**1. Introduction**

370 The Complex World of Polysaccharides

Springer. 300 p.

pp. 405-420.

June 2012, Dubrovnik, Croatia.

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> Microbial biofilm development is observed on virtually all submerged surfaces in natural and industrial environments. Biofilms are also observed at interfaces as pellicles, or in the bulk of aquatic environments as flocs or granules [1, 2]. A biofilm is a complex structure made of aggregates of microbial cells within a matrix of extracellular polymeric substances (EPS) (Figure 1). The matrix structure constitutes the elastic part of the biofilm. Interstitial voids and channels separating the microcolonies contain a liquid phase, mainly constituted by water. This liquid phase is the viscous part of the biofilm. The EPS matrix provides the biofilm with mechanical stability through these viscoelastic properties [3].

> All major classes of macromolecule, i.e., polysaccharides, proteins, nucleic acids, peptidoglycan, and lipids can be present in a biofilm. Although extracellular polysaccharides are considered as the major structural components of the biofilm matrix, extracellular DNA plays an important role in the establishment of biofilm structure [4]. Moreover, nucleases can be regulators of biofilm formation [5]. To get a better understanding of the role of extracellular polysaccharides in the biofilm architecture and mechanical properties, it is necessary to take a look at the properties of a limited number of components, which can be isolated. Most microbial exopolysaccharides are highly soluble in water or dilute salt solutions, and capsule-forming polysaccharides are attached to the cells surface through covalent bonds to other surface polymers. Many of the extracellular polysaccharides produced in biofilms are insoluble and not easily separated from the cells, complicating the precise determination of their chemical structures and physical properties. Jahn et al. extracted a mixture of polymers from *Pseudomonas putida* biofilm material and found it to be very heterogeneous [6]. Most bacterial exopolysaccharides can exist either in ordered or disordered forms. Elevated temperatures and extremely low ionic concentrations favour the disordered forms. Polysaccharide molecules can interact with themselves or with

© 2012 Di Martino et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

heterologous ions and molecules to yield gels, often with multivalent cations playing a significant role in the process. Polysaccharides also interact with proteins molecules both as solutes and when attached to the surface of the microbial cells. The polysaccharide - protein interactions in the matrix induce both structural and functional properties. Indeed, some of these proteins are enzymes constituting an external digestion system [7].

Biofilms in differing environments can be exposed to a very wide range of hydrodynamic conditions, which greatly affect the matrix and the biofilm structure [8]. The shear rate determines the rate of erosion of cells and regions of the matrix from the biofilm. Polysaccharides of the matrix exhibit flow and elastic recovery; because of the flexibility of the matrix its shape can change in response to an applied force. The shear stress to which a biofilm is exposed also affects the physical morphology and dynamic behaviour. Biofilms grown under higher shear are more strongly adhered and have a stronger EPS matrix than those grown under lower shear [9]. Biofilm density can be influenced by the fluid shear during growth [10]. *Pseudomonas* biofilms grown under laminar flow generally consist of hemispherical mound-shaped microcolonies, which form an isotropic pattern on the surface [9]. The biofilm microcolonies grown in turbulent flow are elongated in the downstream direction to form filamentous streamers. The streamers are attached to the glass substratum by an upstream head while the downstream tails are free to oscillate in the flow. Thus, hydrodynamics conditions influence both the structure and the material properties of biofilms [9]. This may be related to the physical arrangement of individual polymer strands in the biofilm EPS matrix [11]. The constitution of the biofilm matrix of *S. enteritidis* varies with pressure forces applied to the biofilm. Indeed in the absence of pressure, the sugars in the biofilm matrix are mainly composed of glucose and very little fucose. However in the presence of power flow, the share of fucose in the biofilm matrix is increased from 11% to about 30% [12].

In this chapter, after the presentation of exopolysaccharides extraction and purification from the biofilm matrix, the structural and physical properties of bacterial alginates, cellulose and other exopoysaccharides related to biofilm formation are discussed. An illustration of the complexity of the biofilm matrix architecture and the role of exopolysaccharides in the properties of the matrix is given through biofilms formation at the surface of nanofiltration membranes used for drinking water production.
