# Role of Nutribiotics in Skin Care
## 2 The Skin Microbiome: A Unique Environment  
The skin is a protective organ that performs important barrier functions against external agents in addition to preventing the loss of body fluids. The cells of the epidermis, but also the microorganisms present on its surface, intervene with barrier function. The microbiome and the skin are part of a whole that coexists and interrelates with each other. This invisible ecosystem of microorganisms performs important functions in the health of the skin, protecting it against external aggressions and acting as a second genome, interacting with other parts of the body to ensure healthy functioning. Its main role is the defense of the skin and interrelation with the environment that surrounds it. Furthermore, the skin microbiome has been found to play an important role in pathogen protection, inflammatory regulation, and overall health [35]. The skin microbiome also helps maintain skin homeostasis and the epidermal barrier, aiding in the process of epidermal renewal by the production of protease enzymes. The secretion of lipases by the microorganisms present on the skin surface also plays a regulatory role, since they break down the lipids secreted by the sebaceous gland. In addition, the skin microbiome produces bacterios [36]. Also, quorum sensing seems to exert a critical role on skin barrier function, with a recent study showing that interspecies quorum sensing among bacteria in human skin is considered a necessary defense mechanism to suppress the ability of _Staphylococcus aureus_ to damage the epidermis [37]. Over the recent few years, several studies have focused on the composition of the skin microbiome and how it changes with development or how external factors may affect its diversity. On the other hand, it is well known that the composition of the skin microbiome varies according to the areas of the body that constitute various ecological and physicochemical niches, mainly related to moisture and sebum content on the surface of the skin [38]. These differences influence resident bacteria and fungi; oily surfaces such as the forehead harbor lipid-loving bacteria that differ from dry areas, such as the forearm, in which there is lower microbial density [39]. In this regard, _Cutiobacterium_ spp., _Staphylococcus_ spp., and _Streptococcus_ spp. are the most abundant bacteria on dry sites; _Staphylococcus_ and _Corynebacterium_ spp. prefer moist areas; on sebaceous sites, lipophilic _Cutibacterium_ species (spp.) are the most abundant [40; 41]. _Malassezia_ spp. is the most abundant fungus throughout the body, except in the areas of the foot that present greater diversity [42; 43; 44]. On the other hand, it has been observed that two phyla, _Bacteroidetes_ and _Frimicutes_, tend to predominate in the microbiome of adults, while _Actinobacteria_ and _Proteobacteria_ constitute a smaller portion. Even so, variations can be found in the proportions of these phyla and the species from person to person [45]. Mites are also found in the skin microbiome. _Demodox_ spp. are characteristic of sebaceous glands and hair follicles, with the most numerous representatives being _D. follicularorum_ (hair follicles) and _D. brevis_ (sebaceous and meibomian glands) [45]. The skin virome has also been explored; it is very heterogeneous and complex with various polyomaviruses (Polyomaviruse), circumoviruses (Circoviridae), and papillomaviruses (Papillomavirusidae) [46; 47]. Figure 2 summarizes the relative abundance of bacterial, fungal, and viral components of the microbial community in different skin microenvironments. Studies of the skin microbiome (microbial and genomic components) in different age groups have shown that skin microbial communities exhibit dynamics that vary throughout life, developing in the early stages of life after exposure to the maternal microbiome, and are followed by changes in terms of diversity and community structure until old age [48].