Figure 2.

Strategies for extracellular digestion of complex carbohydrates. (A) The ‘sharing’-strategy: extracellular enzymes or enzyme complexes (e.g., Cellulosomes) process complex carbohydrate substrates into simple sugars and small oligosaccharides. These substrates can be readily utilized by any proximal bacterium. In Gram (+) bacteria products are bound by solute binding proteins (SBPs) and transported across the cell wall by ATP-dependent transporters MNA₂. (B) The ‘selfish’-strategy: minimal extracellular processing of the substrate. Complex oligosaccharides are transported into the periplasm through the Sus-like transport system where the bulk of depolymerisation occurs. Monosaccharides enter the cytoplasm through a major facilitator superfamily (MFS) transporter. Variations of this pathway exist as come complex oligosaccharides released during extracellular processing may be selectively used by other bacteria in the ecosystem (indicated with a yellow star); however, they remain inaccessible to the majority of bacteria within proximity.