34 SKIN MICROBIOME
degree. Its composition changes when our skin changes. Conversely, our skin changes when the composition of the skin microbiome changes. Our skin microbiome changes as we age. The skin microbiome plays a role in the sensitivity of skin. It is everywhere, and nowhere. We cannot see the skin microbiome and we
know that it is important to us, but we only know little about why this is the case. As scientists, we are still staring into a black hole. We know that what is happening there is incredibly important, but we do not understand much about how and what. Maybe this is a good time to reboot, to go
back to the beginning and regain an overview of what we are really talking about when discussing the skin microbiome. When we talk about ‘good’ and ‘bad’ microbes, we actually distinguish between a composition of microbes which has a good (or at least not negative) impact on our skin and one that has a negative impact on our skin, which may, for instance, have an impact on the skin aging process, skin sensitivity, dryness etc. In conclusion, the main concern is not individual species of microbes. As detailed scientific knowledge about the
causal relationship between the composition of the skin microbiome and skin issues which are relevant for the cosmetic and personal care industry is still lacking at this time, it may be time to refocus. This topic is important enough for us to continue our quest of making consumer- relevant cosmetic and personal care products, but we seem to have reached a dead end.
Thinking differently Another, more promising approach might be found by a 180-degree different way of thinking. Where does the skin microbiome ‘come from’? How is the ‘good’ or ‘bad’ rating determined in terms of the composition? In all its complexity, the composition of the skin microbiome is dictated by something. By what? In order to answer these questions, it is opportune to zoom out and try to understand the notion of ‘the biome.’ On the world’s scale, a biome is a community of plants and animals that has formed in response to a local environment. Our world shows ten biomes, from hot and humid rainforests, to hot and dry deserts, to the cold tundra of the polar regions, and everything in between. It is not difficult to understand the notion that the local environment – i.e. temperature, humidity, soil, etc. – dictates which species of plants, trees, shrubs and animals can live there. Within this environment these species form
an ecosystem which is in a natural balance. Interestingly and luckily, this notion of the environment means that the composition of species can easily be translated to the human microbiome, including our skin’s. The composition of the skin microbiome is dictated by its environment. This begs a next question: What is the exact
location of the skin microbiome? After all, if we want to understand how the local environment dictates the composition of the skin microbiome, we need to know where it lives and thrives. Do the microbes just live ‘on top’ of skin? A distinction needs to be made between the follicular microbiome and the interfollicular, epidermal microbiome.
PERSONAL CARE September 2023 The follicular microbiome largely lives inside
the follicles, mostly close to the sebaceous glands which produce their food source. Such species as Cutibacterium acnes, which is anaerobic, live as far away as possible from oxygen and as close as possible to the place where their main food source, sebum, can be found. C. acnes is one of the main species in the core microbiome of the follicles of our skin. The epidermis has a core microbiome as
well. A number of species are always present on the skin. It is indeed true that, at the surface of skin, species ‘fly on and off,’ depending on our external environment. At the surface of skin, the composition of the microbiome fluctuates rapidly depending on the external environment. A large body of scientific evidence shows that
the composition of the skin microbiome over a longer period of time is rather stable, however. The epidermal skin microbiome therefore consists of a group of microbial species which can be considered to be its core microbiome. This core microbiome can be found inside the top layers of the stratum corneum, the top layer of skin where dead skin cells are in the process of moving upward and which are finally released from the surface of skin in the process of desquamation. Strong evidence exists that the core
epidermal microbiome resides in the lower levels of the upper half of the stratum corneum. Here they multiply and flourish, adhere to dead skin cells which are moving upward and then shed from the skin together with the dead skin cells. As the epidermis itself is in a constant process of self-renewal, so is our epidermal core microbiome. Both our epidermis and our epidermal microbiome are in a constant process of self-replenishment.
Getting to the point In summary, the core microbiome of our skin’s follicles is inside the follicles, and the core
epidermal microbiome resides inside the stratum corneum. Now that we have defined their locations, the next question is: What provides the local environment in which our skin’s core microbiome resides? For the follicular core microbiome, the answer to this question was already given above: The follicles in the skin provide a food source and an oxygen-depleted environment for sebum-loving and anaerobic microbes such as C. acnes to thrive. For the epidermal core microbiome, the
answer is somewhat more complex, but, in essence, similar: During the epidermal renewal (i.e. differentiation) processes, for instance, foodstuffs for the core microbiome – molecules – are produced which play a role in maintaining a low pH level and antimicrobial peptides with which the epidermis supports the core microbiome in its ‘war’ against pathogenic bacteria. This means, the beneficial environment for our ‘good’ core microbiome is produced during the epidermal differentiation process. The above nicely links back to known skin diseases such as acne and atopic dermatitis. Acne is a consequence of the increased production of sebum, i.e. a change in local environment in the follicles. In the cascade of reactions that lead to the
formation of pimples etc., first, virulent strains of C. acnes start to grow due to the increase of sebum production. These virulent strains then ‘take over’ and start to form the problem which ends up with acne. It is important to realize that this cascade of events starts with a problem caused by skin itself, i.e. by ‘us.’ Something similar is described for skin
suffering from atopic dermatitis. For this type of skin, it is known that it has a somewhat higher pH level than healthy skin. Skin pH is a consequence of molecules produced by skin itself. It is not dictated by the external environment. Following this somewhat higher skin pH, Staphylococcus aureus finds a welcome
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