Functions of the skin microbiome

The skin microbiome, defined as the genetic content and theatre of activity of the microbes inhabiting this unique environment, plays a crucial role in maintaining the skin barrier function. The skin forms various ecosystems based on sebum content, hydration, pH, and temperature. In addition to the dry, sebaceous, and moist skin sites—which are acidic, mostly dry, and aerobic—follicles are anaerobic and richer in lipids.

The skin microbiome is essential for an intact skin barrier, contributing to key physiological processes. This barrier forms a nearly impermeable layer that protects against pathogenic microorganisms and toxins. Microbes can produce molecules that activate the aryl hydrocarbon receptor (AHR) in keratinocytes, promoting epithelial differentiation and supporting epithelial integrity. For instance, S. epidermidis, which dominates dry skin areas, secretes sphingomyelinase. This enzyme helps the host acquire essential nutrients for the bacteria and assists in producing ceramides, crucial for skin hydration. C. acnes and Corynebacterium, enriched in sebaceous regions, secrete lipases that hydrolyze free fatty acids from sebum. These fatty acids maintain a low pH, fend off pathogenic bacteria, and induce skin immunity by stimulating the expression of antimicrobial peptides (AMPs).

Beyond supporting the skin barrier, the skin microbiome interacts closely with the immune barrier, especially when the skin barrier is compromised. It activates the innate immune system by stimulating γδ T cells and upregulating perforin-2, an AMP. S. epidermidis also promotes homeostatic T cell activation. C. acnes secretes short-chain fatty acids (SCFAs), which induce cytokines via the TLR-2 or TLR-3 pathway and limit biofilm formation by S. epidermidis.

Besides microbe-host interactions, microbe-microbe interactions additionally act as barriers against pathogenic invasion and infection. As a community the microbes sustain their ecological niches and maintain access to nutrients. Although microbial interactions are not fully understood, it is known that coagulase-negative Staphylococci (CoNS) such as S. epidermidis, S. capitis, S. caprae, S. hominis, S. lugdunensis, and S. haemolyticus secrete AMPs to inhibit S. aureus colonization. On the other hand, C. acnes defends its pilosebaceous niche against Staphylococcus colonization by releasing the AMP cutimycin. Through quorum sensing, staphylococci communicate to prevent S. aureus colonization and infection by blocking a regulator of S. aureus and reducing its toxin production.

Therefore, wound management needs a makeover, reducing disinfection to enhance wound healing. The skin microbiome significantly contributes to wound healing and the skin’s defense against pathogens (2).

Potential influence of cosmetics and textiles on the skin microbiome

The skin microbiome is subject to many external and internal influences. It is influenced by the host genetics, diet, temperature, environment, geography, co-habitants of the host, medication and hygiene behaviour (3). Products like cosmetics, detergents and textiles used on a daily base over years have different effects on the skin´s microbiome. However, it is not easy to prove, since, luckily, our skin´s microbiome is able to regenerate after disturbance. Moreover, the majority of our skin´s microbiome resides in the follicles and invaginations of the skin, where it is protected against external influences (4).

However, Bouslimani et al. found that different products used in his study altered the microbiome and the chemistry of the participant’s skin. Compounds of the tested products such as lotions, deodorants, moisturizer or foot powder were detectable for more than 2 weeks after application, probably only completely disappeared after one cycle of skin renewal (21-28 days). Thus, a single application of some of these products has the potential to significantly alter the microbiome and skin chemistry for extended periods. Regular use can lead to an accumulation of these effects, further enhancing this potential (5). One component that was accumulated in the study was propylene glycol, which can cause contact dermatitis in a subset of the population (6).

Dr. Ian Myles from National Institute of Allergy and Infectious Diseases (NIAID) found that pollutants and textiles cannot only change the microbial composition but also the metabolism of skin microbiomes. The two chemicals isocyanate and xylene were found in increased amounts in areas in the US with higher Eczema prevalence. Myles exposed S. epidermidis strains to isocyanates and xylene. He found that the tested strains stopped producing ceramides in favor of Lysine, which helps the bacteria protecting themselves against toxins. Isocyanates and Xylene are sometimes used during the production process of certain textiles. Myles tested the effect of bedsheets manufactured with isocyanates or xylene and found that eczema related bacteria like S. aureus proliferated on these materials but not on cotton. Moreover, the production of lipids dropped in bacteria grown on materials made with xylene or isocyanate compared to untreated cotton (7).

It is clear, that every product we use in our daily lives, has an influence on the skin’s microbiota, either on their growth or their metabolism. Research around the skin microbiome is still in its infancy and currently used clinical methods to investigate potential effects must be improved. Until we have a better understanding of products influence on our microbiome, we must at least make sure that the influence is as little as possible – it must be ‘Microbiome-friendly’.

How to substantiate the ‘Microbiome-friendly’ claim

Although there are discussions ongoing around different methodologies for testing the influence of cosmetics on the skin microbiome, such as in vivo, in vitro, or ex vivo, it is even more important to discuss the methods used in each of the different approaches. These must be looked at more in depth than it is currently being done. Depending on the scientific question, each method has its value, but it must be designed in the right way. As a brand owner it can be hard to understand the ups and downs of certain methods, therefore transparency and education by the testing facilities is key.

When it comes to substantiation of a ‘Microbiome-friendly’ claim, the tests must be conducted in a way that is giving the assurance that a product or ingredient is safe for the skin’s microbiome. The term ‘Microbiome-friendly’ is rather passive and means that a product or ingredient does not affect the existing healthy skin microbiome of the customer when applied.

The safest and most sensitive way to find out if a product influences the skin’s microbiome, is rigorous in vitro testing on the most representative strains found on the respective skin area. Since there is no definition for ‘Microbiome-friendliness’, determining whether a product meets this criterion is crucial. Having developed antimicrobials for almost a decade, a 1-log-reduction is rated as clear antibacterial activity, which means 90% reduction of a specific strain. ‘Microbiome-friendly’ means the opposite of antimicrobial and should be at the opposite side, meaning no reduction of the strain’s growth at all. For practical and statistical interpretation for ‘Microbiome-friendly’ claims, a compound should not unproportionally affect the growth of the relevant species. A 30% reduction which is applied in our certification standards, can be considered as a small change which is in line with the suggestion made by van Belkum et al., 2023 (8). However, not only the reduction of growth but also the growth increase of the tested strains must be limited, as this also creates an imbalance of the microbiome, and is not ‘Microbiome-friendly’ in the logic of ‘not-interfering’. The design of the test must be scientifically sound, which means that the relevant strains should be included, depending on where on the body the product is applied, the right controls are implied, the test results are providing the right answer to the question, and interpretation of the results is correct.

The ’Microbiome-friendly’ claim is a widespread claim in the industry. They are based on different approaches though, such as adding certain ingredients such as pro-, pre- or postbiotics, reducing certain ingredients such as surfactants or preservatives or adjustment of the pH. These approaches might help to make products more ‘Microbiome-friendly’ however testing their effect on the microbiota is imperative as predictions from looking at the INCI list only, cannot be made. As a rule of thumb: less is more. When formulating with the microbiome in mind, high quality ingredients should be used in minimal amounts and number. Specifically, surfactants have a strong effect on the microbiota, and it can be a challenge to develop a microbiome-friendly wash but it is possible.

When looking for a ‘Microbiome-friendly’ certification of your products or ingredients, make sure the protocol is of high scientific quality (check the details!) and the evaluation is meaningful.