Microbial Exopolysaccharides
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CHAPTER 3.3 l a i bor c M i
sed i rahccasy l opoxE
Microbial Exopolysaccharides TIMOTHY HARRAH, BRUCE PANILAITIS AND DAVID KAPLAN
Introduction The abundant nature and manifold uses of polysaccharides derived from plants and animals are well established. Application of these materials, notably chitosan and cellulose, in food and healthcare products is widespread. Unfortunately, issues of purity and seasononal availability and variability surround the use of these and other materials when obtained from environmental sources. Although several hydrocolloids used widely in the same sectors (i.e., xanthan, gellan, and dextrans) are currently available only from microbial sources, commercial and academic interest in the synthesis and properties of microbial polysaccharides has continued to grow over the past several decades for a source of more economical, consistent, and higher purity raw material (Giavasis et al., 2000). Continuing focus on environmentally benign production techniques and the desire for customizability in bacterially derived products also motivate continued investigation into the molecular biology, polymer science, and process engineering of microbial exopolymer production. While progress continues in the understanding of biosynthetic pathways, substrate transport, and polymer secretion in a variety of organisms, recombinant DNA techniques have also resulted in the elaboration of new, selectively modified polymers that offer the potential for a wide variety of novel materials with more predictable structure and function. The ongoing application of biotechnology to the synthesis, optimization, and customization of bacterial polysaccharides is the focus of this chapter. While the production of polysaccharides by selected microorganisms was first reported in the 1880s (Whitfield, 1988), large-scale application was not achieved until the middle of the twentieth century, with the production and approval of dextran from Leuconostoc mesenteroides in 1947 and the United States Food and Drug Administration (FDA) approval of food grade xanthan from Xanthamonas campestris in 1969. Today, both materials continue to be important (although xanthan dominates) in the estimated
$253 million market for microbial polysaccharides (Magnani, 1998). In the intervening years, various polysaccharides of microbial derivation have been described and, in a few cases, developed into products. While the production of bacterial polymers continues to expand, a still surprisingly small list of polymers has achieved industrial relevance (Sutherland, 1998; Table 1). This continues to be primarily motivated by factors of organismal productivity, regulatory approval, and thus in both cases, cost. Also at issue are two areas of fundamental relevance, the relatively loose understanding of structurefunction relationships that slows the progress of research and development in this area (Sutherland, 2001) and the lack of template-controlled synthesis characteristic of proteins. It should be mentioned from the outset that this chapter necessar
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