TOPOGRAPHIC COMPOSITION OF THE SKIN MICROBIOTA AND IMMUNE MODULATION MECHANISMS

The skin microbiota is a dynamic ecosystem that plays a critical role in maintaining human health. Topographical differences of the skin create variations in microbial diversity and composition across oily, moist, and dry regions, directly influencing barrier integrity and immune responses. This ecosystem composed of commensal bacteria, fungi, and viruses contributes to the integrated functioning of the physical, chemical, microbial, and immunological barriers. Commensals such as Staphylococcus epidermidis and Cutibacterium acnes regulate lipid metabolism, ceramide production, and antimicrobial peptides (AMPs) synthesis, thereby supporting barrier repair and limiting pathogenic colonization. The composition of the microbiota is strongly influenced by intrinsic and extrinsic factors, including age, sex, hormones, diet, stress, hygiene, cosmetic use, climate, and antibiotic therapy. Dysbiosis plays a critical role in the pathogenesis of various dermatological disorders such as atopic dermatitis, psoriasis, and acne. The bidirectional interaction between the skin microbiota and the immune system occurs primarily through Toll-like receptors (TLRs). TLRs recognize pathogen-associated molecular patterns and activate NF-κB, type I interferon, and inflammasome pathways, thereby mediating both innate defense and the initiation of adaptive immunity. Commensals contribute to “inflammation-free defense” through TLR2 signaling, an essential component of cutaneous homeostasis. Additionally, microorganisms such as Staphylococcus epidermidis promote the activation of Th17 cells and the production of IL-17A, strengthening barrier immunity. The accumulation of Foxp3⁺ regulatory T cells during the neonatal period establishes fundamental tolerance toward commensal microbes. Therefore, preserving the skin microbiota and developing targeted microbiota-based approaches represent promising strategies for the prevention and treatment of dermatological diseases in the future.