Humans lack enzymes for digesting dietary fiber, so this aspect of our physiology has been almost entirely relegated to symbiotic gut microorganisms. Various bacteria that are abundant in the human gut microbiome have evolved mechanisms to sense, degrade and import sugars derived from dietary fiber polysaccharides and other sources, such as mucosal glycans, dietary additives and even other microbes. In collaboration with researchers around the world, we have intensively studied these mechanisms in Gram-negative, human members of the Bacteroidetes phylum and are expanding studies into other phyla. The Bacteroidetes employ a series of systems that are most often centered around TonB-dependent transporters (SusC-like proteins) and associated SusD-like proteins. The prototypic system in this group of organisms was characterized by Abigail Salyers and her lab at the University of Illinois in the 1980s and 1990s and termed the Starch Utilization System (Sus) in the human gut symbiont Bacteroides thetaiotaomicron. Beginning with the full genome sequence of B. thetaiotaomicron by Jeffrey Gordon and co-workers in 2003 and the subsequent advent of increasingly facile genome sequencing, variant "Sus-like" systems have now been identified in gut or environmental Bacteroidetes that target almost all of the polysaccharides known to exist on Earth, including cellulose, chitin and even the most complex polysaccharide in nature, rhamnogalacturonan II.
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