The microbiome comprises bacteria that inhabit the gastrointestinal tract, conditioning the development and homeostasis of the host organism. Colonization of bacteria on epithelial cells is a prerequisite for a healthy host-bacterial interaction, and adhesion is the condition for colonization. The microbiome bacteria compete with the pathogenic bacteria for the place of adhesion. Bacteria adhere to the epithelium at the point of contact between cells. Controlled cell death, apoptosis, and cell reconstruction are strictly regulated processes that add up to a natural barrier against the invasion of pathogenic microorganisms.