# Role of Nutribiotics in Skin Care
## 3 Skin Microbiome: Influence of Intrinsic and Extrinsic Factors  
The skin microbiome depends on internal (or intrinsic) and external (or extrinsic) factors. Among the intrinsic factors, genetics, age, gender, hormones, immunity, sleep and stress factors, and metabolism must be mentioned. The exposure of the skin to external factors (UV, pollution, humidity, environmental bacteria, cosmetics, etc.) also has a great influence on the skin microbiome. [PERSON] et al. [55] reviewed the impact of extrinsic factors (external exposure) on the skin microbiome, and, in short, the most important are climate, sunlight (UV radiation), hygiene and cosmetics routine, and environment (air and water pollution, exposure to chemicals), with physical activity and diet also being factors that can be added. Several studies focused on internal factors, finding that among the genetic factors that determine the skin microbiome, ethnicity seems to be secondary, although not insignificant, since some differences have been found; for example, the number of _Cutiobacterium_ on the armpits and scalp of males in Africa and Latin America is lower than in other ethnicities (Caucasian, African American, East Asian, and South Asian), and differences have also been found in the microbiomes of the arms of different ethnicities [56]. Differences linked to gender were also found; the female skin microbiome is characterized by a higher species diversity than that of males, probably due to several factors, such as sweat production and the influence of hormones [57]. The relationship between the skin and gut (the so-called gut-skin axis) could explain the influence of the stress factors and metabolism in the skin microbiome [58] as well as nutrition [59]. Furthermore, diet and obesity were found to influence the skin microbiome, with high-fat diets favoring the growth of _Corynebacterium_, probably because they promote skin inflammation through the expression of mycolic acid. Furthermore, the balance between _Frimicutes_ and _Bacteroidetes_ in obese people is altered, and, during weight loss, changes occur in the composition of the microbiota, decreasing _Frimicutes_ and increasing _Bacteroidetes_[60]. The environment of a given individual also has great influence on the skin microbiome, with the profession or the type of daily activity also having an effect. Some studies suggested that the time children spend outdoors could be relevant along with other factors (e.g., cultural differences), and the constant and close contact with animals could influence the composition and diversity of the skin microbial communities in healthy people [61; 62]. Some authors postulate that differences in the skin microbiome of urban and rural residents may be related to the exposure to microorganisms from the soil, water, and other factors, such as the biomass used in agriculture or livestock [63]. The external environmental conditions also have an important influence on the skin microbiome, including temperature, humidity, and sunlight. When skin is exposed to UV radiation, several impacts may occur. The exposure of the skin to UV rays inhibits the growth of _S. aureus_ and _C. acnes_; the latter is associated with a decreased production of porphyrins [64]. Furthermore, UV exposure results in a reduction in Lactobacillus and Pseudomonas and an overall increase in Cyanobacteria [65]. Additionally, it has been shown that repetitive and intense exposure to UV radiation may increase the skin's vulnerability to infections and worsen associated symptoms, e.g., herpes simplex virus (HSV) [66]; on the contrary, _Staphylococcus aureus_ was reduced by UVB radiation. However, there are benefits derived from exposure to light; thus, the antimicrobial effects of photodynamic therapy (APDT) were demonstrated [67]; some studies suggest that blue light treatment and conventional UV phototherapy may act beneficially in acne vulgaris by reducing _Corynebacterium acnes_ density [68; 69; 70].