Our skin hosts approximately 1 million microorganisms per cm². Understanding the relationship between this "skin microbiome" and fermented ingredients is central to next-generation skincare.
Key players in the skin microbiome
Staphylococcus epidermidis is the most dominant commensal bacterium, producing antimicrobial peptides (AMPs) that prevent pathogen colonization. It also metabolizes glycerin to generate moisturizing factors.
Cutibacterium acnes breaks down sebum to produce propionic acid, maintaining the skin's mildly acidic pH. While overgrowth causes acne, its moderate presence contributes to barrier function.
Malassezia is a fungal genus that feeds on sebum to produce fatty acids. Overgrowth can cause seborrheic dermatitis and dandruff.
Sphingomyelinase and ceramide conversion
The sphingomyelinase produced by S. epidermidis converts sphingomyelin in the stratum corneum into ceramides. Ceramides form the lamellar structure that is fundamental to barrier function. In other words, healthy commensal bacteria maintain the skin's barrier.
Dysbiosis and skin problems
Dysbiosis (disruption of the microbial balance) is caused by excessive cleansing, antibiotic overuse, and stress. S. aureus overgrowth is linked to atopic dermatitis, C. acnes overgrowth to acne, and Malassezia overgrowth to seborrheic dermatitis.
How fermented ingredients affect the microbiome
Postbiotics (fermented ingredients) support the microbiome through the following mechanisms:
1. pH maintenance: Lactic acid and organic acids keep the skin's pH mildly acidic (4.5-5.5), maintaining a beneficial bacteria-dominant environment.
2. Antimicrobial peptide supply: Bacteriocins produced during fermentation selectively suppress pathogen growth.
3. Treg differentiation promotion: Short-chain fatty acids (butyrate, propionate) promote regulatory T cell (Treg) differentiation, suppressing excessive immune responses.
Immunomodulatory action of short-chain fatty acids
Short-chain fatty acids (SCFAs) inhibit histone deacetylase (HDAC), promoting anti-inflammatory cytokine (IL-10) production while suppressing pro-inflammatory cytokines (IL-6, TNF-alpha). This immunomodulatory action is the scientific basis for why fermented ingredients show efficacy for sensitive skin and atopic dermatitis.
References
Key peer-reviewed sources behind the scientific statements in this article.
- Zheng Y, Hunt RL, Villaruz AE, Fisher EL, Liu R, Liu Q, Cheung GYC, Li M, Otto M. Commensal Staphylococcus epidermidis contributes to skin barrier homeostasis by generating protective ceramides. Cell Host Microbe. 2022;30(3):301-313.e9. PubMed
- Furusawa Y, Obata Y, Fukuda S, Endo TA, Nakato G, Takahashi D, et al. Commensal microbe-derived butyrate induces the differentiation of colonic regulatory T cells. Nature. 2013;504(7480):446-450. PubMed