Level 2: local barrier effects
The probiotic effects that are grouped in this level affect processes that take place at the interface between the outside and the inside of the body. At this level most of the probiotic actions affect local microbe ↔ barrier interactions. The nature of these interactions depend on the body site, as the barrier formed by, for example, the skin has different properties than the intestinal or vaginal barrier. Furthermore, in this “3 levels of probiotic action” concept, the barrier is not only formed by a physical barrier consisting of epithelial cells, but also comprises e.g. the overlaying mucus layer, and underlying connective tissue. An example of an important microbe-barrier interaction is the effect of gut microorganism on the permeability of the intestinal barrier. The intestinal barrier is formed by a single layer of epithelial cells that constitutes the largest epithelial barrier that we have in our body. This epithelial cell layer permits the absorption of nutrients, electrolytes and water, while maintaining an effective defence against intraluminal toxins, antigens and microorganisms. The permeability of the intestinal barrier is amongst others regulated by so called tight junctions, protein structures that connect epithelial cells. Studies have shown that the intestinal microorganisms can target various intracellular pathways to change the expression and distribution of tight junction proteins, and thereby regulate intestinal barrier permeability.
Other examples of effects on level 2, are the effects of microbial activities on the:
level of cytoprotective compound production (e.g. defences) by host cells;
regulation of host mucus secretion;
regulation of gut motility.