TonB-dependent transport by the gut microbiota: novel aspects of an old problem

The lower human gastro-intestinal tract is inhabited by an extremely high density of micro-organisms, collectively termed the colonic microbiota. Just two bacterial phyla dominate this habitat, the Gram-positive Firmicutes and the Gram-negative Bacteroidetes. The colon is further characterised by a relative lack of small, easily accessible nutrients such as simple sugars, lipids and amino acids. Instead, a plethora of diet-derived and host polysaccharides constitute the main source of nutrients. Due to their size and complexity, the uptake of such glycans for metabolic utilisation is an energy-dependent process, which in Bacteroides spp. is mediated by an outer membrane protein complex consisting of a SusC-like TonB-dependent transporter and a SusD-like substrate binding protein. In this review, we discuss our understanding of the mechanism of nutrient import by SusCD-like complexes and highlight aspects in which SusCD-mediated uptake differs from classical TonB-dependent transport.