Since the success of the book " The discreet charm of the intestine " by Giulia Enders, and that well-known yogurts were said to do it good, we know that there is an intestinal microbiota . But we are in fact hosts to many other microbiotas, including that of the skin. However, we are gradually discovering that the skin microbiota intervenes in many mechanisms of the skin.

About ten years ago, researchers made a major discovery: we are only half human! What does that mean? As Richard L. Gallo, from the Dermatology department at the University of California, San Diego, explains: "Each of us is actually a community made up of 50% human cells and 50% microorganisms." These microorganisms, which constitute microbiotas, are present everywhere in our body: there are microbiotas in the intestines, but also on and in the skin, in the vagina, the mouth, on the scalp, etc. Microbiotas, these invisible living ecosystems, are in fact part of our identity and are unique to us, just like our DNA, and we live in symbiosis with them.

All of these microorganisms weigh about 1.5 kg for a 75 kg person.
Researchers are now trying to understand the skin microbiota, its interactions with skin cells, its influence on certain dermatological pathologies and how it can influence a new approach to cosmetics.

Microbiota or microbiome?

"The microbiota is the set of living microorganisms detectable on the skin (bacteria, viruses, yeasts), the microbiome is their entire genetic heritage," explains Richard Martin, who has been working on the subject for many years. Indeed, it was by deciphering the human skin genome twenty years ago that Professor Richard Gallo discovered that of the microbiota. Because it is their DNA imprint that allows them to be identified and recognized.

The skin microbiota, an ecosystem in symbiosis with our skin…

It was called skin flora when we did not yet have the adequate tools to study it in detail. It is in fact a real ecosystem that lives on and in the skin, composed of probiotics: mainly bacteria (75-80%), but also viruses (10-20%) and yeasts (5-10%). Each cm² of skin is covered with a million of these microorganisms, or 100 billion in total, which either collaborate or compete. In fact, these numerous microorganisms are not only on the surface, because they slip into all the crevices, from the pores to the pilosebaceous follicles. The most represented are cutibacterium and staphylococcus epidermidis, but there are about 1000 varieties. They live in perfect symbiosis with our cells and feed on skin secretions (lipids from sebum, proteins from dead cells, sugars, water from sweat, etc.).

…which varies depending on areas, age and environment

Transmitted by the mother during vaginal delivery, the skin microbiota is enriched and evolves with each major hormonal change (adolescence, pregnancy, menopause/andropause). But it also differs depending on the part of the body (scalp, armpits, arms, etc.), because, as with plants , some varieties acclimatize better to dry environments (arms, legs), others to humid regions (mucous membranes, armpits, folds, etc.), or oily regions defined by the quantity of sebum (inside the ears, scalp, face, etc.). Age, but also gender and ethnic origin, cause the “populations” to vary. External factors also have a great influence: temperature, humidity, pollution ,diet , and even the people, animals and plants we live with influence its composition. Thus, we do not have ONE but MANY skin microbiotas.

A determining role in skin health

When the balance is broken, pathology sets in.

This discovery leads to the need to rethink our relationship with skin care . "Twenty years ago, we thought that viruses and bacteria caused diseases. However, we now know that the health of the skin depends on the balance between these probiotics within the skin microbiota," explains Richard L. Gallo. In fact, as soon as one variety takes up too much space or takes over, it creates an imbalance called dysbiosis, which often leads to inflammation or even pathology. Indeed, we now know that eczema, acne or dandruff in particular are linked to an imbalance in the microbiota caused and accentuated by what characterizes our post-industrial life: pollution, taking medication, stress , UV rays, excessive hygiene and certain cosmetic components . A study conducted in Hong Kong, comparing 204 women living in two Chinese cities, one polluted, the other not, showed that the microbiota of women living in a polluted environment and showing signs of premature aging strongly resembled that of older women. It is therefore very likely that the microbiota also influences skin aging.
However, a healthy microbiota – and therefore healthy skin – is a microbiota with good biodiversity where the families of bacteria balance each other, “get along” well and interact favorably with the skin cells.

