Text: Stéphanie de Roguin
Photo: DR

"You can identify individuals based on their unique features of their microbiomes"

French science philosopher Thomas Pradeu explains how bacteria make up a part of who we are.

Science tells us that intestinal bacteria are unique to each individual and represent one of the most conclusive ways of identifying someone. Is this method even better than DNA analysis?

Over the past 15 years, the scientific community has come to understand that we can be identified by factors other than our genes. Our intestinal microbiome is one example, as are the microbiomes of our lungs and skin. In the early 2010s, the scientific community started to discuss a number of applications based on the unique characteristics of each person’s microbiome. The microbiome’s relevance to forensics in particular was a subject of much debate. This method consists of identifying individuals based on the unique features of their microbiomes. While twins share the same DNA (except for a few mutations), their microbiomes can contain a number of differences depending on where they live and their personal history. However, this doesn’t mean that genes are no longer useful in understanding what makes us unique individuals. Recent discover-ies only add to our knowledge base about the incredibly diverse range of elements that influence what happens inside us. What is revolutionary is how much our knowledge has improved when it comes to the many factors that determine our identity and individuality.

What other factors define us and make us unique?

In addition to our microbiome and DNA, other factors include epigenetics, the nervous system, and the immune system. The microbiome is considered separate from the body, while the other four are thought to be a part of it. How can we objectively make the distinction (which is often intuitive) between what is intrinsic to the body and what is separate from it? Research into the topic is currently under way. In some cases, can the body transform these external features into internal ones by incorporating elements that originally appeared foreign? These questions lead us to a philosophical understanding of the individual by defining it in both temporal and spatial terms.

Do we still need to distinguish between nature and nurture?

In the 1970s, some people were already speaking out about how this distinction wasn’t useful any more. However, this concept keeps coming up in current research. Generally speaking, biological phenomena are thought to include some innate and some acquired elements. The line between nature and nurture is actually quite thin, even if we have a hard time letting go of the distinction because of our education. In her exceptional book, the American psychologist Susan Oyama described the “ontogeny of information”, emphasising that even DNA is shaped by a person’s context and environment. For example, nowadays, developmental biology is studying how the microbiome can activate certain genes that play an important role in the body’s development. The microbiome can be transmitted vertically between a mother and her child – this is especially the case in insects.

In humans, babies have microbiomes that are distinct from those of their mothers after 18 months. However, the fact remains that the mother’s microbiome influences the foetus during a pivotal stage in his or her development.

Is each person’s identity a combination of what makes them unique, what they share with others, and who they are intrinsically?

Our identity is fashioned by the contact that we have with the outside world. When someone gets a vaccine or interacts with a micro-organism, his or her immune system adapts and changes as a result of this new stimulus. Our identity is also based on our social influences, like our friends and teachers. This is where psychology and sociology intersect with biology, which is rare. The other can become a part of you. In some cases, for example, the immune system can recognize a part of the microbiome, such as a bacteria or virus, then come to tolerate it. It can even play an essential role in the body.

Why is it so important for us to know “who we are”?

This question is a fundamental part of philosophy. It means knowing what defines humanity and discovering what sets us apart from other humans, what makes us unique. Throughout time and depending on the philosopher in question, we have defined humanity through our ability to laugh, reason or use language. The concept of each person being a unique individual has existed since ancient times but became especially popular in sixteenth- and seventeenth-century philosophy. People have always felt a need to understand themselves, find meaning in their lives and realize that they are a part of nature. All the environmental issues that are so popular today come from this same need to understand. Will humanity soon have to pay for all the damage it is doing to the world? The idea of giving purpose to our lives, which we know are temporary, while also standing out from our peers can be seen in the traditions and signs we adopt to show we belong to a specific group. However, these same signals also include a distinct feature. For example, we might get a tattoo to show we belong, but set ourselves apart by using a design that shows our individuality. It is in this narrow space between belonging to a collective and separating oneself from others that the identity of every human being is fashioned. ⁄



Thomas Pradeu is the director of research at CNRS in Bordeaux, where he conducts research into the philosophy of science at the ImmunoConcept Laboratory (UMR5164). In 2010, he published “L’identité: la part de l’autre” in collaboration with the immunologist Edgardo D. Carosella.