Mechanisms by which indigenous microorganisms colonize gastrointestinal epithelial surfaces.

Indigenous microorganisms are known to associate with epithelial surfaces in the gastrointestinal tracts of birds and mammals of many species. In general, the mechanisms by which microorganisms associate with the epithelia are poorly understood. In some cases, in associating with epithelial surfaces, the microbial species involved undoubtedly colonize (i. e., multiply on) them. Some examples of such microorganisms are strains of certain Lactobacillus, and Candida (Torulopsis) species that associate with gastric surfaces, and strains of segmented, filamentous and of oxygen-intolerant anaerobic bacteria of numerous species that associate with intestinal surfaces in mice and rats. Several properties of the microbial cells involved may be important for them to be able to colonize epithelial habitats. Some such properties are a capacity for the cells to adhere to structures on the epithelium, or to be motile and able to move into the mucous gel present on most gastrointestinal surfaces. In addition, microorganisms that can colonize surface microhabitats on gastric or intestinal epithelia must be able to thrive in the environments and nutritional circumstances found in such habitats. In this respect, a capacity to multiply well in environments containing high concentrations of hydrogen ion is important for microbial cells to be able to colonize gastric surfaces, while a capacity enzymatically to digest mucinous glycoproteins and use the degradation products as carbon, energy and nitrogen sources may be important for microorganisms to be able to colonize most gastric or epithelial surfaces. These and related issues are examined in this paper.