Exploring the World of “Touch DNA” and “Touch Microbiome”: One-Shot DNA Collection and Extraction

Microorganisms are present on both the interior and exterior surfaces of the human body, outnumbering human cells at a 3:1 ratio.1 Like human DNA shed from skin cells, microbes are also transferred to the surrounding environment.2,3 Due to the robust structure of bacterial DNA, it withstands degradation more effectively than human DNA. Given the increasing role of microbiome analyses in forensics, and the presence of enough microbial DNA in a fingerprint to generate a reliable profile, this presentation examines the transfer and recovery of touch DNA and microbiomes using 4N6FLOQSwabs.2 At a crime scene, the simultaneous collection of fingermark ridge patterns and related evidence can be challenging. When a fingermark is compromised, investigators are often advised to prioritize DNA recovery over dactyloscopy. Fingermarks frequently contain low quantities of DNA, sometimes below the 0.002ng/μL threshold for obtaining a meaningful Short Tandem Repeat (STR) profile.4 Despite this, forensic DNA analyses can still yield high-quality profiles.5-7 Additionally, a person’s microbial signature can reveal more about their lifestyle and health than human DNA alone. By analyzing both nuclear DNA (nDNA) and microbial DNA from the same swab, investigators can gain further insights, using established forensic genetics methods while maintaining the accuracy of human identification profiles. A total of 40 participants provided fingermarks of their Dominant Hand’s Index (DHI) on a standardized and sterilized glass surface. Prior to deposition, participants washed their hands and allowed natural “recharge” for one hour. Fingermarks were deposited using a cutout mold, applying 500g pressure for 60s. DNA was collected by swabbing the entire fingermark with dried flocked 4N6FLOQSwabs. NDNA extraction was done using the Qiagen QIAamp DNA Blood and Tissue kit.8 DNA quantitation was conducted using the Human Quantifiler kit on a 7500 Real-Time PCR System. Human DNA was amplified by AmpFLSTR Identifiler Plus PCR Amplification Kit on a 9700 GeneAmp PCR System. The amplified samples were processed by capillary electrophoresis and STRs analyzed by GeneMapper ID 3.2 software. Using the same extracts, the V4 region of the 16s rRNA gene was amplified and sequenced using the Illumina MiSeq System. FASTq files were analyzed in QIIME2 (ver. 2024.5.0), were quality filtered using DADA2, and trimmed accordingly. Taxonomic assignments were made using silva-138- 99. Data were further analyzed in R (version 4.2.3) with the “Phyloseq” packages. Results showed that the co-extraction of touch DNA and touch microbiome is possible, and the use of standardized conditions for the deposition and collection of the fingermark significantly improves the success of the analysis when compared with previous studies conducted in less ideal and optimized conditions.3 The microbial profiles obtained from the various fingers from the same donor are distinct one from another one but still more similar than between individuals in terms of bacterial composition, bacterial relative abundance, alpha and beta diversity. Specific taxa (ASVs) unique to specific donors were found in the deposited finger marks after excluding the environmental signature found on the glass slide, highlighting the potential use of touch microbiome as an additional tool to identify the donor of the trace found on touched objects on the crime scene. On the basis of our preliminary results, it is possible to affirm that the obtention of human DNA and microbial DNA with standardized approaches may help forensic analysts in performing a more appropriate personal identification and simultaneously obtain information about habits of a suspect.