**3.1. Bacterial alginates**

Alginate, a polysaccharide which occurs in brown algae and in different bacteria like *Azotobacter vinelandii* [30] and *P. aeruginosa* [44] has been extensively studied. Alginate is an exopolysaccharide with a relatively high molecular mass (104-106 g/ml). It consists of the uronic acid residues β-D-mannuronate (M) and its C-5 epimer, α-L-guluronate (G) [45] (Figure 3).

**Figure 3.** Structure of alginate

Generally, the monomers form a block copolymer with homopolymeric regions of poly-β-Dmannuronate (M-blocks) and poly-α-L-guluronate (G-blocks) as well as heteropolymeric regions (MG-blocks). The absence of G-blocks differentiates alginates produced by *P. aeruginosa* from alginates expressed by algae or by *A. vinelandii* [46]. The functional properties of the alginates strongly correlate with the composition (M/G ratio) and with the uronic acid sequence.

There are 24 genes located on the bacterial chromosome, involved in the production and secretion of alginate in *P. aeruginosa* [45]. Eight genes are implicated in the exportation of alginic acids (Figure 4), and twelve in the biosynthesis of the polysaccharide [47]. The four remaining genes are involved in the regulation of the synthesis.

Alginates can form a gel in the presence of chelating divalent cations. This structure formed is called a Grant "egg-box" [47]. The alginate gel is formed by ionic bonds between the Grich blocks and divalent cations. The mechanical properties of alginate gels can vary depending on the amounts of guluronic acid present in the polymer. Moreover, alginate gels can be formed in vitro in the presence of proteins such as gelatin [48].

**Figure 4.** Biosynthesis of bacterial alginate
