The skin microbiome

The skin, the largest body organ, has an enormous surface that provides a habitat for diverse microorganisms of different kingdoms of life: bacteria, archaea, and fungi but also viruses are present. The common theme in biology, striving for a balanced equilibrium, is key to health. By implication, impairments of this equilibrium result in pathologies. The driver and main characteristic of a healthy skin microbiome is diversity. Over time, from birth to adulthood, the diversity of the microbiota extends because of environmental influences. This need for resistance against environmental factors triggers an evolution of strategies for bacteria to survive. For example, the body’s skin has different ecosystems within which the microbial composition significantly varies. There are sebaceous sites, like the forehead, moist subset areas like the inner elbow, and dry areas of the skin like the forearm, all of which are composed of different subsets of microorganisms. Furthermore, not only location but also age, body temperature, the immune system as well as extrinsic factors like the climate and the use of antibiotics determine the composition and characteristics of the microbiome and make it very individual.

The skin barrier

The term skin barrier unites different strategies to protect our body from external influences of various origins. Epidermal keratinocytes build the first layer of protection in analogy to a wall to the outside. Interestingly, this physical barrier is safeguarded by skin microbiota that secrete components that stabilize the lipid structure. This leads us to chemical components, that contribute to the skin barrier. Both the epidermis itself and the skin microbiome release lipids and acids as protective shields against intrusion. Moreover, bacterial inhabitants of sebaceous regions produce free fatty acids which are responsible for the characteristic low pH value of the skin. Fatty acids inhibit the growth of pathogenic microorganisms and enhance skin immunity. In analogy to the gut microbiome, also the skin microbiome stimulates immune responses in order to protect against cutaneous and systemic infections and this adds another layer of complexity to the skin barrier function. Last but not least, the skin microbiome itself can start a fight against pathogens, for example by disrupting their communication system or inhibiting their growth. This competitive relationship between the skin microbiome and pathogenic microorganisms is important for maintaining homeostasis.

Pathologies associated with the skin barrier

A good example of a connection between pathology and an unbalanced skin microbiome is acne. The loss of diversity of different Cutibacterium acnes phylotypes is responsible for the pathogenesis of the disease. Strains that trigger the innate immune system and skin inflammation take the lead in those kinds of patients¹.

Moreover, a well-examined disease in the context of microbiome dysbiosis is Atopic dermatitis (AD). AD is with 1-10 % of adults and 15-20 % of children a widespread chronic multifactorial inflammatory skin pathogenesis characterized by dysfunction of the skin barrier. AD lesions display an increased abundance of Staphylococcus aureus next to a generally decreased microbiological diversity. Secreted chemicals of the pathogen reach deeper layers of the skin and interfere with the host’s immune system, which contributes to the pathogenic phenotype¹. Actually, AD patients display an almost twofold increase in skin absorption to different chemicals, including irritants and contact allergens⁴. Among these, contact allergens, preservatives beneath fragrances, emulsifiers, vehicle components, and sunscreen agents contribute to contact hypersensitivity. About 7.3 % of Americans display hypersensitivity against preservatives, which is significant, given the estimated average of using 6-12 cosmetic products on a daily basis.

Preservatives and their influence on the skin microbiome

Preservatives are being used to protect the cosmetic preparation throughout its shelf-life against microbiological contamination which may originate either in air germs during bulk production and filling of the product and/or after the opening of the cosmetic when the content is exposed to air or comes in contact with skin. As long as the formulation has nutritive properties microorganisms start growing and metabolizing. This culminates in the cosmetic product losing its claimed properties and causing health-threatening consequences to the user of the product. Therefore, the need for microbiological-safe cosmetic products is the highest priority, which started the career of preservatives a long time ago. As we learned before, the drawback to the use of preservatives and preserving agents is the high potential for contact hypersensitivity. In this context, preservatives range second (58%) after fragrances for contents with allergic potential throughout the main cosmetic categories². There is also a negative correlation described for preservatives and commensal species of the skin microbiome as S. epidermidis, whose numbers decline following the usage of preservatives, such as phenoxyethanol, parabens, and methylisothiazolinone^3,4. This negative impact on the skin microbiome is exacerbated by the fact that even after regular showering, personal care and hygiene products are shown to stay on the skin for weeks after their first use, which has an enormous impact on the bacterial ecosystem of the users⁵.