How to maintain a healthy microbiota?

• Limit periods spent underwater, i.e. not too long or too hot: because water has a basic pH, greater than 7, which destabilizes the microbiota.
• Washing with the right products : the discovery of these "beneficial bacteria" has changed the "hygienic" vision that we have had for over a hundred years. No more question of "bactericidal" products at all costs. Because shower gels with overly aggressive surfactants unbalance the microbiota which takes over 4 hours to rebuild after each shower. So, one shower per day maximum, and with the gentlest possible cleanser.
• No soap or product with the wrong pH: the famous Marseille soap, or any other soap, has a basic pH (above 7). However, the skin has an acidic pH. We therefore prefer a superfatted bar. Similarly, we choose products called "physiological pH" and not "neutral pH", which means that it respects the pH of the skin while a neutral pH is 7.
• Do not touch your face too much so as not to bring in “external” and potentially pathogenic bacteria.
• Avoid cosmetics that are too aggressive or at least limit their use, such as grain scrubs, cosmetics that contain a high level of alcohol or fruit acids.
• Remove make-up gently, without being harsh, without rubbing, avoiding the aggressiveness of cotton wool . And in the morning, do not remove it: a suitable floral water is enough.

“Cosmebiotics” to preserve the microbiota

The discovery of the skin microbiota also leads to a new vision of cosmetics. Now, we look a little more closely at cosmetic formulas and try to formulate them in such a way as not to disrupt it. To do this, we limit or even eliminate certain ingredients that could cause an imbalance, such as mineral oils and silicones, but also emulsifiers (which foam like sulfates) and emulsifiers (which bind emulsions, creams or fluids). Preservatives have also been blamed, but it would seem that they ultimately have no impact on the microbiota. A step further, some brands seek to regulate and rebalance the skin microbiota. To do this, we now find prebiotics, probiotics and postbiotics in certain ranges:

• Probiotics : these are the living microorganisms (bacteria, yeasts in particular) that make up the microbiota and promote its good balance. But because a cream is not a yogurt, it is not kept cool and it is not consumed immediately after opening, it cannot contain real living probiotics. However, for marketing convenience, cosmetic brands indicate "with probiotics". These are in fact fragments of probiotics deactivated either by heat or by fermentation that are recognized and assimilated by the skin's probiotics. Most of the time they are the same as those used in food for the intestinal microbiota: lactococcus, bifidobacterium, lactobacillus, etc. They have an anti-oxidant and anti-inflammatory role and help to reconstitute an ecosystem beneficial to the microbiota.
• Prebiotics : these are simple or complex sugars, fatty substances or amino acids, which nourish and promote the healthy development of skin bacteria. For the microbiota, prebiotics, particularly interesting in cosmetics, act a bit like a fertilizer. They are found in more and more products. The most common is inulin, from chicory. There is also yakon. But we are gradually discovering that certain ingredients, long used in cosmetics, also have a prebiotic role: this is the case for glycerin, but also squalane and polyphenols.
• Postbiotics : these are molecules secreted by probiotics (their waste products, so to speak, metabolites or membranes of these probiotics), which mimic them and promote their good activity. They also promote the ecosystem in which a healthy microbiota lives. Lactic acid is one of them. But some dermo-cosmetic brands develop their own, tailor-made, often from bacteria cultivated in their thermal water, to target a specific problem: acne, atopy, dry skin, etc.

If we are only at the beginning of cosmetics linked to the skin microbiota, it is because the discovery of the skin microbiota is only about ten years old, and even if its study is progressing rapidly, we are far from understanding all of its mechanisms and the extent of its influence.

Not to mention the discovery of the links between the microbiotas of the brain, intestine and skin, which are closely connected. Cosmetics personalized according to the microbiota are still (science)fiction, the application of live bacteria can only be done in the laboratory… But taking into account the balance of the skin microbiota by hygiene & care products is a step forward that contributes to making formulas cleaner .