Biosynthesis and Chemical Composition of Exopolysaccharides Produced by Lactic Acid Bacteria
Patricia Ruas-Madiedo, Nuria Salazar, and Clara G. de los Reyes-Gavilán
from: Bacterial Polysaccharides: Current Innovations and Future Trends (Edited by: Matthias Ullrich). Caister Academic Press, U.K. (2009)
Exopolysaccharides (EPS) produced by lactic acid bacteria (LAB) can be classified according to the chemical composition and biosynthesis mechanism as homopolysaccharides (HoPS) and heteropolysaccharides (HePS). HoPS are composed of a single monosaccharide type, either glucose or fructose, and they are polymerised extracellularly by means of glycansucrases, sucrose being the donor of the corresponding sugar moiety. These α- or β-D-glucans and β-fructans are polymers of high molar mass (≥ 1 x106 Da) which can have different degrees of branching and they are usually produced in amounts higher than 1 g/L, although this value varies according to the strain and the production conditions. The glucansucrase and fructansucrase enzymes present a conserved functional structure in which four domains can be recognized, although enzymes produced by different strains do not share similar sequences. The HePS are built up of repeating unit structures of two or more types of monosaccharides, substituted monosaccharides and other organic and inorganic molecules. To date, around 42 different unique structures comprising from di- to octasaccharides have been described. Galactose, glucose, rhamnose and to a lower extent N-acetyl-glucosamine and N-acetyl-galactosamine are present in their composition. The HePS biosynthesis is a complex process not fully understood, that involves the activity of several specific, as well as housekeeping, enzymes which also participate in the metabolism of carbohydrates and in the synthesis of some components of the cell-wall. The genes involved in HePS synthesis are organised in eps clusters that share a common structural organization in which genes involved in regulation, export, polymerisation and chain length determination, as well as glycosyltransferases responsible for the intracellular assembly of the repeated units, can be recognised. This report summarises an overview of the chemical composition, structure and physical properties, as well the genetics and biosynthesis of EPS produced by LAB and some bifidobacteria and propionibacteria strains read more ...