There is a huge and diverse array of articles on the Secret Life of Skin platform, with many focusing on the unique compliment of microbes that live on and within our body.
But what happens when these microbes leave us?
The importance of surfaces
Interestingly, some microbes make our skin itch, and subsequent scratching causes these microbes, still attached to dead skin, to be shed. In one of the previous Secret Life of Skin articles – Skin Mycobiome and Fungal Acne – it is nonchalantly suggested that certain keratinophilic fungi, which are commonly found in the skin microbiome, soil and on many surfaces, could have strategically evolved to encourage us to scratch so that we deposit skin on surfaces. The strategy being that this skin debris is the fungi’s harvest for their food supply.
The pandemic has made us hyperaware of the role of surfaces as conduits for microbes. The SARS-CoV-2 virus, the natural habitat of which is the inner body, can remain viable on non-porous surfaces for days to weeks. When a person touches a surface, they deposit sebum, sweat, skin microbes, skin cells and a whole cocktail of biological molecules from human DNA to enzymes. The next person who touches the same surface will pick up some of this mixture and – in the case of SARS-CoV-2 – can then go on to transfer the viable virus to their nose or eyes, where it can enter the body and go on to cause COVID-19 in some individuals [1].
In forensic science, as Locard’s exchange principle famously says, ”…whatever he touches…will serve as a silent witness against him”. Every time we touch the office door, our phone, computer keyboards, the mouse, pens, staplers, coffee mugs and so on, we leave skin microbes. In the forensic world, the microbiota, which is readily transferred from skin to surfaces, is known as the ‘touch microbiome’, and human DNA deposited from skin onto surfaces is known as ‘touch DNA’.
Trace microbial signature versus trace DNA
Extensive research funded by the National Institute of Justice (NIJ) in America confirms that each of us has a distinct microbial signature, which is left on the objects we touch. In a recent NIJ-supported study – titled ‘Surveying the Total Microbiome as Trace Evidence for Forensic Identification’ – the principal scientist, Associate Professor Yong Jim Lee of Albany State University, used a variety of different materials to swab human traces from common office equipment. Working with 7 individuals, it was shown that the microbial profiles in the recovered samples could be linked to the individuals that touched each item. Human DNA was not included in the study, and so it was concluded that microbial trace evidence alone could be used for forensic identification [2].
Another small pilot study, this time with 11 volunteers, supported the claim that the touch microbiome could be reliable even when touch DNA failed. Human and microbial DNA isolated from fingerprints left on glass and stored for 30 days at room temperature was then sequenced and quantified. The researchers used the twenty-first genetic marker (the amelogeningene, AMEL), 16 human short tandem repeats (STR) and the fourth hypervariable (V4) region of the 16S rRNA gene, and the Illumina MiSeq platform. They showed that only 5 of the 22 STR profiles could be typed from the touch DNA, whereas a microbiome profile was obtained for 20 out of 22 touch microbiome samples. Six common skin microbiome taxa were identified, as well as one taxon unique to the donor, which could be used to identify the individual [3].
Touch microbiome identification – in practice
A burglary was deliberately set-up and carried out by a local sheriff’s office in the US to investigate whether perpetrators could be identified using their touch microbiomes in a real-life situation. Drawers were opened and rifled through, a coke was taken from the fridge and the TV was unplugged and removed. Later, forensic specialists took swabs from the surfaces that were most likely to have been handled. Just like traditional fingerprints, all the touch microbiomes of legitimate residents, including the cat, had to be taken so they could later be eliminated.
Before we go on with the results of this particular burglary, let us introduce the use of databases…
Databases
Since 1840, fingerprints have helped link criminals to the scene of their crimes, and today there are huge and easily referenced databases for fingerprint matching. But unlike the touch microbiome, fingerprints must be matched to identify the linked individual. As the touch microbiome is influenced by gender, age and lifestyle, touch microbiomes can be used to help build pictures of the culprits to help police find the offenders. It is envisaged that in the future, like fingerprints and DNA profiles, these highly individual touch microbiomes will also be stored in databases for reference purposes.
However, because the skin microbiome is dynamic, with a constant gradual ebb and flow between species, as well as the potential for the microbiome to be dramatically affected by antibiotics, a touch microbiome database would be more useful as a data source for identification of the changes in the touch microbiome due to lifestyle, location, health and other attributes. Such databases would not only be helpful in the search for offenders, but they would also be very useful to dermatologists and cosmetic scientists who are looking for the microbiome elements to target, so they could help skin achieve its optimum health. By improving the accuracy of predictions for such changes, touch microbiome databases could become invaluable tools for researchers in many different disciplines.
In the case of the set-up burglary, the touch microbiomes were analysed by Professor Jack Gilbert, then at the University of Chicago (UC), and compared with UC’s skin microbiome database, which is based on specific taxonomic groups in the skin microbiome of a few thousand people. He correctly predicted from the database that one intruder drank at least 10 units of alcohol a week and the other took medication for migraines [4]. These are early days for the touch microbiome in forensics, but it is nevertheless an exciting area of work. It is amazing that we are already able to identify a person’s gender and describe how they live, their species of pet, the particular medicine they take, how much alcohol they drink and – no doubt – much more, just from the trace microorganisms left on surfaces.
The future
Analysis of touch microbiomes using targeted 16S rRNA or shotgun metagenomic sequencing has its drawbacks. In the future, these limitations will be overcome by using techniques such as the more focused clade-specific markers, i.e. the strongly conserved coding sequences, originating from the clade’s ancestor’s genome, and common to all the members of the clade. This approach, along with supervised machine learning, would more accurately match touch microbiomes to individuals.
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References
- Meyerowitz, E. A. et al. Transmission of SARS-CoV-2: a review of viral, host, and environmental factors. Ann. Int. Med. 174, 69–79 (2020).
- Knight, R. et al. Evaluating the Skin Microbiome as Trace Evidence: Final report to the National Institute of Justice Grant no. 2014-R2-CX-K411 (NCJ, 2018).
- Procopio, N. et al. “Touch microbiome” as a potential tool for forensic investigation: a pilot study. J. Forensic Leg. Med. 82, 102223 (2021).
- Kupferschmidt, K. How your microbiome can put you at the scene of the crime. Science (8 March 2016).
