The skin microbiota represents a veritable invisible army that inhabits our skin. Did you know that every square centimeter of our skin can harbor up to 10 million bacteria? Even more impressive, the average adult's skin harbors a trillion bacteria, or around 50 million per cm², belonging to more than 500 different species.
This skin flora is not just limited to bacteria. Indeed, our skin microbiome also includes viruses, microscopic fungi, yeasts, archaea and even mites. Together, these microorganisms form a complex ecosystem that plays a fundamental role in the protection and functioning of our skin.
In this article, we will explore in depth what the skin microbiome is, its essential functions, and how an imbalance can be associated with various skin conditions like acne, atopic dermatitis, or psoriasis. We will also see how to preserve and strengthen this precious ally to maintain healthy skin. Because understanding our skin microbiota means learning to better protect our skin.
What is the skin microbiome?
Our body is home to a complex mosaic of microorganisms that coexist in symbiosis with our cells. This microscopic world, particularly present on our skin, constitutes a fundamental element of our skin health.
Definition and composition of skin flora
The skin flora, also called skin microbiota, is the set of microorganisms that permanently colonize the surface of our skin. This microbial community forms a true ecosystem that develops from birth and evolves throughout our lives.
Unlike parasites which live solely at our expense, the micro-organisms of the microbiota maintain a symbiotic relationship with our body, providing us with essential mutual services.
This micro-ecosystem mainly includes:
- Bacteria (majority)
- Microscopic fungi and yeasts
- Viruses
- Archaea (formerly “archaebacteria”
- Mites (especially of the Demodex genus)
The density of this population is impressive: there are between 1000 and 1 million microorganisms per cm² of skin, with more than 1000 different species of bacteria. More specifically, it is estimated that between 500 and 1000 different bacterial species are associated with a human body at any given time.
Differences between microbiota and microbiome
Although often used as synonyms, these two terms designate different realities:
The skin microbiota corresponds specifically to all the microorganisms living on our skin. It is the microbial community itself.
On the other hand, the skin microbiome encompasses a broader concept: it designates not only these microorganisms, but also all of their genetic material, their functions and the environment in which they evolve. This distinction is important for understanding scientific studies on this subject.
Body areas and microbial diversity
The composition of the microbiota is not uniform throughout the body. It varies considerably between anatomical regions, creating different “microbial landscapes”. This distribution is influenced by local conditions: humidity, temperature, pH and presence of sebum.
There are mainly three types of microbial habitats:
· Sebaceous areas (forehead, back, torso): rich in sebaceous glands, they are densely populated with bacteria such as Cutibacterium (formerly Propionibacterium), capable of degrading sebum triglycerides.
· Wet areas (armpits, folds, groin): they harbor more Staphylococcus and Corynebacterium, microorganisms tolerant to salt and heat.
· Dry areas (forearms, legs): they have a greater bacterial diversity but less stable over time.
Furthermore, we observe a variable microbial density depending on the region of the body. The areas richest in bacteria are, in descending order: the hands, scalp, armpits, forehead, limbs and back.
This microbial biogeography reflects the remarkable adaptation of species to very different local conditions. Furthermore, it is interesting to note that the composition of the microbiota also varies according to age, sex, genetic and environmental factors.
The essential functions of the skin microbiota
Our skin microbiota is not just a passenger on our skin - it is an active partner that performs several vital functions. This true living ecosystem works tirelessly to keep our skin healthy and protect us from external aggressions.
Role in protection against pathogens
The skin microbiota constitutes our first living barrier between the body and the external environment. Its diversity is associated with broad colonization of the skin, which leaves little room for potentially pathogenic germs to take hold. This protection works in several ways:
First, through space occupation – commensal bacteria compete for nutrients and available space, thereby preventing pathogenic microorganisms from establishing themselves. This mechanism is comparable to already occupied land where no new arrival can settle.
Then, commensal bacteria communicate with our immune system to enhance antibody production and stimulate immunity cells. They also play a crucial role during skin injuries by attracting epidermal cells (keratinocytes) to the site and promoting their multiplication, which accelerates wound repair.
Production of antimicrobial molecules
Beyond a simple physical barrier, our skin microbiota actively produces antimicrobial substances. Certain commensal bacteria secrete bacteriocins, true “natural antibiotics” which inhibit the growth of other potentially dangerous species.
For example, Staphylococcus epidermidis produces antimicrobial peptides capable of specifically inhibiting the growth of the dreaded Staphylococcus aureus, implicated in many skin infections. Furthermore, this same bacteria also stimulates the production of antimicrobial peptides by keratinocytes, thus strengthening our natural immune defenses.
On the other hand, Cutibacterium acnes (formerly Propionibacterium acnes) acts differently by producing propionic acid, which lowers the pH of the skin environment and thus prevents the development of competing microorganisms.
