The microbiome and the skin: COSSMA

The microflora, or as we now call it more precisely microbiota, is in fact the assemblage of microorganisms which are present in a defined environment3. The human microbiota is composed of a huge diversity of bacteria, yeasts, fungi and viruses, and their total number is estimated to be 10,000 different species. Bacterial communities are extraordinarily complex, and this applies to the bacterial communities of our body as well, inhabiting all of our organs, including the gut, the mouth, the nose, the vagina, skin and even blood. These organs provide various conditions for bacterial life – such as moisture or dryness, available nutrients, pH and temperature.

For instance, typical bacterial compositions have been identified as altered lung microbiomes that are associated with cystic fibrosis, chronic obstructive pulmonary disease, or asthma. There are also associations between unbalanced intestinal tract microbiomes and individuals with cystic fibrosis and diabetes^{4, 5}.

Our skin provides many niches with different ecological conditions (humidity, pH, available nutrients) and is also exposed to a large variety of external parameters, such as outside temperature, UV-exposure, pollution and even contact. This wide variety of skin ecosystems results in many differences in cutaneous microbial composition and abundance depending on the body area, and all of this constitutes the skin microbiome^{6, 7}.

Elizabeth Grice and Julia Segre in 2011 did a publication on the diversity of the microbiome on the skin, our largest organ8. Microbes on our skin also communicate with our body,9 and in particular play an important role in our immune system, as explained by Julia Segre ¹⁰. As a prolific expert in the field, Richard Gallo also demonstrated the link between cutaneous microbiota and skin health¹¹.

Probiotics are living microorganisms that are supplied to the gut (via ingestion) to rebalance an altered microbiome and modify its equilibrium towards a healthier one. The classical Bifidobacterium or Lactobacillus we can find in yoghurt, for instance, are typical probiotics. Prebiotics, on the other hand, are food ingredients that promote the growth of beneficial microorganisms in the intestines³.

“The skin’s composition is the consequence of our genetics, diet, life style and the area we live in”

Dr Bernd Becker, Consultant, MolCareConsulting

Beauty and the skin microbiome

The skin microbiome is without doubt a full player in our beauty and our well-being. Understanding the direct effects of cosmetic ingredients on the skin microbiome links the skin microbiome directly to the attainment of beauty. But how do we assess it, how do we evaluate the impact of a cosmetic ingredient or end-product on the Stratum microbium?

At companies such as Givaudan Active Beauty for example, the R&D team is experienced in both techniques – assessing the skin microbiome and evaluating the impact of ingredients on it. After all there is a particular knowhow on cutaneous microbiota sample collection, as well as proprietary genes databases that are exploited with home-made bioinformatics pipelines. Indeed, after many years spent in developing procedures and databases to explore soil and gut microbiomes, the company focused on skin metagenomics techniques and were one of the first to publish on this very specific topic^{13, 14.}

Cosmetics and skin microflora

One additional role of microbes living on our skin is the production, or modulation, of body odours, in particular to produce axillary malodours by decomposing apocrine sweat (usually by Corynebacteria), or foot odours from eccrine sweat^{3, 12}. Deodorants were thus the first cosmetic products developed to have an effect on our skin microbiota, as were new therapies to treat atopic dermatitis or rosacea, or even dandruff³.

How does acne develop?

Acne is a disease of the sebaceous glands with purulent pustules and blackheads. Many factors lead to the creation of this skin disease. During adolescence of females and males there is an increase of androgens in the blood. The increased androgens lead to an increase in sebum, which leads to greasy skin and hair. Simultaneously, the follicles are blocked by terminally differentiated dead keratinocytes, the horn cells. This induces the formation of a blackhead. Microbiota from the skin flora, particularly Propionibacterium acnes, proliferate very rapidly and provoke an inflammatory reaction of the follicle^15.

The impact of the colon’s microbiota on the skin

The interaction between colon and skin is easily comprehensible¹⁵. The skin does not stop at the lips but links the skin of the face to the inside of the body through the entire digestive system. Therefore, skin and the colon mucosa are in direct and tight connection. The skin as well as the colon mucosa define the borderlines between our environment and our bodies both inside and out, protecting us from harmful impacts.

This is why the flora of the colon is of major importance. The microbiota of the colon, “the microbiome” not only decide whether we have a thick or thin body but very often about its health status. The colon’s wide-ranging effect reaches far inside the body, influencing nearly every organ. The colon behaves in a way like a serviceable, yet outgoing friend. The microbiota produces messenger molecules and hormones and in addition coaches the immune cells. Keeping the colon healthy is a major prerequisite for maintaining one’s health. A healthy colon leads not only to a good body mass index and general happiness, but also and even more to improving the skin e.g. promoting a proper complexion, fewer pustules, full hair, less allergic reactions and alleviation of skin diseases.

The skin microbiome

The compounds of the skin flora are permanently changed and regularly complemented, e.g. by shaking hands. Microbiota are regularly exchanged between humans. However, the incredibly huge number and diversity of microorganisms on our skin protects us against harmful pathogens.

The inflammatory process has a very important function in the immune system. An inflammation protects against the attempts of pathogens to enter the body. What is interesting is the fact that in advance of a disease and a strong inflammation, the bacterial diversity often decreases weeks, months or even years before the disease occurs. Harmful bacteria can spread more easily and suppress protecting microbiota. Correlations were found between neurodermitis, allergies and diabetes. It is known, that the colon’s flora during normal ageing deteriorates and turns into an impotent monoculture.

Polyphenols – the florist of the colon

Polyphenols are secondary compounds from plants responsible for the colour, taste and smell of food products. Many studies demonstrate that polyphenols effectively absorb harmful substances (free radicals) and support and increase the intrinsic protection mechanisms of the skin against sunlight (UV-radiation), delay of wrinkling and senile lentigo formation. They also lead to a radiant complexion. Due to their dichotomy, polyphenols help to improve health¹⁶with their biotransformation of their metabolites and the modulation of colon’s microbiota.

Most of the polyphenols pass together with food through the small intestine without being absorbed. They reach the colon and have an impact on the colon’s microbiome,¹⁷ so that a reciprocal connection and interaction between polyphenols and the colon’s microbiome is created.

Skin microbes induce hair growth

Early in the life of neonatal skin, there is a short period of time during which the first steps for hair growth are started by the accumulation of regulatory T cells (Tregs) that mediate tolerance to commensal skin bacteria¹⁸. The Treg migration into the neonatal skin drives hair follicle morphogenesis¹⁹. The commensal bacteria enter the hair follicle, and together with invading Tregs, enter the new hair follicle. The hair follicle (HF) development facilitates the accumulation of Tregs in neonatal skin and organises a primary reservoir for skin commensals.

In the skin, there is a constant dialogue between the immune system and commensal bacteria. The skin is the interaction area between commensal microbes fundamental to skin physiology and immunity. Regulatory T (Treg) cells play a critical role in establishing and maintaining immune homeostasis in peripheral tissues. In the colon, commensals themselves facilitate the generation of the peripherally induced Treg cells that are critical for proper immune homeostasis.

Treg cell accumulation in neonatal skin leads to hair follicle development. However, the molecular mechanisms that allow us to tolerate the presence of skin commensals without eliciting destructive inflammation are still unknown. A defined period of neonatal life is characterised by an abrupt influx of highly activated regulatory T (Treg) cells into neonatal skin. During this period of time the host-commensal relationship in the skin relies on a unique Treg cell population that mediates tolerance to bacterial antigens¹⁸.