## 2 Human Microbiome
### Microbiome Types
#### 2.1.3 Skin  
Being the largest organ in the human body, the skin's main function is to serve as a physical barrier, protecting the organism against external aggressions and potential colonization by microorganisms, given that it is in daily contact with a myriad of them [47]. The skin represents a human habitat, where environmental factors and lifestyles can shape the microbial community differently in different specific regions of the human body. One of the main characteristics of the human skin microbiome is its high diversity and interpersonal variation, with a slightly higher concentration in hair follicles. This is extremely important for the health of the skin, as it plays an essential role in its protection against external and potentially harmful substances [15; 48; 49]. In this way, the low pH of the skin, resulting from the production of acids during keratinization and the excretion of acids by epithelial cells and microorganisms, preventsnew microorganisms from surviving due to those inhospitable conditions. This makes the skin a highly effective barrier against microbial colonization. Additionally, temperature is another factor that can influence the microbiome. Due to the increase in body temperature, sweat production alters the humidity percentage on the skin's surface. The subsequent evaporation of sweat increases the skin's salinity, preventing the proliferation of certain microorganisms, especially Gram-negative bacteria [47]. In this way, by combining all the described factors with the various microenvironments, we have different microbial populations in the various regions of the skin. The cutaneous microbial community is specific to each region of the human body, each individual, remaining quite stable over time, despite constant exposure to different environments. Instead of acquiring the prevalent microorganisms from the environment, hosts maintain their microbiome, which ends up functioning as a unique microbial fingerprint for each person [50, 51]. [PERSON] and [PERSON] published that the four dominant phyla residing on the skin are Actinobacteria, Proteobacteria, Firmicutes, and Bacteroidetes, the most-predominant bacterial genera being _Staphylococcus_, _Cutiobacterium_, and _Corynebacterium_, each of which is concentrated differently among various skin locations [48]. Areas with a high density of sebaceous glands, such as the back, face, chest, and the area behind the ears, tend to be colonized by large quantities of lipophilic microorganisms, such as _Cutiobacterium_. This is because these bacteria hydrolyze the triglycerides found in the sebaceous glands, subsequently releasing free fatty acids on the skin, which acidify and soften it, acting as an embilient. Moreover, these areas tend to have lower levels of diversity compared to dry and moist regions [48, 52, 53]. The role of sebum in defining the cutaneous microbiome can be reflected in age-related changes in the composition and diversity of this microbiome. After birth, babies are colonized by more bacteria of the phylum Firmicutes than Actinobacteria. Generally, as sebaceous glands mature during puberty, there is a shift in the skin microbiome with an increase in Actinobacteria, including _Corynebacterium_ and _Cutibacterium_. This age-associated change may be a significant factor in reducing the incidence and severity of common childhood skin diseases, such as atopic dermatitis. The fungi present in sebaceous areas tend to be less diverse than the bacterial communities, with the dominant species being _Mallasezia_[54, 55]. Humid or occlusive areas of the skin typically have a microbial community composed of Gram-positive bacteria such as _Staphylococcus_ and _Corynebacterium_, which prefer high moisture concentrations in their habitat, as found in the belly button, armpits, groin, behind the knee, and inner elbow areas. In these locations, the high humidity, elevated temperature, and concentration of skin lipids promote the growth of the microbiome.