**1. Introduction**

Alginate is a linear anionic biopolymer consisting of β-D-mannuronic acid units (M) and its C5-epimer α-L-guluronic acid (G) linked by 1 → 4 glycosidic bonds. It can be found as a homopolysaccharide (poly-M or poly-G) or as a heteropolysaccharide containing M/G in different ratios and sequences. Alginate has been known to be a major component of brown algae cell walls since the late nineteenth century when scientists were searching for utile products from kelp [1], and the gelatinous material isolated from these seaweeds was then named alginic acid [2]. The molecular backbone of alginate, however, was only described decades later, after the work of Fisher and Dörfel in 1955 [3], and the structural analyses of alginate have been an extensive field of research ever since due to the immense variability of M/G content and sequential organization found throughout the years in many macroalgae [4, 5]. The structural variability presented by this polymer and the ability to form gels with

different physicochemical properties is nowadays reflected in its many biotechnological uses in the food sector, cosmetic industry, wastewater treatment, pharmaceutical, and biomedical applications, among others [6–9].

Even though all commercial alginate produced today is extracted from seaweed, a handful group of specialized microorganisms can also produce this polymer as an exopolysaccharide [10]. Two genera of bacteria, *Pseudomonas* and *Azotobacter*, have been extensively studied over the past years on their ability to exudate alginate [11–13]. The production of alginate by *Pseudomonas aeruginosa* has been first reported by Linker and Jones in the mid-1960s [14, 15], and most of the knowledge on bacterial alginate production and biosynthesis emerged from the studies with this opportunistic human pathogen for patients afflicted by cystic fibrosis [16, 17], since the exopolymer represents major virulent factors during the lung infection process [18, 19]. Shortly after, alginate was described as an exopolysaccharide produced by *Azotobacter vinelandii* [20], a nitrogen-fixing soil bacterium found in association with plants. Over the years, other species belonging to these two genera have also been reported to produce alginate, such as *P. putida* and *P. fluorescens* [21], *P. mendocina* [22]; *P. syringae* [23] and *A. chroococcum* [24, 25].

The biosynthesis of bacterial alginates exhibits highly comparable mechanisms among the different producing species but differs from the seaweed polysaccharides in one particular characteristic. The microbial polymers are often O-acetylated, submitted to esterification at O-2 and/or O-3 on the D-mannuronate units [26–28], which affects their physicochemical properties in comparison to the alginate extracted from macroalgae [29–32]. Furthermore, as well as different M/G compositions and sequences are found on alginates from seaweeds, the gel formation ability and viscosity of bacterial alginates are also affected by the arrangement of M-, G- and MG-blocks along their structures [7, 33]. Some mucoid strains of *P. aeruginosa* biosynthesize great amounts of alginate containing mainly M- and MG-blocks (**Table 1**), often O-acetyl-esterified, which leads to an increasing interaction of the polymer with water molecules, polymer extension, and water capacity, thus allowing the development of a thickly natured and highly structured biofilm matrix [34–36]. *Azotobacter* spp., on the other hand, produce alginates that are closely associated with the cells and able to capture divalent ions such as Ca2+ that form a hard and brittle gel due to the high content of L-guluronate residues (G-blocks) [37, 38], allowing the formation of desiccation-resistant cysts under adverse environmental conditions [39, 40].


*GulA (G): guluronic acid; ManA (M): mannuronic acid.***\*** *ManA units are found O-acetylated on C-2 or C-3 only in bacterial alginates.*

#### **Table 1.**

*Most commonly found structural disaccharides (blocks) of alginates produced by seaweeds, Pseudomonas spp. and Azotobacter spp.*

#### *Bacterial Alginate Biosynthesis and Metabolism DOI: http://dx.doi.org/10.5772/intechopen.109295*

The production of tailor-made alginate in order to obtain polymers with different molecular weights, M/G ratio, and O-acetyl content to suit specific applications has been intensively investigated [41–43] as it poses as a potential commercial interest for the use-driven design of these polysaccharides in different industrial and medical applications [44–46]. Altering fermentation parameters and growth conditions in order to induce alginate production and or structural modifications during microbial cultivation has been extensively evaluated, especially for *Azotobacter vinelandii* [47–52] due to its non-pathogenic characteristics, which makes this bacterium suitable for biotechnological processes. However, the genome sequencing of both *P. aeruginosa* [53] and *A. vinelandii* [54], together with the use of molecular tools for gene cloning, have expanded the possibilities for *in vivo* fine structural adjustments of alginate, and the isolation of alginate-modifying enzymes is allowing the on-demand chemical tailoring of these bacterial polymers [55–58], including the suppression or degradation of alginate produced by *Pseudomonas* spp. for commercial and medical purposes [59–62].

Understanding the molecular basis involved in bacterial alginate biosynthesis, transport, and metabolism, which are usually under strict regulatory control [63–66], is paramount in order to develop new strains and mutants guided to the production of the idealized alginate structure in large-scale bioreactors or to be used therapeutically on clinical patients in opposition to alginate-overproducing pathogens [67, 68]. In this chapter, we are going to explore the main steps involved in the biosynthesis of microbial alginate, from the assembly of its precursor units to the polymerization, chemical and structural modifications, and transport through the cytoplasmic and outer membranes until the secretion of the mature polymer by the bacterial cells and the extracellular modifications that may follow. Gene and enzyme regulation will also be briefly addressed when relevant to each of the biosynthetic phases.
