Probiotic agents are defined as live microorganisms, which confer a health benefit to the host when administered in adequate amounts. However, also molecules produced by both probiotic and non-probiotic bacteria and administered alone seem to provide beneficial effects similar to those obtained with live cells. These molecules can stimulate different signalling pathways by activating pattern recognition receptors and modulate different immune responses in the host. This review focuses on bacterial components, such as surface molecules and DNA, with beneficial immunomodulatory properties. Lipopolysaccharides from Gram-negative bacteria, lipoteichoic acid from Gram-positive bacteria and DNA of specific strains are the most effective molecules in modulating the host immune response, as they can modify the pattern of released cytokines. Bacterial components with beneficial properties could be an interesting substitute to traditional probiotics, as they could eliminate some of the risks associated with the consumption of live bacteria. Data presented in this review show that the concept of probiotic agent should change, as a large portion of the beneficial effects of probiotic bacteria does not depend on their viability and integrity. A more clear understanding of molecular mechanisms resulting in the health benefits provided by probiotics could be the basis for a more rational application of probiotics in functional foods or as supporting therapy for specific disorders.
Keywords: Probiotic, surface molecules, bacterial DNA, lipopolysaccharides, polysaccharide A, lipoteichoic acid, bacterial components, probiotic bacteria, live microorganisms, Gram positive bacteria
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