Microbial Production of Alginate: Biosynthesis and Applications

Abstract

Alginate is the main representative of a family of polysaccharides that neither show branching nor repeating blocks or unit patterns and this property distinguishes it from other polymers like xanthan or dextran. Although only consisting of two different components, β-D-mannuronic acid and its C5-epimer a-L-guluronic acid, its unique and random structural pattern has attracted a lot of scientific and commercial interest over the past decade. Besides its production by brown algae, it is only produced by the two bacterial genera Pseudomonas and Azotobacter, which played a major role in the unravelling of its biosynthesis pathway. The pathway involves the generation of the cytosolic precursor GDP-mannuronic acid its polymerization to poly-mannuronic acid while traversing the cytoplasmic membrane. In the bacterial periplasm it can undergo enzymatic modification in form of acetylation or epimerization before the polymer is finally exported through the outer membrane and released into the environment. The degree of variability in the polymer and the possibility of genetic engineering of its producing bacterial hosts have been increasingly considered as an option to tailor-make alginates as biomaterials for numerous applications. Like DNA, alginate is a highly negatively charged polymer, which in combination with its random pattern imparts material properties ranging from viscous solutions to rigid gel-like structures in the presence of divalent cations. Traditionally, the self-assembly processes of algal alginates were mainly used in biotechnology for encapsulation purposes but given the option of fine-tuning its material properties, bacterial alginates are more and more considered for the production of micro- or nanostructures suitable for medical applications.