The power of microbes

Every salesperson's worst nightmare is that the goods on offer will spoil before they are sold, or that the customer will complain about a product's short expiry date. This is precisely why food and cosmetics (the latter also fall under food law, by the way) are preserved. This is because water-based media in particular offer ideal living conditions for the invisible culprits lurking everywhere, whose work manifests itself in the form of mould, a foul odour and the decomposition of a cream.
We are talking about microorganisms, which elsewhere serve constructively to purify water, dispose of waste and, physically, aid digestion and protect the epidermis. So how can we encourage them to work to our advantage and keep them happy, instead of thoughtlessly killing them all with chemical weapons?

Notable features

From a macroscopic point of view, cosmetics behave very differently on the skin. Some of their components disappear after a relatively short time, while others hardly change until the next skin cleansing, when they are removed. This is due to several factors relating to their structure:

• Lipophilic (fat-soluble) ingredients penetrate the skin more easily than hydrophilic (water-soluble) ingredients.
• Non-ionic substances (without charge) are absorbed more easily than ionic (salt-like) substances.
• Substances with structures that interact with intercellular components are absorbed more quickly. One example is acid amides, which, due to their structure (-CO-NH-), often have anti-itching properties.
• Substances with high molecular masses of more than 500 Da (Dalton) remain on the skin surface¹, for example paraffins.

Enzymes

However, there are also skin care ingredients that have a size of more than 500 Da and yet slowly disappear from the skin's surface. These include, above all, vegetable oils with their triglyceride structure. 
The explanation for this phenomenon is quite simple when you consider that the skin microbiome located at the "gateway" has an extensive range of enzymes that enable it to break down triglycerides using hydrolases (esterases) and partially break down the released fatty acids using oxidases. This breakdown also contributes proportionally to the surface acid mantle (pH < 6).  

Undesirable germs

It can be assumed that the metabolic capabilities of the skin flora are similar to those of the epidermis, but differ in terms of quantity. As with the intestinal microbiome, the resulting metabolites then diffuse into the body and are further processed there. 
However, paraffinic hydrocarbons are not broken down in the same way as petroleum pollution in water. Their occlusive effect changes the ratio of germ populations among themselves. This gives unwanted, anaerobic germs better opportunities to develop, as atmospheric oxygen is kept away. As a result, for example, inflammatory reactions occur in rosacea skin that has been (incorrectly) treated with paraffins after a few days at the latest. Similar conditions can be expected in cases of skin blemishes and acne prevalence. Medical measures include the use of the antibacterial agent metronidazole (rosacea, perioral dermatitis) and the oxidising agent benzoyl peroxide (acne).

Pollutants

High concentrations of antioxidants in skin care products also have a negative effect by inhibiting the above-mentioned oxidases in the skin flora. Further examples of the damaging effect on both the skin flora and the microbiome are:

• Preservatives: They ensure that the product is germ-free, inevitably kill parts of the skin microbiome and have an allergenic effect on the epidermis. As with antibiotics, the resulting resistance of pathogenic germs also leads to intolerance of preparations.
• Surfactants: These destroy the skin barrier and the microbiome through their washing-out effects. This typically leads to subsequent inflammatory flare-ups in atopic skin due to the easier penetration of facultative pathogenic germs. For people with this condition, it is advisable to reduce intensive skin cleansing (excessive hygiene).

An intact microbiome influences the formation of NMF (Natural Moisturising Factor), as it is involved in breaking down epidermal proteins into amino acids. Remember: NMF not only moisturises the skin, but also protects it from free radicals.² 

Water-based skin care products

Over time, aqueous emulsions and dispersions have become the standard in skin care because they are easy to apply, leave a cooling effect and water (manufacturer jargon: profitol) is inexpensive.
However, water requires many additives to stabilise the multi-phase products chemically, physically and microbiologically, as well as to optimise their sensory and haptic properties. Most of these additives are useless for the skin and its care and are not conducive to the coexistence of the microbiome.
With the development of barrier-affine products, a rethink began, with more and more focus on the physiology of the skin.³ Lamellar, liposomal and nanodisperse formulations based on native and hydrogenated phosphatidylcholine were developed. Both are cell membrane components and, like vegetable oils, are substrates for the microbiome. 

Reduction of additives

Their use excludes many additives that are counterproductive for resilient microorganisms, including synthetic, physiologically non-degradable emulsifiers and preservatives. The latter are largely inactivated by phospholipids⁴ and, if used, would result in a higher rate of allergic reactions due to the penetration-enhancing effect of phospholipids. 
Hypertonic aqueous phases and/or alcoholic hydroxyl groups in the form of low-dose ethyl alcohol, glycols and/or glycerine are responsible for the microbiological stabilisation of these products. In the concentrations that occur locally during application, they are biodegradable and tolerated by the skin flora.

Water-free cosmetics

Oleogels are ideal for high doses of plant-based care oils. They are water-free and require minimal or no additives. As they usually contain phospholipids, unlike pure plant oils – such as avocado oil used in children's care products – they penetrate the skin comparatively quickly. 
Oleogels behave haptically like high-fat W/O emulsions. Just like the epidermis, the skin flora copes very well with these compositions. In addition, they can accommodate physiological components that can have both lipophilic and hydrophilic properties. With the appropriate formulation, they can be used to treat conditions such as atopy and even rosacea and perioral dermatitis.^{5 6}

Developments

A working hypothesis for the development of microbiome-compatible skin care does not necessarily require an endless list of suitable ingredients, but only a few basic considerations:

• The epidermis and skin microbiome form a well-coordinated team.
• What is harmful to one team member is also detrimental to the other.
• Conversely, what the epidermis tolerates well is also tolerated by the skin flora.

In other words, the chemistry must be right. In addition to the appropriate physiological components, their dosage is also important. Too much of a good thing can easily prove counterproductive – just like in real life.

References

1. Bos JD, Meinardi MM, The 500 Dalton rule for the skin penetration of chemical compounds and drugs, 2000; 9 (3):165-9
2. Dröge W, Free Radicals in the Physiological Control of Cell Function, Physiol Rev 2002; 82: 47-95
3. Lautenschläger H, Die Haut und ihre Pflege – Physiologie und Chemie im Einklang? Chemie in unserer Zeit 2021; 55 (5): 306-319
4. Wallhäusser KH, Praxis der Sterilisation, Desinfektion, Konservierung, Keimidentifizierung, Betriebshygiene, Georg Thieme Verlag, Stuttgart 1995: 43, 394
5. Lautenschläger H, Trend zu physiologischen Inhaltsstoffen – Phosphatidylserin in der Hautpflege, Chemie in unserer Zeit 2024; 58 (5): 93-97
6. Lautenschläger H, Kloss J, Kosmetische Zusammensetzung, DE 10 2017 002 125 (8.3.2017) (Offenlegungsschrift)

Dr. Hans Lautenschläger

published in
Beauty Forum
2025 (8), 88-90