Skin in balance

The results obtained in ten years of scientific research (e.g., DOI: 10.1097/PRS.0000000000007621, 10.1186/s12915-019-0660-6, 10.1016/j. cell.2016.04.008) are unambiguous: unless a cosmetic product is designed to make life of the microbes on and in human’s skin difficult or impossible (e.g., deodorants, antiperspirants), cosmetic products do not have a significant negative impact on the skin microbiota (all bacteria and fungi living on and in the skin) over an extended period of time. In light of the prevalent “microbiome friendly” claims, conventional cosmetic products are clearly not “unfriendly” to the skin microbiota. It is important to emphasise that this describes the real-life situation (skin of human volunteers) and not the situation in a petri dish where microbial species normally living on the skin can indeed suffer from exposure to cosmetic formulations.

It would be too far to go into detail, but the following simple explanation for this discrepancy may suffice: In a real-life situation, microbes do not just live on the skin, they live inside the skin. From the inner layers of the epidermis they slowly move upward and are eventually shed from the skin together with dead skin cells. Anything that happens to them at the skin surface is, therefore, essentially irrelevant. At the surface of the skin, they might be killed, but their successors will reach the skin surface soon afterward, accounting for the fact that, in essence, the skin microbiota is rather stable over longer periods of time, despite the normal human habits, like using cosmetic products and showering regularly.

Skin and microbiome are one

In recent years, marketing claims concerning the skin microbiome have become prevalent. Numerous skincare, shower and bathing products claim to be “microbiome friendly,” and many brands boast that they support, balance, or protect the skin microbiome. The skin microbiome appears to be here to stay, and rightfully so. The human skin’s microbes play an essential role in skin health and disease as well as in many important cosmetic features of the skin, such as moisture level, smoothness, and evenness. The skin and the microbes residing in and on it essentially can be seen as one entity where different cells, human and microbial, interact intensively with each other.

Over longer periods of use, cosmetic products do not harm, are not “unfriendly” to the skin microbiota. This implies that conventional cosmetic products are inherently “microbiome friendly”, but this is in the eye of the beholder. Many people agree that “microbiome friendly” implies more than just being harmless to the microbiota. “Microbiome friendly,” for them, means that the skin microbiota benefit from the application and use of the cosmetic product. This is most probably the reason why the use of “prebiotics” is prevalent in the cosmetic industry.

“Prebiotics,” in essence, are food for “good” bacteria. But is it really that simple?

Healthy skin and its microbiota

We must distinguish between two possible situations of the human skin: a healthy and an unhealthy situation. In a healthy situation the skin and its microbiota live in synergistic harmony where both support each other. This situation needs to be preserved or even improved, if possible. In an unhealthy situation the healthy synergistic harmony with the skin’s microbiota is disturbed and both have a negative influence on each other. This is the case, for instance, with people who are prone to acne or dandruff.

In a healthy situation, the skin provides its microbes with an environment and food in which they can thrive. The microbes on and in healthy skin interact with skin cells in such a way that the skin benefits from their presence: there is real synergistic balance.

This balance can be disturbed or change with the ageing process, and cosmetic products can help regain and keep the balance. They can do so by supporting the skin in the processes which are necessary to provide the microbiota with the optimal environment and food. “Prebiotics” might be helpful in this context, but we must realise that in a healthy situation the skin microbiota is in balance and that supporting the growth of certain bacteria over others might disturb this balance.

Unhealthy skin and microbiota

Most people have healthy skin even though they may occasionally experience problems with their skin. It might be dry, oily, or sensitive, but these types of skin are not diseased. It is when problems like acne and dandruff occur that skin can clearly be deemed unhealthy.

With these types of skin, the balance between the skin and its microbiota is disturbed. The exact nature of this problem is something which has only recently be recognised. Where, in the past, Cutibacterium acnes (C. acnes, former name: Propionibacterium acnes) was seen as the acne bacterium, we now know that C. acnes largely plays an important positive role for the skin. Only certain strains of C. acnes have a causative function in the development of comedones, pustules and pimples. It is therefore incredibly important to not kill C. acnes as a species but to focus on the C. acnes strains which cause the specific problem.