**5. Conclusion**

386 The Complex World of Polysaccharides

ability of the material to store energy, whereas the loss modulus characterizes energy dissipation in the material under dynamic excitations. If the material is perfectly elastic then the resultant stress wave is exactly in phase with the strain wave. By contrast, when the rate of change of the sinusoidal oscillation is a maximum and the strain is zero, for purely viscous systems, the resultant stress wave will be exactly 90° out-of phase with the imposed deformation. For NF biofilms, during oscillation analysis, values of storage modulus (*G'*) stay higher than values of loss modulus (*G"*) over the entire range of frequencies covered, indicating that the NF biofilm behave like a highly elastic physical gel [74]. Polysaccharides alone, like alginates, are known to realize a sol-gel transition under adequate physicochemical conditions [75]. The physicochemical microenvironment inside NF biofilms may be permissive to exopolysaccharides sol-gel transition. The gel state is resistant enough and presents a micro porosity favourable for resistance to flow forces, microcolonies development and cell nutrition inside the biofilm structure. This model of sol-gel transition of polysaccharides inside biofilms is consistent with rheological properties previously demonstrated for other biofilms: *Streptococcus mutans* biofilms have elasticity and viscous behaviour analogous to NF biofilms for a range of frequencies between 0.1 to 20 Hz [76]. The rheofluidification behaviour and gel-type rheological properties shared by different type of biofilms and purified polysaccharides suggest that the critical components of the biofilm

The time-dependent strain response observed in the creep curves clearly indicated that NF biofilms exhibited viscoelastic behaviour. Viscoelasticity is thought to be a general mechanical property of biofilms. A very wide range of elasticity and viscosity values has been previously observed for a wide sample of biofilms formed artificially in laboratory experiments or coming from natural aquatic environments [4, 72, 76]. Thus, it wasn't surprising to observe that the rheological properties of NF biofilms are different from the ones of natural biofilms from different aquatic environments like Nymph Creek (Yellowstone National Park) and Chico Hot springs (Montana) algal biofilms [4]. These differences in viscosity and elasticity between biofilms can be related to different exopolysaccharide contents and to different shearing strains. Bacterial and algal alginates are known to have different monomeric composition leading to a stronger binding of cations for bacteria, a property involved in the formation of a

The specificity of NF biofilms may be the necessity to resist shear forces applied to the membrane during the filtration process. In the Méry-sur-Oise plant, NF membranes are operated at feed pressure of approximately 10 bars [78]. The high membrane feed pressures may influence the rheological properties of NF biofilms by increasing cohesive forces in the biofilm bulk, increasing forces, which keep the exopolymers to the membrane surface, and thus strengthening the mechanical stability of the biofilm. This may explain at least in part

Shaw et al. have previously shown that the elastic relaxation time varied much less between biofilms of different origins. was estimated to be the time required for viscous creep length to equal elastic deformation length (so that memory of initial conditions is lost), i.e., ≈ *η* /*G*.

matrix determining the biofilm texture are polysaccharides.

stable gel in the presence of ambient Ca2+ cations [77].

the NF biofilms resistance to industrial cleaning [65].

Extracellular polysaccharides are considered as the major structural components of the biofilm matrix. A large variety of polysaccharides required for bacterial adherence and biofilm formation have been described. Polysaccharide molecules can interact with themselves or with ions and proteins. These interactions result from electrostatic attractive forces, repulsive forces, hydrogen bonds, van der Waals interactions and ionic attractive forces. All these forces influence the structure and the stability of the biofilm matrix and the way EPS and polysaccharides can be extracted from the biofilm bulk. A universal protocol for extracellular polysaccharide extraction from the biofilm matrix does not exist. Each study may adapt usual extraction procedures to biofilm specificities and to the nature of the polysaccharide studied. The viscoelasticity nature of biofilms is universal but biofilms in differing environments exposed to different hydrodynamic conditions will encounter changes in the structure, composition and then physical properties of their matrix. Biofilm science is highly exciting since it is a mixture of biology, biophysic, chemistry and much more.
