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Microbial Communities on Skin Leave Unique Traces at Crime Scenes

Investigators in two NIJ-supported studies have demonstrated that people carry unique microbial communities on their skin, and traces of those communities, left on touched objects, can be linked to the individual.
Date Published
December 10, 2018

When a crime scene is searched for evidence, new research indicates that by swabbing surfaces that were likely touched by the person who committed the crime, investigators can find an identifiable skin microbial signature the person left behind. Although the work is preliminary and considered a "first step," one of the researchers noted that the work has demonstrated that "skin microbes can ... serve as individual evidence in a variety of scenarios."

Two related NIJ-supported studies evaluated the possibility of using an individual's skin microbiome — a community of microorganisms that inhabit a specific environment — as a form of trace evidence from evidence found at a crime scene. The first study, led by Dr. Rob Knight of the University of California, San Diego, examined whether the sequence in which surfaces are touched by an individual, along with the number of times the surfaces are touched, influences the detection of the person's microbiome.

The study also looked at whether an individual's skin microbial signature is recoverable from an object that had been touched by multiple people, and how long such signatures last on a surface. And finally, the researchers collected samples from deceased individuals at crime scenes and later took samples from the bodies at morgues to determine if the signatures changed after death.

The genetic signatures of these microbes were captured by swabbing and then extracting DNA from the bacteria in each of the samples. The 16s ribonucleic acid (RNA) gene was sequenced so comparisons could be made between bacteria from different samples.

The researchers found that they could distinguish individual microbial signatures, but the results were highly variable based on what type of surfaces were touched (plastic and ceramic surfaces yielded the best results), and how many times they had been touched. Surfaces that had been touched 20 or 30 times gave the best results, and the researchers found that the signatures could persist for at least one day.

The researchers concluded that their data about the "transferability, stability, and individuality of human skin microbiomes" provided "basic information about the conditions under which criminal investigators can match an individual's skin microbial signature to objects or surfaces at crime scenes."

A follow-on study staged two mock residential burglaries to determine if researchers could distinguish the burglars from the residents of the homes. "We sampled the hand and nasal microbiome of 'home occupants' (including cats or dogs) and of the 'invaders', and up to 10 home surfaces," the researchers said. The surfaces included doorknobs, counter tops, and miscellaneous surfaces. After the residents left the home, two researchers, playing the role of burglars, entered and for 30 minutes touched as many random surfaces as they liked. One was wearing nitrile gloves, and the other was not wearing gloves.

Following each 30-minute burglary, another team, wearing shoe covers, facemasks, and hair covers, entered and began swabbing surfaces. Bacteria were gathered, RNA was sequenced, and the researchers tried to distinguish the burglars from the residents.

Analysis showed that some samples could be traced to an individual, "that individual being the possible burglar or entrant to the crime scene," the researchers said. This implies that in the future, extra caution should be taken to protect crime scenes from contamination by police, crime scene personnel, and others, the scientists said.

The researchers concluded that sampling for microbes with the routine swab method is not too onerous and relatively inexpensive, and probably should be conducted at most crime scenes.

About This Article

The research described in this article was funded by NIJ awards 2014-R2-CX-K411 and 2015-DN-BX-K430, with lead researchers from the University of California, San Diego, and the University of Chicago, respectively. This article is based on the grantee reports "Evaluating the Skin Microbiome as Trace Evidence" (pdf 14 pages) by Rob Knight, and "Characterizing Microbial Assemblages as Trace Evidence as Following Residential Burglaries" (pdf 10 pages) by Jack Gilbert.

Date Published: December 10, 2018