**1. Introduction**

The intestinal microbiota contributes significantly to metabolic, trophic, and protective functions. In this regard, intestinal bacteria are responsible for metabolism of many complex substances into simple components; thus, intestinal microbiota contributes to the digestion of nutrients and has a key role in the nutrition of the host, to the control of certain pathogens and to the improvement of the functions of the local immune system, preventing or participating in some pathologies such as colon cancer [1]. In addition, intestinal bacteria are involved in vitamin synthesis and also in ion absorption [2, 3]. Through their trophic effect, the intestinal bacteria stimulate the proliferation and differentiation of cells on intestinal epithelium [4]; also, these bacteria may contribute to the maturation of immune cells, regulating the proliferation of pathogenic microorganisms and their toxins [1]. In addition

#### **Figure 1.**

*Microbiota-GALT-brain axis. The vagus nerve makes the connection of the intestine to the brain and vice versa. Dysbiosis causes local alterations in the GALT and in the brain. When there is no dysbiosis, the bacterial metabolites participate in the state of local and systemic health and even more so over the brain.*

to the functions above described, it has been proposed that intestinal microbiota exerts indirect functions in other organs such as the liver and brain. Studies performed in humans and in animal models suggest that intestinal dysbiosis has an important role in the development of mood disorders such as depression, anxiety, and Parkinson's disease [5–7]. For these reasons, the interest in exploring the interactions between immune system, intestinal microbiota, and central nervous system (CNS) has increased (**Figure 1**).

On the other hand, when bacterial probiotics are administered in adequate amounts, they confer benefits on host health. The main functions of probiotics are to prevent and ameliorate several digestive and allergic disorders. Also, the microbiota modulates ontogeny and immune system functions, as well as the interactions of the intestine-brain axis to regulate some neurological functions. However, the microbiota effects are not only in intestine but also in peripheral tissues, such as in immune system modulation and interacting with the gut-brain axis to regulate some neurologic functions.
