EMERGING THERAPEUTIC APPLICATIONS OF BACTERIAL EXOPOLYSACCHARIDES - Biomaterial Applications: Macro to Nanoscales - page 146

8.1 INTRODUCTION
Bacterial exopolysaccharides (EPS) are high molecular weight (10-
1000 kDa) carbohydrate polymers produced by bacteria as secondary me-
tabolites [1]. Exopolysaccharides have critical physiological function in
bacterial cells ranging from cell adhesion to imparting adaptive response
to environmental adversities such as, extreme temperature, pH, salinity
etc. [2]. Most bacterial EPS are synthesized intra cellularly and exported
to the extracellular environment as macromolecules. But, a few polysac-
charides such as, levans and dextrans are synthesized and polymerized
outside the cells by the action of secreted enzymes that convert the sub-
strate into the polymer in the extracellular environment [3]. Secreted EPS
separated from the bacterial cell serves as an intact biopolymer that can be
processed and leveraged for various commercial applications.
Structural and chemical diversity of the EPS is expressed in terms of
its monosaccharide composition, spatial arrangement of the monosac-
charides, stereochemistry of the functional groups, molecular weight and
the net charge. Most of the monosaccharides present in EPS are made up
of pentoses (D-arabinose, D-ribose, D-xylose), hexoses (D-glucose, D-
galactose, D-mannose, D-allose, L-rhamnose, L-fucose), amino sugars
(D-glucosamine and D-galactosamine), acidic sugars (glucuronic acid and
mannuronic acid) [4]. Rare monosaccharides like 4-amino-4, 6-dideoxy-
2-O-methylmannose and bacillosamine (2, 4-diamino-2, 4, 6-trideoxy-D-
glucose) are limited to a few species of Vibrio and Bacillus, respectively
[5, 6]. Exopolysaccharides can be either homopolysaccharides or hetero-
polysaccharides. Homopolysaccharides are polymers made up of a single
type of monosaccharide unit (e.g., dextran-a polymer of glucose) and
heteropolysaccharides are composed of more than one type of monosac-
charide as repeating unit (e.g., alginate-a polymer of guluronic acid and
mannuronic acid). Polysaccharides are formed by the glycosidic linkages
between the monosaccharide units. Exopolysaccharides formed by β-1, 4
or β-1, 3 linkages between the monosaccharide units are characterized to
be rigid (e.g., xanthan) and EPS formed by α-1, 2 or α-1, 6 linked mono-
saccharide units are likely to be more flexible (e.g., dextran) [7]. These
linkages build the polymeric structure of the EPS resulting in the forma-
tion of high molecular weight complexes [1].
Possible application of the EPS is determined by the functional
groups present in it. The nature and degree of functionalization impart