In biofilms, flocs and similar multispecies microbial communities, exopolysaccharides (EPSs) are always present, frequently as the major component other than water. The EPSs vary widely in their composition, structure and properties and thus it is impossible to generalise about their contribution to biofilm or floc structure. Relatively few of the polymers obtained from biofilms and flocs have been adequately purified and analysed but such evidence as is so far available suggests that the polysaccharides closely resemble those synthesised by the corresponding planktonic bacteria. From a knowledge of the physical properties of these, it is now possible to present a reasonably accurate picture of some of the factors which they may contribute to the structure and stability of complex aggregates of micro-organisms in biofilms and flocs.

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