The principle of “every contact leaves a trace,” formulated by French criminologist Edmond Locard, has long been fundamental to forensic science. Traditionally, these traces were visible particles like pollen, sand, and fibers, but such evidence often lacks a direct link to a specific individual. Recent research, however, shows that the unique population of bacteria on a person’s skin can leave microbial traces on clothing that last for months, providing a novel way to identify the wearer.

Microbial traces offer new forensic possibilities in crime scene investigations. For instance, if a piece of clothing found at a crime scene does not belong to the victim, traditional evidence such as skin cells or hair might provide clues. However, the unique community of microorganisms that live on and within the human body can also serve as a valuable identifier. These microbes are specific to different body parts, can persist for long periods, and transfer to clothing, other people, and the environment, making them particularly useful in forensics.

Forensic microbiology began in the early 2000s, originally focused on defending against bioterrorism. Today, it plays a broader role, helping to identify individuals, determine their health status before death, and understand how and when they died. The updated version of Locard’s principle could be described as “every contact leaves a microbiological trace.” The concept of the “touch microbiome” was first described in 2021 by researchers Procopio and Gino, along with colleagues from the University of Central Lancashire and the University of Eastern Piedmont. This unique bacterial population found on individuals’ skin was shown to persist for up to a month on non-porous surfaces like glass slides. The team also analyzed DNA from samples of long-frozen bodies, identifying microbial populations linked to the manner of death and the decomposition stage, suggesting that microbial signatures can help solve cold cases when DNA extracts are still available.

In recent work, researchers studied microbial traces on clothing, which are often collected as evidence at crime scenes. Cotton T-shirts were worn by two individuals for 24 hours and then stored in a controlled environment for six months. Samples from these worn and unworn T-shirts were frozen, shipped to Italy for microbial DNA extraction, and then sequenced in the UK. Results revealed distinct microbial signatures on the worn garments, unique to each individual. The microbiome remained stable for up to 180 days, and there was evidence of microbe transfer between worn and unworn items stored close together.

Clothes collected from a crime scene can reveal much about the people involved, including indicators of gender, occupation, income, and cultural affiliations, as well as clues about the crime itself, such as the manner of death and the location. 

The discovery that unique microbiomes can be used to identify individuals marks a significant advancement in forensic science, providing yet another valuable tool for reconstructing events and establishing identities.

This content is republished from The Conversation (original article can be found here). 

Original authors: Misha Ketchell (Editor, The Conversation)

Note content has been edited