Polysaccharide utilization colon bacteria

Our approach to these and similar question has been to investigate the mechanisms by which pure cultures of colon bacteria utilize individual polysaccharides in vitro, with a view to determining what factors affect the organism s decision to utilize a particular type of carbohydrate. This approach is based on the assumption that information about the specific features and limitations of polysaccharide-catabolizing systems in colon bacteria will permit us either to make predictions about the extent to which catabolism of a particular class of polysaccharides can occur in vivo or to develop specific methods for detecting metabolic states in bacteria in vivo. 

Human colonic bacteria are quite versatile with respect to their ability to ferment polysaccharides. Moreover, individual species do not seem to have a consistent strategy for degrading polysaccharides. In some cases, cell-associated periplasmic enzymes are used. In other cases the enzymes are extracellular or even membrane-bound. It may turn out to be the case that the catabolic strategy is specific for the type polysaccharide rather than for the species, i.e. different species may utilize a particular type of polysaccharide in the same way. 

Non-starch polysaccharides have little nutritional value in their own right, as they are compounds that are not digested or absorbed to any significant extent. Nevertheless, they are a valuable component of the diet, and some of the products of fermentation by colonic bacteria can be absorbed and utilized as metabolic fuel. Together with non-starch polysaccharides, we have to consider that proportion of starch that is (relatively) resistant to digestion in the small intestine (section 4.2.2.1), because it too is a substrate for bacterial fermentation. 

Because of the variety of polysaccharides which can be fermented by some Bacteroides species, it is difficult to predict with certainty which polysaccharides in the complex mixture of dietary and host-produced carbohydrates that enter the colon will be degraded most rapidly and most extensively. Further information about how these organisms make choices between different polysaccharides i vitro may help to clarify this issue. However, nutritionists who are interested in catabolism of dietary fiber components iji vivo should be aware that the bacteria may prefer other sources of carbohydrate, such as mucopolysaccharides from host secretions or even Maillard products, to the dietary polysaccharide under study, and that this preference may influence catabolism of a particular polysaccharide in ways which we cannot at present predict. Effects of this sort may be responsible for some of the individual-to-individual variation which is encountered in nutritional studies of dietary fiber utilization. 

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