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 iso lated polysaccharide-degrading enzymes, relatively little is known abo ut how intact bacteria degrade polysaccharides, This review summarizes recent findings suggesting that there are at least three different st rategies. The most familiar one is the excretion of extracellular poly saccharidases, which diffuse to and degrade nearby polysaccharides. An example of this type of strategy is provided by the plant pathogen, E rwinia spp, A second strategy is to have the enzyme exposed to the ext racellular medium but attached to the surface of the cell. Examples of this strategy are provided by the pullulanase system of Klebsiella ox ytoca and the cellulosomes of Clostridium thermocellum. A strategy tha t could be seen as a combination of the extracellular enzyme strategy and the surface organelle strategy is provided by Vibrio harveyi, whic h attaches to its substrate, chitin, via proteins that appear to be sp ecialized for attachment and produces extracellular enzymes that attac k the chitin. A third strategy is to import the polysaccharide, as app ears to be done by Bacteroides spp. In this instance, the polysacchari de is bound to an outer membrane receptor, then passes into the peripl asm where the degradative enzymes are located. The ecological advantag es and disadvantages of these systems are discussed, and areas where f urther research is needed are defined.