BIOLOGY-Xanthan gum

Xanthan gum is a complex exopolysaccharide produced by the plant pathogenic bacterium Xanthomonas campestris pv. campestris. It consists of D-glucosyl, D-mannosyl, and D-glucuronyl acid residues in a molar ratio of 2:2:1 and variable proportions of O-acetyl and pyruvyl residues. Because of its physical properties, it is widely used as a thickener or viscosifier in both food and non-food industries. Xanthan gum is also used as a stabilizer for a wide variety of suspensions, emulsions, and foams. We are interested in the biosynthesis and degradation of xanthan. The biosynthetic pathway of xanthan resembles that of succinoglycan

The polysaccharides are classified according to their uses as viscosity-increasing agents and as gelling agents.

A third category includes polysaccharides with specific applications such as tailor-made dextran and pullulan and polysaccharides used as substrates for the preparation of rare sugars.

Three other industrially useful bacterial polysaccharides have been developed. S−130, the extracellular, high viscosity polysaccharide produced by a strain of Alcaligenes, has excellent suspending and heat stability properties useful in oil field drilling, workover and completion, and enhanced oil recovery fluids. S−194 has excellent suspending properties and unusual compatibility with salts, making it useful in agricultural applications, particularly flowable pesticides and liquid fertilizers. S−198 has excellent stability to shear and has potential application in the developing market of water−based lubricants.

Microbial polysaccharides are complex carbohydrate polymers produced by a variety of microorganisms. Those of commercial interest are generally produced extracellularly in substantial quantities. Commercial applications can be based on specific structural features or on physical properties. Applications based on physical properties include use as water-soluble thickeners, gelling agents, and emulsion stabilizers. Applications based on specific structural features include the use of bacterial capsular antigens as vaccines, and the use of polysaccharides as metal complexants. Economically, the most important microbial polysaccharides are xanthan gum, dextran, and gellan. Others, such as pullulan, curdlan, and scleroglucan, are used less in the United States, but significant quantities are used overseas. Other microbial polysaccharides that show promise are bacterial cellulose, emulsan, liposan, levan, and succinoglucan.