A review of skin microbiome and new challenges to cosmetic microbiome-friendly formulations.

Human skin is a complex ecosystem that hosts diverse species of microorganisms. Unbalanced conditions caused by intrinsic and/or extrinsic factors can lead to dysbiosis, presenting symptoms, such as dryness, high transepidermal water loss, reduced barrier protection, premature ageing, and in severe cases, inflammatory dermatoses. Strategies to maintain the skin microbiome balance are becoming increasingly suggested, with prebiotic, probiotic, or postbiotic ingredients promoting the diversity and relative abundance of important microorganisms. Topical products directly influence this balance, both traditional ingredients and specific active ingredients. The concentration and combination of these ingredients, as well as the pH of the final product, are extrinsic characteristics that can affect homeostatic skin condition. Focused on repairing or preserving the skin microbiota, microbiome-friendly cosmetics are gaining prominence in the cosmetics industry, with a focus on reducing or replacing ingredients with adverse effects on skin microbiota or adding positive compounds for the microbiota. This review approaches the main characteristics of the skin microbiome, in symbiosis and dysbiosis, elucidates strategies for skin microbiota rebalance, and addresses the challenges of developing microbiome-friendly products through studies of the interaction between skin microbiome and substantial classes of cosmetic ingredients, such as surfactants, lipophilic compounds, preservatives, fragrances, vitamins, and UV filters. The presented findings elucidate the relationship between the host, the skin microbiome, and the use of cosmetics, which could serve as a tool for the development of microbiome-friendly cosmetics. Given the growing popularity of this topic, we also highlight the need for further research focused on the dynamics between the skin microbiome and cosmetic ingredients.