specific amplicon sequence variants were observed within high relative abundance bacterial taxa (e.g., Staphylococcus epidermidis, Streptococcus mitis, and Streptococcus parasanguinis), but the number of skin individual-specific amplicon sequence variants detectable in "touched" bacterial communities was low. Only a small fraction of skin-derived taxa was detectable on objects and detectability couldn't be explained by robust individual-specific signatures. The dominant bacterial genera and species identified in the shared amplicon sequence variants between skin and "touched" bacterial communities were among the relatively high-abundance taxa within the skin microbiome, like Streptococcus, Staphylococcus, Corynebacterium, Staphylococcus epidermidis, Streptococcus mitis, Corynebacterium bovis, Streptococcus parasanguinis, etc. Such high relative abundance bacterial species showed both individual-specific, easy-to-transfer, and easy-to-deposit in touch events, which may be new marks in forensic identification.