The Interactions between Innate Immunity and Microbiota in Gastrointestinal Diseases

The gut microbiota refers to the collection of microbial populations that reside in the gastrointestinal tract. It is characterized by an interplay between different cell types and their defense systems, food particles, molecules derived from digestion, and the vast array of residing microbial species with their secretory products. These microorganisms present in the gut lumen, which can be classified as probiotics, commensals, or pathogens, form the microbiota, which exerts several physiological functions, that is, the absorption and digestion processes, tolerance to non-self-food antigens, and defense from pathogens. The human microbiota can weigh up to 2 kg in total and contains tens of trillions of microorganisms, a number 10 to 20 times greater than the total number of cells in the human body, and includes at least 1000 different bacterial species [1]. This rich gut microbial community has coevolved in a symbiotic relationship with the human intestinal mucosa in such a way that the indigenous microbiota is essential for gut homeostasis. This ecosystem acts as a functional unit; thus, microbiota is considered a “superorganism” and is an integral part of the gastrointestinal tract [2].