# Role of Nutribiotics in Skin Care
## 4 Skin Microbiome and Dermatological Disorders  
It is very well known that the skin microbiome plays an important role in developing and maintaining homeostasis and in the regulation of the host immune system. [PERSON] and [PERSON] [39] summarized the \"dialogue between skin and immune system\" as follows: Microorganisms present on the surface and skin appendages (bacteria, fungi, viruses) can produce antimicrobial peptides and regulate the production of antimicrobial peptides by keratinocytes as well as the production of immune mediators, such as complement and IL-1. These molecules can directly or indirectly improve skin immunity by improving cellular microbicidal function and by promoting cytokine production and the recruitment of effector cells. Furthermore, IL-17 production by the microbiota may promote the effector function of keratinocytes against invading microbes. Additionally, skin-residing microbes can release defined metabolites that can be captured directly by skin-residing dendritic cells [39]. Skin disorders such as acne, atopic dermatitis, and psoriasis have all been associated with dysbiosis of the skin microbiota. Dysbiosis is the alteration in the composition,  
Figure 3: Intrinsic and extrinsic factors that influence the skin microbiome. activity, or distribution of the cutaneous microbiota. According to [PERSON] et al. [80], three mechanisms can be involved: (1) overgrowth of a microbiota member (e.g., _Ctiubacterium acnes_ in acne); (2) elimination of a microbiota member; and (3) invasion by non-member microorganisms (e.g., _Staphylococcus aureus_ in atopic dermatitis) [80]. Dysbiosis is associated with various dermatological conditions, such as psoriasis, atopic dermatitis (AD), seborrheic dermatitis, acne, rosacea, vitiligo, hidradenitis suppurative, lepra, and others linked to viruses, but it is not entirely clear if the changes in the microbiota cause diseases or whether certain conditions cause an imbalance in microbial communities [81]. The gut microbiome also plays a role in some skin disorders; there appears to be a bidirectional link between the gut and the skin, which in turn is linked to the body's homeostasis, the so-called gut-skin axis. The gut microbiota modulates the functionality and composition of the innate and adaptive immune system, and vice versa. This fact could explain why some skin diseases are linked to intestinal dysbiosis and an imbalance of skin homeostasis, suggesting a role of the intestinal microbiota in the pathogenesis of several inflammatory skin diseases [82]. Multiple studies support a connection between them, and several skin diseases associated with gastrointestinal disorders, but more studies are needed to attribute a cause-and-effect relationship between the gut microbiome and dermatological conditions. For example, between 10% and 25% of patients with gastrointestinal diseases, such as [PERSON]'s and celiac disease and ulcerative colitis, also have associated skin disorders, specifically skin ulcers and psoriasis [83]. [PERSON] et al. [81] postulated that the intestinal microbiota contributes to the allostasis and homeostasis of the integumentary system after any inflammatory process due to the relationship with innate and adaptive immunity. Proinflammatory cytokines could damage the intestinal barrier, and severe intestinal dysbiosis provokes inflammation beyond the intestinal, and therefore low-grade systemic [PERSON] with skin involvement. Many studies have shown that the overgrowth (or decline) of pathogens on the skin is a common occurrence in various skin diseases and conditions. The main changes are summarized in Table 1.