Introduction to mechanisms of association of indigenous microbes

Indigenous microorganisms of numerous types associate with epithelial surfaces in the gastrointestinal tracts of mammals and birds. Some of the microbial types, e.g., Lactobacillus sp. in the stomachs of laboratory rodents, adhere to a surface without altering it ultrastructurely. In such cases, the adherence is mediated undoubtedly by macromolecules on the bacterial surfaces, possibly polysaccharides in most cases, interacting in specific ways with receptor macromolecules on the epithelial surface. Other microbial types that associate with epithelial surfaces without altering them ultrastructurally, e.g., Clostridium 109-2 in the mouse large bowel, may adhere to the surface only weakly or not at all, and maintain the association because they are motile and attracted to the epithelium by chemotactic substances. Microbial types that alter the ultrastructure of the epithelial cells to which they attach interact intimately with the membranes of the epithelial cells. In such cases, the microbes have specialized segments or ends for adhering to the membranes, and probably elaborate systems for stabilizing the membranes and cytoplasm at the site in the epithelial cell to which they attach. Some such organisms may have evolved unique reproductive mechanisms to maintain their populations on the epithelial surface.