**1. Introduction**

#### **1.1 Polysaccharides**

Polysaccharides are biopolymers that are widely distributed in nature. Certain microorganisms have the ability to produce a large amount of polysaccharides in the presence of a surplus carbon source. Some of these polysaccharides (e.g. glycogen) serve as storage compounds while others are excreted by the cell. Different monosaccharides (hexoses and pentoses) including some complex sugars are linked glycosidically to form long chains of the polymer. These polysaccharides exhibit a wide range of chemical structures with greatly differing physical properties. A considerable expenditure of energy is incurred by the microbial cell to synthesize these biopolymers.

Microorganisms produce a diverse range of biopolymers with varied chemical properties by using both simple as well as complex substrates. These biopolymers could be either intracellular or extracellular depending upon their cellular location. Though the intracellular biopolymers are limited, nevertheless, the range of the extracellular biopolymers is vast and may be categorized into four major classes; polysaccharides, inorganic polyanhydrides (such as polyphosphates), polyesters, and polyamides to be collectively termed as extracellular polymeric substances or exopolysaccharides (EPS).

### **1.2 Exopolysaccharides**

Exopolysaccharides are high-molecular-weight polymers that are synthesized and secreted by the microorganisms into the surrounding environment. These exopolysaccharides are mainly polysaccharidic in nature, that is, they are generally composed of monosaccharides and some non-carbohydrate substituents such as acetate, pyruvate, succinate, and phosphate. They are either covalently linked or loosely attached to the cell surface or can be released into the surrounding environment [1]. These exopolysaccharides are categorized into two groups: homopolysaccharides and heteropolysaccharides [2]. The homopolysaccharides consist of only one type of single structural unit whereas the heteropolysaccharides are composed of high-molecular-mass hydrated molecules made up of different sugar residues [3]. The composition of the EPS, however, varies with the type of microorganisms.
