Metagenomic analyses of bacteria on human hairs: a qualitative assessment for applications in forensic science

Abstract

Background: Mammalian hairs are one of the most ubiquitous types of trace evidence collected in the course of forensic investigations. However, hairs that are naturally shed or that lack roots are problematic substrates for DNA profiling; these hair types often contain insufficient nuclear DNA to yield short tandem repeat (STR) profiles. Whilst there have been a number of initial investigations evaluating the value of metagenomics analyses for forensic applications (e.g. examination of computer keyboards), there have been no metagenomic evaluations of human hairs-a substrate commonly encountered during forensic practice. This present study attempts to address this forensic capability gap, by conducting a qualitative assessment into the applicability of metagenomic analyses of human scalp and pubic hair.

Results: Forty-two DNA extracts obtained from human scalp and pubic hairs generated a total of 79,766 reads, yielding 39,814 reads post control and abundance filtering. The results revealed the presence of unique combinations of microbial taxa that can enable discrimination between individuals and signature taxa indigenous to female pubic hairs. Microbial data from a single co-habiting couple added an extra dimension to the study by suggesting that metagenomic analyses might be of evidentiary value in sexual assault cases when other associative evidence is not present.

Conclusions: Of all the data generated in this study, the next-generation sequencing (NGS) data generated from pubic hair held the most potential for forensic applications. Metagenomic analyses of human hairs may provide independent data to augment other forensic results and possibly provide association between victims of sexual assault and offender when other associative evidence is absent. Based on results garnered in the present study, we believe that with further development, bacterial profiling of hair will become a valuable addition to the forensic toolkit.

Keywords: 16S DNA; Bacteria; Forensic; Metagenomics; Next-generation sequencing; Pubic hairs; Scalp hairs; Sexual assaults.

Figure 1

Principal coordinate plots (PCoA). Clustering of microbial taxa from each individual at each collection time point. The lilac circle represents post-SI bacterial sequences, whilst the pale blue and yellow circles represent non-SI bacterial sequences—both circles relate solely to the co-habiting couple. Panel A represents pubic hair microbial taxa from male (orange) and female (red) participants. Panel B represents scalp hair microbial taxa from male (green) and female (blue) participants. Panel C represents microbial taxa present in male and female scalp and pubic hair samples.

Figure 2

Microbial data extracted from scalp and pubic hairs. Diagrams illustrating core and transient (Tr) bacterial taxa on male and female scalp and pubic hair samples.

Figure 3

Personalised microbial data. Diagrammatic summary of unique bacterial taxa found in male and female scalp and pubic hair samples. Male individuals left to right: Individuals M2 (cohabiting male), M3 and M7. Female individuals left to right: Individuals F1, F4, F5 (cohabiting female) and F6.