Regulation of skin pH
Maintaining an acidic pH (between 4.1 and 5.8) is fundamental for the balance of our skin. Our microbiota actively contributes to this, in particular thanks to certain bacteria such as Cutibacterium acnes which secrete fatty acids helping to maintain this low pH.
This acidic pH plays a crucial role because it:
· Prevents the proliferation of pathogenic bacteria sensitive to acidity
· Preserves the integrity of the skin barrier
· Avoids dehydration and hypersensitivity
Contribution to hydration and the lipid film
Hydration and the skin microbiome have a complex symbiotic relationship. Good hydration promotes a diverse and balanced skin microbiome, while a healthy microbiome helps maintain skin hydration.
The microbiota actively participates in the formation and maintenance of the hydrolipid film, this water-in-oil emulsion present on the surface of our skin. This protective film is composed of a mixture of sebum, lipids released by keratinocytes and sweat.
Triglycerides, the most abundant compounds in sebum, are hydrolyzed by the lipases of commensal flora bacteria. This transformation produces free fatty acids (such as lauric acid and sapienic acid) which have antimicrobial properties against certain bacteria.
In addition, the microbiome participates in the production of NMF (Natural Moisturizing Factors), a set of water-soluble compounds that retain water in the epidermis, thus ensuring hydration and elasticity of the skin.
Thus, our skin microbiota not only protects us against external aggressions - it also helps to keep our skin supple, hydrated and healthy.
Factors influencing the balance of the skin microbiome
The delicate balance of our skin ecosystem is not frozen in time. There are many factors that can modify its composition and functioning. Understanding these influences allows us to better preserve this protective flora.
Age, sex and genetics
From birth, our skin microbiota begins to form and evolve. In the newborn, the flora is not very diverse and largely influenced by the method of delivery: babies born vaginally initially inherit the maternal vaginal microbiota, while those born by cesarean section develop a microbiota closer to that of the maternal skin.
At puberty, hormonal changes profoundly alter the composition of the microbiome. The increase in sebum production then promotes the development of certain bacteria such as Cutibacterium acnes, which can sometimes be associated with the appearance of acne.
Furthermore, hormonal differences between men and women also explain certain variations in the composition of the microbiota. Thus, men generally have less microbial diversity but greater temporal stability of their microbiota than women.
Our genetic heritage also plays a fundamental role in determining our microbiota. It notably influences the chemical composition of our sweat and our sebum, creating an environment that is more or less favorable to certain microbial species.
Hygiene and cosmetics
Our daily hygiene and care habits have a major impact on our skin microbiota. Cleaning too frequently or with products that are too aggressive (alkaline soaps, antiseptics) can severely disrupt the balance of the microbiome by eliminating beneficial bacteria.
Likewise, antimicrobial deodorants, by targeting the bacteria responsible for body odor, can also eliminate beneficial species and promote the development of resistant bacteria.
On the other hand, certain cosmetic products can have a positive effect on the microbiota. Formulations adapted to the physiological pH of the skin (around 5.5) or containing prebiotic ingredients make it possible to preserve and even strengthen microbial diversity.
Environment and pollution
Our skin microbiota is in constant interaction with the external environment. The climate (temperature, humidity) modifies the living conditions of micro-organisms on our skin and therefore the balance of populations.
Air pollution also poses a threat to our microbiota. Fine particles can not only settle on the skin but also penetrate hair follicles, creating an environment conducive to the development of potentially harmful species.
Stress and diet
Chronic stress can affect our skin microbiota in several ways. First, it changes the chemical composition of our sweat, creating a less favorable environment for certain beneficial bacteria. In addition, stress activates neuroendocrine pathways that can dysregulate sebum production and the skin's barrier function.
As for diet, its influence on the skin microbiota mainly involves its effect on the intestinal microbiota. A diet high in refined sugars and saturated fats promotes systemic inflammation, which can impact the skin and its microbiota. Conversely, a diet rich in fiber, antioxidants and omega-3 fatty acids appears to have a protective effect on the microbial balance of the skin.
Thus, our skin microbiota turns out to be a dynamic ecosystem, constantly remodeled by a multitude of internal and external factors.
Imbalance of the skin microbiota: causes and consequences
Sometimes the delicate harmony of our skin's microbial ecosystem breaks down. This imbalance, far from being trivial, can cause various complications for our skin health.
What is dysbiosis?
Dysbiosis refers to an imbalance in the skin microbiota, characterized either by a loss of bacterial diversity or by an excessive proliferation of certain species. This phenomenon occurs when commensal bacteria become opportunistic pathogens, disrupting the established microbial balance.
Several factors can cause this dysbiosis, including taking antibiotics, stress, an unbalanced diet and exposure to pollution. As a result, the protective biofilm that lines our skin weakens, weakening our skin barrier in the face of external aggression.
