Microbial exopolysaccharides

Another delivery alternative is a hydrogel-based system which entraps the drug and releases it afterwards in the human body due to its swelling properties in the presence of water [261]. Doxorubicin and dopamine [262] have been efficiently incorporated into the dextran hydrogels for therapeutic purposes [263]. Last but not least, dextrans have been investigated for their utility in medical imaging [264]. 

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I. W. Sutherland, Biotechnology of Microbial Exopolysaccharides, Cambridge University Press, Cambridge, U.K., 1990. 

Commercial applications for polysaccharides include their use as food additives, medicines and industrial products. Although plant polysaccharides (such as starch, agar and alginate) have been exploited commercially for many years, microbial exopolysaccharides have only become widely used over the past few decades. The diversity of polysaccharide structure is far greater in micro-organisms compared to plants and around 20 microbial polysaccharides with market potential have been described. However, microorganisms are still considered to be a rich and as yet underexploited source of exopolysaccharides. 

Figure 7.1 Structures of some of the components of microbial exopolysaccharides.

The presence of uronic acids in microbial exopolysaccharides results in their polyanionic nature. 

Some microbial exopolysaccharides contain the inorganic substituents phosphate and sulphate. Phosphate has been found in exopolysaccharide from bacteria of medical importance, including Escherichia coli. Sulphate is far less common than phosphate and has only been found in spedes of cyanobaderia. In addition to these inorganic components, which form part of the structure of some exopolysaccharides, all polyanionic polymers will bind a mixture of cations. Exopolysaccharides are, therefore, purified in the salt form. The strength of binding of the various cations depend on the exopolysaccharide some bind the divalent cations calrium, barium and strontium very strongly, whereas others prefer certain monovalent cations, eg Na ... 

As with all polysaccharides, microbial exopolysaccharides can be divided into... 

The unique physical properties of microbial exopolysaccharides (considered in Section 7.7), which determine their commercial importance, arises from their molecular conformation. This, in turn, is determined by the primary structure and from associations between molecules in solution. 

From a commercial point of view, xanthan gum is the most important microbial exopolysaccharide currently being manufactured. Therefore, we shall consider the fermentation of this product by Xanthomonas campestris in some detail. 

Figure 7.5 Production of xanthan gum in batch culture using X. campestris. Bacterial dry weight ( ) xanthan gum ( ) residual glucose ( ) residual glutamate (A). Adapted from Microbial exopolysaccharide, Yenton etai pp 217-261. In biomaterials Novel Materials from Biological Sources, D Byrom (Ed), MacMillan Academic Professional Ltd, 1991.

Microbial exopolysaccharides are widely used in industry as viscosifiers and as gelling agents. In this section we will consider, in general, the rheology of exopolysaccharides in solution and their ability to form gels. Specific properties of individual microbial exopolysaccharides and applications which exploit these characteristics are considered later in this chapter. 

The viscosity of a solution of microbial exopolysaccharide must, therefore, be defined as a function of the shear rate (see Figure 7.7). 

One of the most striking and useful properties of exopolysaccharides is that they can form gels at relatively low concentration (typically around 1%). Gels are distinct from viscous solutions that flow readily and are used widely in the food industry and in some personal care products. The mechanism of gel formation depends on the type of microbial exopolysaccharide ... 

At temperatures below Tm, there is relatively little change in viscosity of microbial exopolysaccharides (Figure 7.8). 

At temperatures above Tm, chemical and enzymatic degradation of microbial exopolysaccharides is enhanced. The apparent enhanced stability of microbial exopolysaccharides in their ordered confirmation is thought to be due to the glycosidic bonds in the backbone of the polymer which raises the activation energy. This restricted movement would also restrict access of enzymes and chemicals to the backbone. 

Figure 7.9 Interconversion of sugar phosphates and sugar nucleotide phosphates. Adapted from "Biotechnology of microbial exopolysaccharides". IW Sutherland, Cambridge University Press, 1990.

False. D-xylose is a pentose sugar which are very rarely found in microbial exopolysaccharides. 

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