Skin Microbiome as a “Protective Barrier”
The skin hosts a diverse ecosystem of microorganisms, including bacteria, fungi, viruses, and mites. These microorganisms do not simply reside on the skin; they play important roles in skin immunity and in maintaining the skin barrier.
The most abundant microbiome on the skin is bacteria. Representative genera include Staphylococcus, Corynebacterium, and Propionibacterium. These microbiomes inhabit specific niches of the skin such as sebaceous glands, hair follicles, and sweat glands. Among fungi, the genus Malassezia is commonly found in sebaceous areas, while arthropods such as Demodex mites inhabit hair follicles and sebaceous glands.
This microorganism helps prevent the colonization of pathogens through physical and competitive mechanisms. Bacteria on the skin compete with pathogenic microorganisms for space and nutrients, and some microbiome produce antimicrobial substances that inhibit pathogen growth. For example, Cutibacterium acnes is known to produce substances that suppress the growth of certain pathogens.
In addition, the skin microbiome helps regulate the host immune response through metabolic byproducts such as free fatty acids, antimicrobial peptides, phenol-soluble modulins, and cell wall components. Some microbiomes also influence skin lipid production, contributing to the strengthening of the skin barrier.
Dysbiosis and Skin Diseases
Many studies have reported that various skin diseases—including acne, atopic dermatitis, psoriasis, and seborrheic dermatitis—are associated with imbalances in the skin microbiome.
For example, while Cutibacterium acnes can inhibit the growth of certain pathogens, its overgrowth may contribute to the development of acne.
In addition, Staphylococcus aureus, Streptococcus pyogenes, and Malassezia have been reported to be associated with atopic dermatitis and psoriasis. However, it remains unclear whether changes in the skin microbiome are a cause of these diseases or a consequence of them.
Research on the scalp microbiome has shown that seborrheic dermatitis is a polymicrobial condition arising from ecological imbalances between fungi and bacteria. In particular, the high activity of sebaceous glands influenced by androgens is closely associated with seborrheic dermatitis, which is considered one of the reasons why the condition is more prevalent in men.
Both the overgrowth and the reduction of certain microbiomes can affect the skin. What matters most is the overall balance of the microbial community.
The skin’s microbial environment constantly changes depending on external factors and health conditions, moving between states of balance and imbalance. For example, ethanol disinfection can temporarily disrupt microbial balance, but the ecosystem generally returns to equilibrium over time.
The skin microbiome is highly complex and remains an active area of research. Since only a limited number of cosmetic ingredients have been shown to continuously disrupt microbial balance, there is generally no need for concern. However, if skin conditions suddenly worsen after using a particular cosmetic product, it may be related to microbial imbalance, and discontinuing its use is advisable (this is not a “healing crisis”).
[Referenced multiple studies]