The Sus operon: a model system for starch uptake by the human gut Bacteroidetes

Abstract

Resident bacteria in the densely populated human intestinal tract must efficiently compete for carbohydrate nutrition. The Bacteroidetes, a dominant bacterial phylum in the mammalian gut, encode a plethora of discrete polysaccharide utilization loci (PULs) that are selectively activated to facilitate glycan capture at the cell surface. The most well-studied PUL-encoded glycan-up-take system is the starch utilization system (Sus) of Bacteroides thetaiotaomicron. The Sus includes the requisite proteins for binding and degrading starch at the surface of the cell preceding oligosaccharide transport across the outer membrane for further depolymerization to glucose in the periplasm. All mammalian gut Bacteroidetes possess analogous Sus-like systems that target numerous diverse glycans. In this review, we discuss what is known about the eight Sus proteins of B. thetaiotaomicron that define the Sus-like paradigm of nutrient acquisition that is exclusive to the Gram-negative Bacteroidetes. We emphasize the well-characterized outer membrane proteins SusDEF and the α-amylase SusG, each of which have unique structural features that allow them to interact with starch on the cell surface. Despite the apparent redundancy in starch-binding sites among these proteins, each has a distinct role during starch catabolism. Additionally, we consider what is known about how these proteins dynamically interact and cooperate in the membrane and propose a model for the formation of the Sus outer membrane complex.

Microbiota, metabolome, and immune alterations in obese mice fed a high‐fat diet containing type 2 resistant starch

“…It is notable that at least some of the mice contained high levels of S24‐7 prior to the introduction of the HF diet and therefore RS supported at least a partial‐restoration of that family to the intestine. These findings are in agreement with enrichment of the Bacteroidetes phylum with greater fermentable fiber intake in humans and the broad presence of polysaccharide utilization loci (PUL) that enable these bacteria to bind and catabolize extracellular carbohydrates, including starch . In rodents, both S24‐7 and Allobaculum proportions were increased with the introduction of RS and other fermentable carbohydrates and were lowered with the introduction of obesogenic diets .…”

Identification of a New Lipoprotein Export Signal in Gram-Negative Bacteria

“…To date, most studies have focused on identifying and characterizing the functions of these Bacteroidetes surface complexes (5, 7, 16–18, 22, 23), but little is known about how they assemble (24) and particularly about how lipoproteins reach the bacterial surface. In Gram-negative Proteobacteria , lipoprotein synthesis and transport have been well studied in model organisms such as Escherichia coli (25).…”

Anaerobic utilization of Fe(III)‐xenosiderophores among Bacteroides species and the distinct assimilation of Fe(III)‐ferrichrome by Bacteroides fragilis within the genus

“…Although progress has been made in recent years in the understanding of the structures and functions of the SusC‐like protein family of TBDTs (Foley, Cockburn, & Koropatkin, ; Martens et al., , ), very little is known about Bacteroides TBDTs role in the assimilation of iron‐chelate complexes. Bacteroides species contain an extensive number of predicted TBDTs potentially involved in iron acquisition but their substrates and regulatory controls have not been well defined compared to the classical TBDTs in aerobic and facultative bacteria whose cognate substrates and regulation of the transport mechanisms are well understood (Koebnik, ; Schalk & Guillon, ; Schalk et al., ; Schauer et al., ).…”