The ability to produce polysaccharides is widely found among different species, but despite many sources of these biopolymers, those from algae and higher plants are dominant on the world market. Polysaccharides derived from microorganisms, including bacteria, yeasts and moulds, have still not been exploited enough. The main reasons for that are associated with costs of production, because of specific substrate requirements in certain cases, bioreactors demands or provision of aseptic conditions. Nevertheless, polysaccharide production from microorganisms has many advantages: it takes significantly less time compared to plants; in case of some algae species, it is more energy efficient because of the use of solar energy for production, and a lot of industrial waste and raw materials can be used as carbon sources, which is probably the greatest advantage of all (Donot et al. 2012). MPSs are synthesized and accumulated mostly after the growth phase, and, in regard to their location in the cell, they can be divided into three main groups. Inside the cell, carbon and energy sources are cytosolic endopolysaccharides. The second group is made of those that make up the cell wall. Polysaccharides exuded into the extracellular environment are known as exopolysaccharides (EPSs), and they appear in the form of capsules or slime. They are also involved in biofilm formation, where they have many significant roles: participation in attachment to a surface, formation and stabilization of biofilm structure, enhancement of resistance to environmental biotic and abiotic stresses and antimicrobial agents, preventing of desiccation and assumption of nutrients (Shia and Zhua 2009). MPSs are divided into two groups: homo-polysaccharides, made up of a single type of monosaccharide (e.g. pullulan, dextran or levan) and heteropolysaccharides, made up of several types of monosaccharide, with complex structures (e.g. xanthan or gellan). MPSs are also classified on the basis of their microbiological origin on bacterial and fungal polysaccharides. Commercially, the most important are following glycans: from bacteria—heteropolysaccharides (xanthan, alginate, hyaluronan, gellan); homopolysaccharides (dextran, cellulose, curdlan, levan) and from fungi—homopolysaccharides (beta glucan, pullulan, scleroglucan). Structures of some MPSs are shown in Fig. 17.1. In many cases, they are mainly composed of glucose, galactose and mannose but many other neutral amino sugars and uronic acids are often present, too. Also they can contain some organic ester-linked substituents and pyruvate ketals, which give them anionic character and increase their lipophilicity (Freitas et al. 2011).
展开▼
