The mechanism(s) by which prebiotics promote the relatively specific proliferation of bifidobacteria remain speculative. It is probably due to the efficient utilization of these carbohydrates as carbon and energy sources by bifidobacteria relative to other intestinal bacteria, and their tolerance to the SCFA and acidification of the microenvironment resulting from fermentation. Additionally, many bifidobacteria adhere to large granular substrates such as resistant starch and these may provide a site for colonization as well as a substrate (13,169). The ability of bifidobacteria to use a wide variety of oligosaccharides and other complex carbohydrates reflects their evolution in the hind-gut of humans and animals where the ability to metabolize a diverse range of food and host-derived complex carbohydrates and glycoproteins provides a competitive advantage. Analysis of the Bif. longum genome has revealed a large number of proteins specialized for the catabolism of carbohydrates (170).
Interestingly, while many bifidobacteria grow well when cultured with prebiotic oligosaccharides as their sole carbon and energy source, they often do not grow when supplied only with the monosaccharides from which these oligosaccharides are composed (74,171,172). This physiology may be another consequence of their evolution in an environment with a limited availability of simple sugars. It suggests that bifidobacteria lack transport mechanisms for many monosaccharides and import prebiotic oligosaccharides before hydrolyzing and metabolizing them. This presumably minimizes the availability of released simple sugars for cross-feeding by other intestinal bacteria and may be another factor contributing to the specific bifidogenic effect of NDOs.
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