The mammalian gut Bacteroidetes display a greatly expanded capacity for glycan degradation, with many having the ability to flexibly forage on at least a dozen complex polysaccharides. This glycolytic potential is packaged into discrete polysaccharide utilization loci (PUL) that encode the necessary machinery for the degradation and import of a distinct glycan structure. PUL-encoded protein complexes are referred to as starch utilization (Sus)-like systems after the first such system for starch import described in Bacteroides thetaiotaomicron. Sus-like protein complexes are present in nearly every gut Bacteroidetes yet restricted to this phylum, and their glycan specificity dictates the bacterial metabolic niche.
The Sus of B. thetaiotaomicron is perhaps the simplest and most well characterized of these glycan uptake systems, though more recently many other systems that target host mucins, hemicellulose, and even peptides have been elucidated. All are comprised of a putative TonB-dependent transporter and two classes of carbohydrate-binding proteins: the SusD-like proteins and the surface glycan-binding lipoproteins (SGBPs). Within individual Sus-like systems there is some redundant polysaccharide binding between the SusD-like proteins and the SGBPs, yet each protein plays a distinct role in carbohydrate import. Much of our work with the complexes for the acquisition of starch and xyloglucan has revealed that the presence of these proteins, and likely their interactions with the TonB-dependent transporter, are more important than their ability to bind glycan. Through a detailed understanding of how human gut bacteria acquire carbohydrate nutrition in the highly competitive gut ecosystem, we can develop prebiotic and probiotic strategies to manipulate the composition of this community towards improved human health.