Microbial exopolysaccharides: Main examples of synthesis, excretion, genetics and extraction

Exopolysaccharides (EPSs) produced by microorganisms represent an industrially untapped market. Some microorganisms can produce and excrete over 40 g L-1 of EPS in simple but costly production conditions. Approximately thirty strains of eukaryotic and prokaryotic microorganisms are notable for their EPS production. EPSs are produced in response to biotic and abiotic stress factors and/or to adapt to an extreme environment. The main function of EPSs is to aid in protection against environmental pressures. Heteropolysaccharides and some homopolysaccharides are synthesised in microbial cells and then secreted into the extracellular environment. More currently, homopolysaccharide synthesis occurs outside of the cells after specific enzymes are exuded. Although natural secretory mechanisms exist in microorganisms, it is often necessary to resort to physical or chemical extraction methods to improve the yield of EPSs at an industrial level. In light of growing interest, our basic understanding of microbial EPSs needs to be improved. (C) 2011 Elsevier Ltd. All rights reserved.