Solving the problem of how to eat something as big as yourself: Diverse bacterial strategies for degrading polysaccharides

Polysaccharide digestion by bacteria is an important activity in many ecosystems, and a number of bacterial genera can perform this function. Although many papers have been published about the properties of isolated polysaccharide-degrading enzymes, relatively little is known about how intact bacteria degrade polysaccharides. This review summarizes recent findings suggesting that there are at least three different strategies. the most familiar one is the excretion of extracellular polysaccharidases, which diffuse to and degrade nearby polysaccharides. An example of this type of strategy is provided by the plant pathogen,Erwinia spp. A second strategy is to have the enzyme exposed to the extracellular medium but attached to the surface of the cell. Examples of this strategy are provided by the pullulanase system ofKlebsiella oxytoca and the cellulosomes ofClostridium thermocellum. A strategy that could be seen as a combination of the extracellular enzyme strategy and the surface organelle strategy is provided byVibrio harveyi, which attaches to its substrate, chitin, via proteins that appear to be specialized for attachment and produces extracellular enzymes that attack the chitin. A third strategy is to import the polysaccharide, as appears to be done byBacteroides spp. In this instance, the polysaccharide is bound to an outer membrane receptor, then passes into the periplasm where the degradative enzymes are located. The ecological advantages and disadvantages of these systems are discussed, and areas where further research is needed are defined.