Modulation by Probiotics

In general, probiotic bacteria do not colonize the human intestinal tract permanently, but specific strains are able to transiently colonize or persist for some time in the intestine and may modulate the indigenous microflora. The rationale for modulating the gut microflora by means of probiotics derives from the demonstration that this microflora is important to the health of the host. Specific probio-tics have been shown to colonize temporarily the human intestinal tract, thereby modulating the intestinal microflora both locally and at the commensal level. Such modification has not been reported to be permanent; rather it is related to a balancing of aberrant or disturbed microflora to assist it to return to normal metabolic and physiological activities. Such modulation and restoration of the normal state of the microflora activity is a key target for probiotic action. However, the state of the microflora should be well characterized to enable the selection of specific probiotics to counteract the aberration or disturbance in question.

Specific probiotic bacteria can modulate both the intestinal microflora and local and systemic immune responses. Activation of immunological cells and tissues requires close contact of the probiotic with the immune cells and tissue on the intestinal surface. Interestingly, both lactobacilli and bifidobacteria, which colonize mainly the small and large intestine respectively, when given as probiotic supplements were able to modify immunological reactions related to allergic inflammation, whereas lactobacilli were ineffective in protection against cows' milk allergy. In this respect, preferential binding of probiotics on the specific antigen-processing cells (macrophages, dendritic, and epithelial cells) may be even more important than the location of adhesion. It is also known that the cytokine stimulation profiles of different Bifidobacterium strains vary and that strains isolated from healthy infants stimulate mainly noninflammatory cytokines.

Results of an increasing number of clinical and experimental studies demonstrate the importance of constituents within the intestinal lumen, in particular the resident microflora, in regulating inflammatory responses. Probiotic bacteria may counteract inflammatory processes by stabilizing the disturbed gut microbial environment, forming a stable healthy microflora and thus improving the intestine's permeability barrier. Another mode of action comprises enhancing the degradation of enteral antigens and altering their immunogenicity. Yet another mechanism for the gut-stabilizing effect could be improvement of the intestine's immunological barrier, particularly intestinal IgA responses. Probiotic effects may also be mediated via control of the balance between pro- and anti-inflammatory cyto-kines. Such effects may be mediated through changes in the intestinal microflora, especially by modulation of the bifidobacteria microflora.

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