Associated diseases: acne, eczema, psoriasis
Many dermatological conditions are directly linked to specific dysbioses. Acne vulgaris, which affects up to 9.38% of the world's population, is caused by the proliferation of certain strains of Propionibacterium acnes (now called Cutibacterium acnes) which secrete lipases and proteases causing inflammation.
Atopic dermatitis, for its part, is characterized by a loss of microbiota diversity and excessive colonization by Staphylococcus aureus. This bacteria exacerbates inflammation and promotes hyperpermeability of the skin.
Furthermore, psoriasis, affecting 2-3% of the population, is also associated with an altered skin microbiome. Although considered difficult to infect thanks to the increased expression of antimicrobial peptides, this condition nevertheless seems involved in specific dysbioses.
Link between microbiota and healing
The microbiota plays a fundamental role in wound healing. A wound represents an entry point for microorganisms, requiring a complex repair process. However, certain pathogenic bacteria such as Staphylococcus aureus can cause infections, cause prolonged inflammation and delay healing.
Indeed, specific strains (notably Staphylococcus and Pseudomonas) produce virulence factors which hinder the tissue reconstruction process. Conversely, a diverse microbiome promotes healing by strengthening the skin's barrier function.
Impact on body odor
Body odor mainly results from the bacterial breakdown of the constituents of our sweat. Dysbiosis of the skin microbiota can amplify these odors, particularly in the armpits where bacteria such as Corynebacterium and Staphylococcus hominis transform sweat into odorous compounds.
At puberty, a significant reorganization of the skin microbiota takes place, explaining why the body odor of adolescents differs from that of children. If children have a rather sour odor, adolescents give off more of a sulfur odor, particularly in the armpits.
How to preserve and strengthen your skin microbiota?
Taking care of our skin microbiota is not a luxury but a necessity to maintain healthy skin. With a few adjustments to our daily routine, we can preserve this valuable ecosystem.
Good hygiene practices
Excessive cleansing risks decimating our good bacteria, leaving our skin vulnerable. I therefore avoid washing my skin more than once a day to allow the microbiota to renew itself properly. The water temperature should never exceed 37°C. Furthermore, I carefully pat my skin dry, without forgetting the folds, to avoid residual humidity which could promote imbalances.
Choice of suitable products (pH, alcohol-free)
To preserve the integrity of my microbiome, I favor:
· Gentle cleansers, without harsh sulfates or drying alcohols
· Products with a physiological pH (around 5.5), close to that of my skin
· Moisturizing textures that maintain the favorable environment for beneficial microorganisms
· Treatments enriched with emollient ingredients to restore the lipid film
I also avoid frequent use of antimicrobial products that could disrupt my skin flora.
Role of prebiotics and probiotics
Prebiotics are dietary fibers that nourish the good bacteria in our skin. Fructo-oligosaccharides and galacto-oligosaccharides are among the most common.
As for probiotics, these living microorganisms such as Lactobacillus and Bifidobacterium, they contribute to the balance between good and bad bacteria. Their application in topical treatments helps to naturally rebalance the skin microbiome.
Link with the intestinal microbiota
Our skin microbiota partly reflects the health of our intestinal microbiota. The gut-skin axis constitutes a bidirectional communication system where metabolites produced by intestinal bacteria influence inflammation and skin barrier function.
To support this balance, I favor a diet rich in fiber, colorful vegetables, fatty fish and fermented foods. Some studies show that oral probiotics can also improve skin condition by balancing the gut microbiome and reducing inflammation.
Conclusion
Our understanding of the skin microbiota has evolved considerably in recent years, revealing its complexity and importance for our health. This invisible but omnipresent ecosystem plays a fundamental role in protecting our skin against pathogens, regulating skin pH and maintaining optimal hydration.
Imbalances in this microbiome can certainly contribute to various skin conditions like acne, eczema or psoriasis. Conversely, a balanced and diverse microbiota promotes healthy and resilient skin. Our modern lifestyle, characterized by excessive use of antimicrobial products and constant exposure to pollutants, unfortunately puts a strain on this delicate balance.
Our skin microbiome therefore represents a precious ally that must be preserved. To do this, I adopt gentle hygiene practices, I choose products with physiological pH and I avoid too frequent or aggressive cleaning. Furthermore, the use of treatments containing prebiotics or probiotics can also help to strengthen this ecosystem.
Notably, the health of our skin is closely linked to that of our intestine. Thus, a balanced diet, rich in fiber and fermented foods, supports not only our intestinal microbiota but also, by extension, our skin microbiota.
Ultimately, taking care of our skin microbiome is not just a simple aesthetic gesture. This approach constitutes a holistic approach aimed at preserving the natural balance of our skin. Our skin protects us every day - let's learn to protect the billions of microscopic organisms that, in turn, protect it.
