Polymer exocellular polysaccharide

Xanthan is the extracellular (exocellular) polysaccharide produced by Xanthomonas campestris. As with other microbial polysaccharides, the characteristics (polymer structure, molecular weight, solution properties) of xanthan preparations are constant and reproducible when a particular strain of the organism is grown under specified conditions, as is done commercially. The characteristics vary, however, with variations in the strain of the organism, the sources of nitrogen and carbon, degree of medium oxygenation, temperature, pH, and concentrations of various mineral elements. 

Most of the exocellular polysaccharides produced by bacteria are synthesized inside the bacterial cell, with the use of membrane-bound enzymes. Both types of chain assembly were observed for these polymers. In many cases, the mechanism of the assembly remains unidentified, and the nature of the glycosyl acceptors in the process is not clear. 

The material presented in previous sub-sections clearly shows that both of the possible mechanisms of polysaccharide chain-assembly may operate in the biosynthesis of bacterial polysaccharides. There is no clearcut, mechanistic difference in the biosynthesis of O-specific chains of lipopoly-saccharides, exocellular polysaccharides, and carbohydrate chains of Grampositive, cell-wall polymers for every class of polymer, the existence of both mechanisms of chain assembly was demonstrated. 

Acidic polysaccharides (see Table IV) that contain uronic acid residues are, perhaps, the most prevalent type of exocellular polysaccharide. Often, these acidic biopolymers contain other sugars, including pentoses, hexoses, and heptoses, also found in neutral polysaccharides (see Tables V and VI). In many instances, these polymers possess alkali-labile O-acyl substituents, such as acetic, formic, ma-lonic, pyruvic, and succinic acids. Positively charged biopolymers that contain free amino sugars are rare, but have been found (see Table VII). More often, these amino sugars are N-acylated, generally with acetyl groups. 

Bacterial polysaccharides represent a large variety of polymers biosynthesized by bacteria. Their chemical structures and also their physical properties in solution or in the solid state may vary widely. They often contain uronic acid and then become polyelectrolytes (190). Many new polysaccharides have been developed from bacteria for industrial purposes. Exocellular polysaccharides are produced on a large scale by the usual techniques of microbiology and fermentation. This procedure allows good control of the characteristics of the polymers and allows purification of the polysaccharides more easily than from other natural sources (191-194). Extension of such production also allows reducing the price and extends the range of applications. A good example remains the hyalmonan previously produced by extraction from animal somces but in which some fraction of proteins remained. Bacterial hyaluronan can be prepared in a very pme form (195). 

Formation of L-guluronic acid, a component of the alginic acid-like polysaccharide produced by P. aeruginosa and Azotobacter vinelandii, requires special comment. In this case, a polymer built from /3-(l- 4)-linked D-mannosyluronic acid residues serves as an intermediate in the biosynthesis.204,205 Part of the D-mannosyluronic acid residues in the polymer is subjected to an epimerization at C-5 catalyzed by an exocellular enzyme of the micro-organism,205-207 producing a polysaccharide composed of structural blocks that contain only D-mannosyluronic acid or only l-gulosyluronic acid residues, as well %s some having both. The mechanism of the epimerization remains unclear. 

Hansenula holstii NCYC 560 produces cell-wall, and exocellular, phosphorylated mannans of mol. wt. 10°. The cell-wall mannan contains two branched polysaccharides having (l- 2) and (1— 3) linkages, with /3-d, in addition to a-D, linkages. Phosphoric diesters are present. The exocellular phosphonomannan is composed of 3 polymers, each... 

Singh RP, Shukla MK, Mishra A, Kumari P, Reddy CRK, Jha B (2011) Isolation and characterization of exopolysaccharides from seaweed associated bacteria Bacillus licheniformis. Carbohydr Polym 84 1019-1026 Stanford PA (1979) Exocellular, microbial polysaccharides. In Tipson RS, Horton D (eds) Advances in carbohydrate chemistry and biochemistry, vol 36. Academic, London/New York, pp 266-303 Stingele F, Neeser JR, Mollet B (1996) Identification and characterization ofthe eps (exopolysaccharide) gene cluster from Streptococcus thermophilus Sfi6. JBacteriol 178 1680-1690... 

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