## 2 Human Microbiome
### Microbiome Types
#### 2.1.4 Gastrointestinal Tract  
The large and diverse colonization of the human gastrointestinal tract begins early in life. The gastric and intestinal bacterial ecosystem of an adult contains a complex array of microorganisms with over 100 trillion microbial cells, including more than 1000 bacterial species [63]. The colonization of the gastrointestinal tract begins shortly after birth, through maternal contact. Over the first few weeks, it becomes more diverse and changes until it stabilizes around 2 to 3 years of age, by which time it is already very similar to the gastrointestinal microbiome of an adult [64]. Regarding the stomach's microbiome, the fact that this organ has a highly acidic pH and secretes pepsinogen in its mucosa prevents any potential colonization, as microorganisms are destroyed. As a result of these adverse environmental conditions, the population remains very low. Some species resistant to the stomach's acid, hydrochloric acid, are  
Figure 1: Factors contributing to skin microbiome variation. included in the _Lactobacillus_ and _Streptococcus_ genera; _Heliobacter pylori_ is also resistant. However, the latter can cause chronic active gastritis, peptic ulcers, or even neoplasms [57]. The intestinal microbiome has several functions and interacts with the host beyond being merely a physiological support in the food digestion process. It is part of and regulates the intestinal mucosal barrier, controls nutrient absorption and metabolism, aids in the maturation of immune tissues, and prevents the proliferation of microorganisms. Under physiological conditions, the microbiome continues to stimulate the immune system since it is an effective defense mechanism against foreign agents in the body [65; 66; 67]. The composition of the intestinal microbiome is extraordinarily diverse and dynamic in short periods because the intake of food or medications significantly affects the microbial community. However, its composition appears to remain stable over time among individuals and their closest family members. Despite the daily intake of microorganisms through food, this population remains relatively constant, and exogenous factors are needed to disturb the balance of the commensal microbiome, such as the presence of pathogenic bacteria or the use of antibiotics [68; 69; 70]. The majority of nutrients that we obtain from the food we eat go through various human enzyme processing before they get absorbed in the small intestine. However, the intestinal microbiome contributes to the metabolism of fibers that are typically not digested by these enzymes. In the large intestine, a group of microorganisms, including _Lactobacillus_ and _Bifidobacterium_, contribute to the metabolization of plant-derived polysaccharides, fibers, oligosaccharides, undigestible proteins, and intestinal mucins to provide energy sources for the host [71; 72; 73; 74]. In addition to its role in digesting certain foods, the intestinal microbiome plays a vital role in preventing the invasion of certain pathogens by creating microbial resistance. It contributes to the education and stimulation of the immune system, maintains the integrity of the intestinal epithelium and homeostasis, and enhances the motility of the gastrointestinal tract. The bacteria that make up the colon's microbiome are also responsible for synthesizing vitamins such as B12, K, biotin, thiamine, and folic acid [68; 75]. The majority of microorganisms that make up this community are anaerobic bacteria from the phyla Bacteroidetes and Firmicutes [76], which play a fundamental role in food digestion and occupy about 90% of the gastrointestinal microbiome. Additionally, there are microorganisms from the phylum Actinobacteria (family Bifidobacteriumaceae) and Proteobacteria (family Enterobacteriaceaeae), although in much smaller quantities. Other bacteria from different phyla can also be found, such as Fusobacteria and Euryarchaeota (Kingdom Archaea), representing a small percentage of the intestinal microbiome.