‘What are the limits/potential of using the skin microbiome in forensic identification?‘
Refers to: http://aem.asm.org/content/83/22/e01672-17
- Back in 2010, one of the first papers to moot the idea of using skin microbiome in forensic identification compared and found a statistical correlation between microbiome signatures recovered from nine computer mice and the palm surface of the dominant hand of their owners ( ).
- A more recent study on 25 to 105 samples from different body sites also mooted the forensic idea ( ).
- Meantime, the latest study referenced in the question included sampling a mere 12 healthy individuals ( ).
Could a case for forensic application of skin microbiome be made on such slim pickings? Available evidence suggests not since skin microbiome varies both across body sites as well as across time while microbiome analysis itself remains unstandardized, method standardization being an essential attribute of forensics.
Skin Microbiome Varies Across The Body & With Time & Age, A Crucial Limitation For Forensic Application
Forensic signatures require individual differences to be consistent, considerable and reproducible. We are far from such certainties when it comes to the human microbiome. Where the skin is concerned, studies show skin microbiome varies in different parts of the body as well as with time/age/infection (, , , , , , ). This implies full body sampling of an individual would be necessary to match microbiome found on a given surface. How practical and feasible is such a proposition in routine evidence analysis, which would likely not be limited to what’s detected on computer mice?
Thus, merely examining the research portion of the question yields a near-insurmountable obstacle. Zooming out to look at the larger perspective, two other intrinsically intertwined issues also loom large to make clear forensic microbiome analysis may still be firmly rooted in the realm of science-fiction.
Both Forensics & Microbiome Analysis Still Lack a Basic Attribute of the Scientific Method, Rigorous Quality Control
One, the fact that, popular TV shows notwithstanding, forensics is not a science but rather a motley bag of tools shaped not by dictates of the scientific method but by the imperative of problem-solving as mandated by the justice system, with the most critical proviso being in the words of Nathan J. Robinson in the Boston Review (see below from)
‘Forensic science works when prosecutions are successful and fails when they are not.’
The very basis for forensics thus clashes with the very premise of science itself, namely, (see below from),
‘neutral, open-ended inquiry’
Seen in such a light (see below from),
‘…one of the key problems with evaluating forensic science. The measures of its success are institutional: we see the failures of forensics when judges overturn verdicts or when labs contradict themselves. There is a circularity in the innocence cases, where the courts’ ability to evaluate forensic science is necessary to correct problems caused by the courts’ inability to evaluate forensic science. At no point, even with rigorous judicial review, does the scientific method come into play. The problem is therefore not that forensic science is wrong, but that it is hard to know when it is right.’
Recognizing it to be a potentially fatal weakness, the US(NAS) has long advocated the need for basic quality control across all types of forensics ( ). However such warnings have gone largely unheeded even as some of the scientifically more dubious forensic techniques such as hair or bite-mark analysis have recently lost their patently thin and brittle sheen of impartial, rigorous evidence analysis. As Robinson points out in his 2015 critique (see below from , also Google FBI hair analysis if interested in other contemporaneous news reports),
‘This past April, the FBI made an admission that was nothing short of catastrophic for the field of forensic science. In an unprecedented display of repentance, the Bureau announced that, for years, the hair analysis testimony it had used to investigate criminal suspects was severely and hopelessly flawed.’
Similar high-profile reversals have tarnished the validity of bite-mark analysis (, , ).
Even techniques such as DNA analysis, presumed to rest on arguably more scientifically rigorous basis, haven’t escaped serious and valid criticism (; also see below from , emphasis mine),
‘DNA failures can border on the absurd, such as an incident in which German police tracked down a suspect whose DNA was mysteriously showing up every time they swabbed a crime scene, from murders to petty thefts. But instead of nabbing a criminal mastermind, investigators had stumbled on a woman who worked at a cotton swab factory that supplied the police. That case may seem comical, but a 2012 error in New York surely doesn’t. In July of that year, police announced that DNA taken off a chain used by Occupy Wall Street protesters to open a subway gate matched that found at the scene of an unsolved 2004 murder. The announcement was instantly followed by blaring news headlines about killer Occupiers. But officials later recanted, explaining that the match was a result of contamination by a lab technician who had touched both the chain and a piece of evidence from the 2004 crime. Yet the newspapers had already linked the words “Occupy” and “murder.” The episode demonstrates how the consensus surrounding DNA’s infallibility could plausibly enable government curtailment of dissent. Given the NYPD’s none-too-friendly disposition toward the Occupiers, one might wonder what motivated it to run DNA tests on evidence from protest sites in the first place.’
Systematic analysis of even a historical mainstay of forensics such as fingerprint analysis is still a rarity and, when undertaken, shows clear need for improvement (see below from, emphasis mine)
‘The interpretation of forensic fingerprint evidence relies on the expertise of latent print examiners. The National Research Council of the National Academies and the legal and forensic sciences communities have called for research to measure the accuracy and reliability of latent print examiners’ decisions, a challenging and complex problem in need of systematic analysis. Our research is focused on the development of empirical approaches to studying this problem. Here, we report on the first large-scale study of the accuracy and reliability of latent print examiners’ decisions, in which 169 latent print examiners each compared approximately 100 pairs of latent and exemplar fingerprints from a pool of 744 pairs…Five examiners made false positive errors for an overall false positive rate of 0.1%. Eighty-five percent of examiners made at least one false negative error for an overall false negative rate of 7.5%. Independent examination of the same comparisons by different participants (analogous to blind verification) was found to detect all false positive errors and the majority of false negative errors in this study. Examiners frequently differed on whether fingerprints were suitable for reaching a conclusion.’
Two, the intrinsically intertwined part of forensic quality control, procedures to detect and analyze the microbiome remain unstandardized (), a key reason why results from different microbiome studies routinely contradict each other. Obviously, forensic application of any tool is impossible without method standardization, which is still far from the norm for microbiome studies including those sampling skin ( , ).
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