June 17th 2024.
The world of crime scene investigation may soon see some major changes. When you think of a criminal investigation, you likely imagine detectives meticulously gathering and analyzing physical evidence found at the scene: weapons, bodily fluids, footprints, and fingerprints. However, this is just the start of the process to piece together what happened and who was involved in the crime. At the core of this process is the "principle of exchange," which was developed by French criminologist Edmond Locard in the early 1900s. This principle states that "every contact leaves a trace," and it serves as the foundation for reconstructing the events of a crime.
During Locard's time, these traces were typically visible elements that could be seen with a magnifying glass or microscope, such as pollen, sand, and fibers. However, this type of evidence has its limitations as it is not always directly associated with a specific individual. Our latest research has shown that the bacteria population on a person's skin can leave a unique trace on the clothes they wear, and these traces can last for months, making it possible to identify the wearer.
When investigating a crime scene, imagine finding a victim and a piece of clothing that does not belong to them. While pollen or sand may help determine where the clothing came from, how do you identify the owner? Skin cells, hairs, and bodily fluids are all potential contenders, but another unique identifier is the community of microorganisms living on and inside our bodies. These microbes are specific to different parts of the body, can persist for long periods, and can be transferred to other people and the environment, making them valuable in forensic investigations.
The field of forensic microbiology emerged in the early 2000s as scientists sought ways to defend against bioterrorism. Today, it is used to identify individuals after death, understand their health before they died, determine the cause and timing of death, and even determine their origin. In short, we have updated Locard's principle to say that "every contact leaves a microbiological trace."
While this principle has been established, there is still much to learn about the transfer and persistence of an individual's microbiome in their surroundings. We also need to understand how different surfaces may affect microbial populations and how these traces may be contaminated by other items or the environment. In 2021, our team described the "touch microbiome," which refers to the unique bacterial populations on an individual's skin. We also studied how these bacteria can be transferred and persist on non-porous surfaces for up to a month in uncontrolled indoor settings.
In our most recent study, we collaborated with researchers from the University of Central Lancashire in the UK and the University of Eastern Piedmont in Italy to improve the potential for individual identification from clothing. We had two volunteers wear cotton T-shirts for 24 hours in Australia, and then we placed the shirts in a controlled environment for up to six months, alongside unworn control items. Samples were taken from both the worn and unworn shirts at various time points and frozen for analysis.
We found that each volunteer transferred distinct and recognizable microbes onto the clothing, unique to each individual. We could also distinguish between the worn and unworn items, even after an extended period. The microbiome remained stable on the worn garments for up to 180 days. Furthermore, we observed the transfer of specific bacteria from the worn items to the unworn ones stored closest to them, indicating the possibility of microbe transfer between items.
Clothing at a crime scene can provide crucial evidence for investigations. It can help identify individuals by revealing indicators of gender, occupation, income, social status, political, religious, or cultural affiliations, and even marital status. Additionally, it can provide insight into the manner of death, location of the crime, and sometimes even the time since death. Our research shows that clothing can provide even more evidence than previously thought. The discovery of unique microbiomes capable of identifying individuals from clothing marks a significant leap forward in the field of forensic science.
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