The human microbiome: an emerging tool in forensics

Advances in sequencing technology have enabled DNA profiling to become a staple in criminal forensics. Short tandem repeats (STRs) embedded in an individuals’ genetic code enable authorities to take advantage of biological variability to accurately identify and discriminate among people. According to the National DNA Index System (NDIS), CODIS, a DNA database containing more than 12 million profiles, has assisted in more than 340,000 criminal investigations in the USA (CODIS NDIS Statistics, n.d.). However, this still represents a small percentage of total crimes committed. Indeed, many criminal cases still go unsolved despite advances in DNA profiling; for example, in 2015, only 20% of residential burglaries (>1.5 million) were resolved by authorities, according to the 2015 FBI Uniform Crime Reporting statistics (Clearances, n.d.). This can be explained in part by resource allocation, as burglaries are not prioritized for investigation compared to other higher profile crimes (Par e et al., 2007; Coupe, 2016), which can lead to significantly reduced response times, resulting in crime scene evidence contamination or destruction, further impeding investigative efficiency. Therefore, there is a need to improve the lines of evidence that can be acquired to link perpetrators to the crime scene. Improving trace evidence options for criminal investigations is a major focus for forensic research specialists globally. One possible option that has recently emerged encompasses the symbiotic microorganisms that reside in and on our bodies. The NIH-funded ‘Human Microbiome Project’ (HMP) has significantly improved the scientific and public recognition of the vital importance of symbiont ecology to host health and development (Consortium, 2012; Meth e et al., 2012; Grice, 2015). There are approximately as many bacterial cells in our body as human cells (Sender et al., 2016) and the compliment of bacterial taxa, especially at the subspecies level, appears to be unique to each person (Zhu et al., 2015) offering a compelling opportunity to develop a new identifiable marker unique to the individual. The microbiome is even unique in identical twins (Goodrich et al., 2014), theoretically offering an opportunity to increase identity resolution over that possible with human genome evidence. However, the microbiome changes over time in an individual (Oh et al., 2016), so how can it be used to identify a person? While the relative proportions of the bacteria do indeed change, the composition of the community appears to be relatively stable (Caporaso et al., 2011; David et al., 2014), although this stability and continued identifiability are areas of active research. Interestingly, the fluctuations in the structure and composition of the microbiome may contain useful information that could also be used for forensic purposes. Host lifestyle, including diet, occupation, travel, and pharmaceutical use, can influence the composition and structure of microbiome. This suggests that profiling the microbial community in and on our body could also help to reveal details about an individual’s lifestyle (Gonzalez et al., 2016; Kuntz and Gilbert, 2017), which could represent new trace evidence. Profiling the microbiome may be useful in identifying a person or their lifestyle characteristics, but for burglary, the microbiome of the perpetrator would need to be detected at the crime scene, in their absence, while retaining the identifiable characteristics. In support of this, we know that humans shed ~30 million bacterial cells into their vicinity every hour (Qian et al., 2012) and researchers have already demonstrated the forensic potential of the microbiota left behind by people on physical surfaces. For example, the bacterial community found on your finger tips (microbial fingerprint) could be traced on a keyboard, so that which keyboard, and even which keys, a person used could be identified based on the bacterial residue (Fierer et al., 2010). Furthermore, mobile phones carry the personal microbial signatures of the owner (Meadow et al., 2014; Lax et al., 2015). Importantly, these are just preliminary studies, and the results and conclusions cannot be used to justify the application of microbial sequencing to forensic studies. However, the statistical basis for the accurate matching of a person to their microbiota, and evidence that a residual microbial fingerprint could be used to discriminate individuals, does suggest that in the future, it may be possible to use these profiles for forensic investigations. Yet, however intimate the association, because microbial compositions can shift with environmental factors and over time, they cannot be definitively equated