New forensic technique: personal bacteria as the new fingerprint?

Bacteria can be found everywhere, but did you know we each have different bacterial colonies living on our skin? With the help of their distinctive qualities, perhaps they can aid us in solving crimes.

The study of forensic science can be traced back to ancient periods, most evidently with mummification during 3000 BC. The removal of organs was sought to be an example of an autopsy. It wasn’t until the late 19th century that this branch of science started to kick off after the development of fingerprint matching to help solve crimes. As we continue to learn and evolve, a new possible era of using bacterial communities for forensic identification is slowly coming to light. In comparison to the traditional fingerprint method, this new technique has been observed to be more readily available due to bacteria’s ubiquitous nature (found everywhere).

For a long time, forensics has commonly been associated with DNA analysis, alongside the fingerprint method. DNA analysis uses the remnants of the perpetrator’s biological traces such as skin tissue, blood, or even hair. Although these techniques are very effective, they all have one common major setback, that is, viable samples may not always be present. For example, in the case of fingerprints, they are more easily retrieved from smooth surfaces. Therefore, this limits investigations when it comes to traces left on rougher surfaces.

In light of these limitations, scientists have discovered a new possible forensic technique using bacteria. Due to its ability to easily transfer itself through the slightest touch, this favorable trait makes it easier to collect samples, further supporting its forensic possibility.  In addition, it aligns perfectly with Locard’s principle (also known as the basic principle of forensics), which states that “every contact leaves a trace.”

Image by the Author, created with BioRender.com

Skin bacterial communities differ between individuals. Residual skin bacteria left on an object can be used for forensic identification by matching these bacteria to the skin bacteria of the person who touched the item.

Similar to fingerprints, each individual has unique bacterial communities living on the surface of their skin, with only ~13% of hand bacterial communities being shared between individuals. These bacterial communities are also known to be stable over time and can be regenerated within hours after washing one’s hands. In other words, these bacterial communities do not change readily. Using this concept, a study was conducted by Noah Fierer and his colleagues in 2020 to examine the possibility of using bacterial skin communities to identify individuals.

The authors designed a study assessing the similarity between bacterial communities on an object touched by an individual and the individual’s finger. Fierer and his colleagues indicated this by matching bacteria left on individual keys of a keyboard to the fingers of the keyboard’s owner. They found that the bacterial communities on an individual’s fingertip and the keys of their keyboard were more similar than other users’ fingertips and keys. This was verified through the generation of scatter plots which were able to perfectly group the bacteria on the individual’s fingertip and keys. This meant that the skin bacterial community had a ‘high degree of interindividual variability.’ In other words, they are very personalized, which allows for individuals to be differentiated for identification.

Adapted from Fierer et al. (2020)

Bacterial communities found on individual keyboards (the filled colour shapes) and the fingers of the owners (the unfilled colour shapes) match one another. The plot shows the degree of similarity between bacterial communities on the fingertips of three individuals and their keyboards. Points closer together indicate that samples have similar bacterial communities.

Additionally, some bacteria are highly resistant to environmental stress such as humidity. The authors also found that bacteria left behind on an object were able to remain viable for identification for up to two weeks at standard room temperature. This experiment was done by swabbing the skin surface of two individuals and either freezing the swabs at -20°C or leaving them in open containers at room temperature. After two weeks, Fierer and his colleagues realized that the bacterial composition did not change, making them still effective for individual identification. However, their experimental design was under the condition that the object remains untouched.

Although this concept is relatively new, it illustrates the potential that skin bacterial communities could have in forensics, primarily because bacterial composition remains unchanged for days after contact. Further research would be necessary to identify the effectiveness of overlapping contact and the frequency of contact with the object needed to provide accurate results. But regardless, skin bacterial identification for forensics does show promising results.

Overall, the use of bacteria for forensic identification poses an excellent alternative to other traditional methods. Many crimes go unsolved due to the possible lack of DNA samples for identification. With the ease of obtaining bacteria and a unique bacterial community for every individual, bacterial identification could work hand in hand with other forensic techniques to help provide better safety and justice in the world. Perhaps in a few decades or so, we will begin to see the implementation of bacteria to help keep our community safe.

Learn more about bacteria in forensics
  1. Noah Fierer: Forensic use of hand bacteria (YouTube video)
  2. The Forensic Microbiome: The Invisible Traces We Leave Behind

About the Author

Maurene Chin is a fifth-year undergraduate student majoring in Neuroscience and Human Biology at the University of Toronto. She dreams of pursuing a career in Dentistry to help individuals restore their confidence and healthy self-esteem. In her spare time, she loves reading web novels, and during the quarantine, she also found interests in baking and gardening. Maurene likes to go by the motto ‘Life is too short to hesitate, you never know until you try (that is, as long as it’s not dangerous).’

Featured image: Sajad Baharvandi on Unsplash (license).

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