Research paperA probabilistic approach to evaluate salivary microbiome in forensic science when the Defense says: `It is my twin brother'
Introduction
The suitability of salivary microbiota composition to support discrimination between closely related individuals has been investigated in [17]. Microbiota profiles have shown high individuality and stability over time and may represent a valid and precious contribution for forensic investigations when standard genotyping DNA methods fail (e.g. due to degraded material or to close relatedness among individuals, such as homozygous twins). Previous studies have used microbiomes to differentiate an individual from a limited group of people (see, e.g. [6]).
Beta-diversity (β-diversity) indices (such as the Jaccard distance) are commonly used in microbiota studies to highlight taxonomical differences between pairs of samples [10]. They have been implemented to overcome the dimensionality problem that renders the quantification of the ressemblance between microbiota profiles problematic. Their adequacy and feasibility for discrimination between hypotheses of forensic interest have been discussed in [17].
The assessment of the value of a score quantifying the similarity between measurements originating from questioned material and from control material for discrimination purposes is an open problem, as reiterated by [6]. The use of a cut-off value against which such score is compared in order to support or even suggest a conclusion regarding their origin (e.g. whether compared saliva traces originate from the same individual or from closely related individuals) is often suggested. Routine examples come mainly from the toxicology domain where a scientist is often asked to classify a controlled substance in a blood or hair item into a predefined category (e.g. doping athletes, abusive drinkers, and so on). However, as it will be discussed in Section 5, such an approach presents severe limitations and leaves the problem of forensic interpretation open. Most importantly, it renders a binary response, no matter the effective similarity (as measured by a beta-diversity indece) between the compared material. In the current paper, a probabilistic approach to evidence evaluation is proposed. The most efficient way to provide a quantitative measure of the support of the evidence (i.e., a β-diversity distance) to competing hypotheses about the origin of a given salivary material is given by the Bayes’ factor that expresses, in probabilistic form, the ratio between the probability to observe the evidence given each of the hypotheses of interest.
The paper is structured as follows. The scenario of interest and the proposed probabilistic approach are introduced in Section 2. Available data and the chosen statistical models are described in Section 3, while the results of statistical analyses are presented and discussed in Section 4. Section 5, finally, concludes the paper.
Section snippets
A probabilistic approach
Consider a case where a saliva trace is collected from a receptor (e.g. a given object on a crime scene). The salivary microbiome of the trace, as well as that of a saliva sample taken from a known source (say, Mr. X) for comparative purposes, is thus analyzed with the aim of discriminating between the following two competing hypotheses: .
Hp: the saliva trace originates from Mr. X;
Hd: the saliva trace originates from the twin brother of Mr. X,
Data
A longitudinal study on 30 pairs of homozygous twins (29 couples and 1 triplet) has shown the potential of salivary microbiota-based profiles for discrimination between closely related individuals [17]. Four salivary samples have been collected from each participant: the first sample was taken during the first inclusion visit, and the rest in correspondence of the follow-up visits that have been planned 1, 12 and 13 months after inclusion. DNA was then extracted from the native saliva samples
Data analysis
All comparisons, graphical representations and statistical analyses were conduced in RStudio (Version 1.2.5033), supported by the BANOVA package [7].
Conclusion
The discrimination between individuals based on their salivary microbiome profiles has been investigated. Individual profiles have shown high individuality and stability. The fact that self comparisons characterizing the same individual tend to have a modest distance if compared with intra-pair (or inter-individuals) comparisons does not however allow one to conclude that a low value of such distance is suggestive to say that the compared material originate from the same individual. This can be
Acknowledgment
The authors would like to thank the Swiss National Science Foundation for its support through grant n. 10531C-170280.
References (19)
- et al.
Critical analysis of forensic cut-offs and legal thresholds: A coherent approach to inference and decision
Forensic Sci. Int.
(2018) - et al.
Relatedness and DNA: are we taking it seriously enough?
Forensic Sci. Int.
(2005) - et al.
Setting bounds for the likelihood ratio when multiple hypotheses are postulated
Sci. Justice
(1998) - et al.
Integrating the microbiome as a resource in the forensics toolkit
Forensic Sci. Int.: Genet.
(2017) Evaluating DNA profiles in the case where the defence is it was my brother
J. Forensic Sci. Soc.
(1992)Expert evidence and forensic misconceptions of the nature of exact science
Sci. Justice
(1996)The scientist and the scales of justice
Sci. Justice
(2000)- et al.
Statistics and the Evaluation of Evidence for Forensic Scientists
(2021) Interpreting DNA evidence: can probability theory help?
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