banner by Dheeraj Jalluri and Helen Richard

It's Microbial, My Dear Watson

by Helen Richard

“All rise.” The bailiff’s voice booms through the courtroom. The courtroom echoes with the sound of people rising to their feet. The judge enters and sits before everyone else sinks back down into their seats. The trial begins. The prosecution begins their case by introducing its first piece of evidence, Exhibit A: the microbial DNA analysis of the defendant, John Doe.

Many people are familiar with some iteration of this scene through the proliferation of procedural crime shows, which have appeared on television in the last decade. How, they might not recognize that microbial DNA analysis is a potential game changer in the field of forensic science.

This emerging form of forensic analysis is based on investigating the traces of microbes, which are primarily comprised of bacteria, left at crime scenes. While DNA and fingerprints are supposed to provide proof beyond a reasonable doubt, sometimes this is not the case. DNA can be contaminated, denatured or too small to be studied. Fingerprints, on the other hand, can be subject to circumstantial bias and on occasion have been misinterpreted.

Considering these limitations and problems of evidence often considered to be “infallible,” what do microbial signatures bring to the table, or the autopsy table for that matter?

For starters, people have unique microbial signatures. Each microbial signature evolves as someone moves through their surroundings due to the exchange of bacteria between the environment and the person. This means that a pair of identical twins who might share the same DNA can be distinguished with the help of microbial DNA analysis since it’s unlikely they would do everything and go everywhere together. Even differences in diet will alter a person’s microbial signature.

Additionally, a microbial signature is almost indestructible. Unlike DNA or fingerprints, it cannot be cleaned from the scene of the crime. In fact, it’s noted by Claude Roux, the director of the University of Technology Sydney Centre for Forensic Science, that “when you remove traces, you add new ones.” While the bacteria a criminal takes with them from the scene of the crime can change quickly, they’re also leaving behind millions of cells, creating a sort of breadcrumb trail for microbiologists. This is a promising result, as microbial DNA analysis can potentially bear the burden of proof that slips through the cracks of regular DNA analysis and fingerprinting. One such example is if a perpetrator used a public computer to hack into a server. The microbes that he or she leaves behind could tell investigators exactly who did it, even if 10 people touched the keyboard after the cyber attack. The rate at which the bacteria evolve with each new individual that uses the keyboard creates a timeline to filter the suspects down to the actual culprit.

While this technique isn’t quite ready for regular use by law enforcement, there is at least an indication that it can help start investigations off on the right track. Currently, the premier researcher on the topic, Dr. Jack Gilbert, and his team are working to create a database with the goal to tie “skin microbes to diet, race, health or workplaces, giving investigators a rough sketch of their suspect.” The face of forensics is changing and while it may drastically change the plot of our favorite crime shows, it holds promise in increasing the efficacy of the justice system, which is a very worthy cost to pay.