Molecular characterization of the starch utilitization system of Bacteroides thetaiotaomicron
Bacteroides spp. are obligately anaerobic Gram-negative bacilli that reside in high numbers in the colons of humans and animals. It has been estimated that Bacteroides spp. constitute as high as 30% of all colonic isolates. Since mono- and di-saccharides are efficiently absorbed in the small intestine, Bacteroides spp. are probably using polysaccharides as their main sources of carbon and energy. One of the reasons that Bacteroides spp. are so successful at colonizing the colon may be due to the nutritional versatility exhibited by the numerically predominant colonic Bacteroides spp., namely, B. thetaiotaomicron, B. vulgatus, and B. uniformis. All these species are able to ferment a wide variety of dietary polysaccharides as well as host-derived mucopolysaccharides. Another possible reason for their success at colonizing the colon may be due to the intrinsic nature of the way Bacteroides' polysaccharide utilization systems work. Instead of taking a divide-and-conquer approach, that is, secreting extracellular polysaccharidases into the surrounding medium and translocating the breakdown products of enzymes through porins, Bacteroides spp. appear first to bind the polysaccharide at the cell surface and then to transport the molecule into the periplasm, where the majority of the polysaccharide-hydrolyzing enzymes are located. Nonetheless, the preponderance of the starch-hydrolyzing activity is within the cell, and until the precise location and topology of the outer membrane enzymes are found, the current model favors translocation across the outer membrane by a starch binding receptor. In any case, this general polysaccharide uptake strategy (substrate-sequestration) would confer a selective advantage to Bacteroides spp. in competition with those species of colonic bacteria that secrete extracellular enzymes, who most likely lose a large amount of potential nutritional sources to nearby competitors. My work centered on the characterization of several outer membrane proteins (OMPs) involved in the early steps of the starch utilization process with the broad aims of determining the functional roles of the OMPs and developing a general working model of starch utilization in B. thetaiotaomicron, particularly with regard to the early steps.
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign