Chapter 8: Microbial Polysaccharides & Polyesters

Microbial Polysaccharides & Polyesters begins by exploring bacterial extracellular polysaccharides, distinguishing between cell-bound capsules, which serve as virulence factors and protective barriers against phagocytosis or desiccation, and diffuse slime layers. A significant portion of the text examines Xanthan gum, a high-value exopolysaccharide produced by the plant pathogen Xanthomonas campestris. The summary details the molecular structure of Xanthan, which features a cellulose-like backbone of beta-1,4-linked glucose residues and trisaccharide side chains containing mannose and glucuronic acid, modified by acetyl and pyruvate groups. Key physical properties are highlighted, particularly its unique rheological behavior known as shear thinning (pseudoplasticity), where viscosity remains high at rest but decreases significantly under shear stress, and its lack of hysteresis, making it an ideal additive for food stabilization and oil drilling muds. The text outlines the biosynthetic pathway of Xanthan, which utilizes the lipid carrier bactoprenol and specific glycosyltransferases encoded by the gum operon. The chapter then transitions to microbial polyesters, specifically polyhydroxyalkanoates (PHAs), which function as intracellular carbon and energy reserves accumulated by bacteria like Ralstonia eutropha under nutrient imbalances. It compares the material properties of short-chain thermoplastics, such as poly(3-hydroxybutyrate), to polypropylene, noting the bacterial polymer's brittleness and superior biodegradability. The discussion includes the metabolic engineering required to synthesize copolymers like poly(3-hydroxybutyrate-co-3-hydroxyvalerate) to improve flexibility and toughness. Furthermore, the chapter covers the biosynthesis of PHAs through different metabolic routes, including fatty acid beta-oxidation and de novo fatty acid synthesis, and the phenomenon of cometabolism to produce novel polymers. Finally, it addresses the biotechnological challenges of producing PHAs in transgenic plants such as Arabidopsis and corn to reduce costs, analyzing issues like metabolic drain, yield optimization, and life cycle assessments that compare the energy footprint of green plastics against petrochemical alternatives.