This Fleming Lecture reviews bacterial polysaccharides, focusing on their structure, industrial applications, and roles in human health and disease. Polysaccharides are highly hydrated polymers with diverse structures due to varying monosaccharide compositions and glycosidic linkage patterns. They are used extensively in food production as stabilizers and gelling agents, with microbial sources like xanthan offering advantages over plant-derived alternatives. In non-food industries, bacterial polysaccharides serve as thickeners in textiles, rheological modifiers in oil drilling, and supports for cell immobilization. Biomedically, they are crucial vaccine components against pathogens like Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae type b. Capsular polysaccharides protect pathogenic bacteria by resisting complement-mediated killing and phagocytosis. The lecture emphasizes E. coli group II capsule genetics as a paradigm, describing three conserved functional regions flanking serotype-specific regions, with detailed molecular mechanisms for biosynthesis and export. Future prospects include engineering bacteria to produce medically important polysaccharides like heparin and hyaluronic acid, and designing inhibitors targeting pathogenic capsule production.

Key findings

• Bacterial polysaccharides are widely exploited in food, industrial, and biomedical applications due to their unique physical and chemical properties
• Capsular polysaccharides confer virulence by resisting complement-mediated killing and opsonophagocytosis through multiple mechanisms including C3b masking and charge effects
• E. coli group II capsule gene clusters contain three functional regions: two conserved regions flanking a serotype-specific region, with conserved genes encoding ABC-type transporters and biosynthetic enzymes
• Conjugate vaccines linking capsular polysaccharides to protein carriers successfully overcome poor immunogenicity in infants, as demonstrated by H. influenzae type b vaccines
• Genetic engineering of bacteria like E. coli offers potential for producing medically important polysaccharides including heparin and hyaluronic acid through heterologous expression