**2. The role of the intestinal microbiota in the physiology of the human body**

The microbiota of the digestive tract consists of about 3 × 1013 to 40 × 1013 (3–40 trillion) bacterial cells, counting at least 10 times more than their host cells. The groups of *Bacteroidetes* and *Firmicutes* predominate in a numerical proportion of 90%, aside from lower density of *Proteobacteria, Actinobacteria, Fusobacteria*, and *Verrucomicrobia* and small populations of fungi, *Archaea*, and viruses, all exerting major functional effects on different organs. The bacterial microbiota belongs to 1000–1160 types of species [8]. The individual microbiota is evaluated in 150 to 160 species by the 16S RNA (rDNA analysis) ribotyping method [9]. The population composition of the intestinal microbiome stabilizes at the age of 3 years and is determined by various conditions, such as genetic factors, the maternal microbiota, the mode of birth (i.e., natural or by cesarean section), the antigenic exposure during early life, and is reconfigured mostly by diet [10, 11].

The microbiota is considered a virtual organ, whose functions must be integrated into general physiology. The host-microbiota interaction is primarily a symbiotic relationship, in which the host organism provides the ecological niche and nutrients for microbiota survival. The microbiota carries out fermentative and biosynthetic metabolic activities, thereby influencing systemic physiology [12]. The metabolism of the microbiota functions as a bridge between the diet with the human body. The intestinal microbiota increases the energy efficiency of the diet by fermenting the fibrous components, providing essential metabolites for organ systems, especially short-chain fatty acids (SCFA), such as acetic, propionic, and butyric acid. A proportion of 50% of the energy needs of epithelial cells is provided by SCFA [13, 14]. The modern diet is 7–10 times poorer in the fibrous component, compared to the traditional Mediterranean one. Microbiota synthesizes vitamin K and B, synthesizes amines through which it modifies endocrine function, stimulates the inflammatory process, has a protective role against the invasion of enteric pathogens (*Shigella flexneri*), metabolizes some drugs to their active form, ferments indigestible components of the diet (complex polysaccharides, amino acids,

xenobiotics) [15], and modulates the lipid metabolism. The bile acids synthesized in the liver from cholesterol, facilitate the absorption of lipids and fat-soluble vitamins and maintain cholesterol balance. Also, the biliary acids have a signaling function through specific hepatocyte receptors [16–19]. All these functions are impaired in patients with endocrine AIDS.
