36 SKIN MICROBIOME g Host


Biofilm Planktonic bacteria (MAMPs)


Eukaryotic microorganism (e.g. protozoa, fungi etc.)


Host virus Nutrient from host Bacteriophage AMP Secondary metabolite


Microbiota r Impact on functions:


Immune system Pathogen protection Development


Organ morphogenesis Metabolism Reproduction Ageing


Behaviour environment Figure 2: The metaorganism concept11


reduction in erythema, and accelerated barrier recovery.19,20


products as ecosystem optimizers, capable of regulating microbial metabolism through biochemical signaling.


Galenic and product type Formulation architecture - emulsion type, viscosity, and film permeability - affects oxygen diffusion, nutrient gradients, and microbial adhesion. High-viscosity or wax-based systems create microaerophilic microenvironments, promoting C. acnes growth, whereas breathable emulsions maintain microbial diversity. 21,1 Emulsifiers with low irritation potential


(polyglyceryl esters, lecithin blends) and buffered gels help maintain the acid mantle, allowing native microbiota to re-establish quickly after application.21


Cosmetic preservatives Due to their intrinsic antimicrobial properties, it is legitimate to think that cosmetic preservatives can have a deleterious effect on the skin’s resident microbial population. Few studies show this, but the conclusions of some of them are quite edifying. A study evaluating the effect of an emulsion


containing phenoxyethanol applied to the faces of volunteer subjects, showed that the resident flora (mainly C. acnes, C. humerusii and S. epidermidis) had decreased while, consecutively, the number of Pseudomonadota (Proteobacteria), a group of pathogenic Gram (-) whose main genus is Pseudomonas, had increased significantly.22 Another comparative study evaluated the


effect on the resident flora of reconstituted skin (3D skin models - Innovenn Ltd) of various combinations of different preservatives (sodium benzoate, phenoxyethanol, ethylhexylglycerin,


PERSONAL CARE MAGAZINE March 2026 These formulations redefine topical


phenylpropanol, caprylyl glycol, sodium anisate, benzyl alcohol, benzoic acid, dehydroacetic acid, o-cymen-5-ol, tropolone, levulinic acid, potassium sorbate) and other additives such as sequestrants, antioxidants and solvents.23 The strains tested were S. aureus, S.


epidermidis and C. acnes. This study showed that combinations


containing [hydroxyacetophenone, phenylpropanol, propanediol, caprylyl glycol, tocopherol and tetrasodium glutamate diacetate] were potentially the least likely to induce skin dysbiosis or worsen its symptoms, in particular by strongly inhibiting C. acnes and S. aureus without having any significant deleterious action on S. epidermidis. On the contrary, the combinations [sodium


benzoate, 1,2-hexanediol], [1,2-hexanediol, o-cymen-5-ol, ppg-3 benzyl ether myristate] and [1,2-hexanediol, caprylyl glycol, tropolone, levulinic acid, sodium levulinate, glycerin] strongly disrupted the growth of S. epidermidis, which represents a risk of dysbiosis insofar as this commensal bacterium, which is very abundant on the skin, naturally inhibits the hemolytic activity of S. aureus.24 Finally, the study describes the inhibitory


effects of phenoxyethanol, propylparaben, methylparaben, butylparaben, benzyl alcohol, benzalkonium chloride and 70% alcohol on the fungal and bacterial flora of the face of 15 volunteer subjects. The group of parabens with 0.4% alone or 0.8% in combinations had the most marked fungistatic effect to the yeasts C. albicans and M. furfur.25


Microbiome-friendly preservatives Some studies show that not all preservatives


necessarily have a detrimental effect on the microbiome; on the contrary, some of them can even contribute to maintaining a state of eubiosis by inhibiting pathogenic bacteria. According to an in vitro study conducted by


Pinto et al., certain combinations of preservatives, some of which are compatible with the Cosmos standard, had a significant inhibitory effect on certain pathogens without, however, harming the resident bacteria of the skin microbiome.23 Thus, the following combinations of Cosmos-


compatible preservatives, with or without non- Cosmos antioxidant (i.e. hydroxyacetophenone), inhibited moderately C. acnes but strongly S. aureus without inhibiting the growth of S. epidermidis. ■ {Phenylpropanol, propanediol, caprylyl glycol, tocopherol} ■ {Phenylpropanol, propanediol, caprylyl glycol, tocopherol, tetrasodium glutamate diacetate}. ■ The same positive effect was observed for the combination {benzyl alcohol, benzoic acid, dehydroacetic acid}, however at a lower level Therefore, it seems obvious that such


combinations of preservatives are suitable for leave-on skin care products in general as they might help restore skin dysbiosis.


Cosmebio-labelled products and microbiota Cosmebio-labelled products are certified according to Cosmos, which that is a leading international certification for natural and organic cosmetics, ensuring ingredient purity, sustainability, and environmentally responsible manufacturing. The standard defines two levels of cosmetic products: Cosmos Organic, requiring a minimum


www.personalcaremagazine.com


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