Taken together, the balance of the skin microbiome is endangered by the use of preservatives, which should add a layer of consciousness for the cosmetic user and fuel the development of alternative solutions in the cosmetic industry.

Dispenser technology that allows true preservative-free cosmetics

We need to understand that all ingredients that inhibit the growth of germs are preserving agents and not only those that utilize this function primarily. Inhibiting the growth of microorganisms is not always beneficial, thinking of the significant negative impact of preservatives on the human skin microbiome by the use of cosmetics on a day-by-day basis. In contrast to the U.S., where preservatives in cosmetics are not under special regulation, in the EU there is a positive list of preservatives that need to be declared as such (EC Regulation 1223/2009, Annex V). But since some ingredients have several functions and the preserving function is only one of them, for example, pentylene glycol, several alcohols, or even magnolia extract, can be used in cosmetic products in Europe for preserving the formulation without the need to declare them accordingly. The consumer would understand the claim “preservative-free” in those cases as being true, which is not the case, unfortunately. Actually, with most of the currently used packaging solutions and the respective manufacturing processes, a claim like “100% free from preservatives” cannot be achieved.

Therefore, the combination of high-end technologies in dosage systems, such as the COMFORT® system, and innovative manufacturing conditions, such as the “Ultra High Temperature (UHT) process, will become the gold standard for “real” preservative-free products in the cosmetic industry.

The COMFORT® system is an airless dosage system for dispensing sensitive liquid or semi-solid formulations. It permits avoiding any kind of preservatives, even after the product has been opened, as the special design of the system prevents contamination. Both a microbiologically tight closing valve and the bacteriostatic effect of the surfaces used in the nozzle guarantee perfectly protected contents. A microbiologically tight valve is technically very complex and physically requires a high opening pressure because the smoother the valve for the product outlet, the easier it is for germs to overcome the valve and enter the bottle. The bacteriostatic effect is achieved by a silver-based additive. This prevents biofilm formation on the surface. Additionally, the flexible airless inner bag that folds when the vacuum emerges due to pressing the pump, dispenses the content without being in touch with any oxygen. This leads to proven perfect protection of the preservative-free contents.



The innovative “Ultra High Temperature” (UHT) technology that is used during the aseptic manufacturing process rounds off modern technology to provide products that are 100% free from preservatives. UHT sterilization describes heating the contents to temperatures exceeding 275°F (135°C) for one or two seconds, then packaging it in a hermetically sealed container.



Conclusion

Eliminating dispensable ingredients like preservatives and at the same time guaranteeing safety against microbial contamination has a winning effect on the skin microbiome. Impairments in terms of species diversity which directly correlates to the skin barrier can be avoided. The same is true for a substantial proportion of substances prone to contact hypersensitivity. The term “less is more” could not be more applicable to this topic.

References

1) Lee H. and Kim M., 2022, Int. J. Mol. Sci., 23, 13071
2) Travassos A. et al. “Non-fragrance allergens in specific cosmetic products”, Contact Dermatitis, 2011, 65, 276-285
3) Wang Q. et al. “Effect of cosmetic chemical preservatives on resident flora isolated from healthy facial skin”, J Cosmet Dermatol., 2019, 18(2): pp 652-658
4) Pinto, D.,”Effect of commonly used cosmetic preservatives on skin resident microflora dynamics”. Sci Rep, 2021, 11(1): p. 8695.
5) Bouslimani A. et al., „The impact of skin care products on skin chemistry and microbiome dynamics”, BMC Biol. 2019, 17(1):47