Recent developments in the biosynthesis and applications of heteropolysaccharides from lactic acid bacteria

Microbial exopolysaccharides (EPS) occur as capsules or as secreted slime. They represent a small fraction of today's biopolymer market with factors limiting their use being mainly associated with economical production. Efficient production and reduction in recovery costs requires knowledge of biosynthesis and adoption of appropriate bioprocess technology. EPS from generally recognized as safe food grade microorganisms, particularly lactic acid bacteria (LAB), have potential as food additives or as functional food ingredients with both health and economic benefits. Many different heteropolysaccharides (HePS) are secreted by LAB regarding sugar composition and molecular size but they show few common structural features, which raises questions about the relationship between structure and texture. HePS are made by polymerizing repeating units formed in the cytoplasm. These are assembled at the membrane by specific glycosyltransferases (GTF) through the sequential addition of sugar nucleotides. The latter are delivered as building blocks and attached to the growing repeating unit that is anchored on a lipid carrier. After completion, the repeating unit is externalized and polymerized. The enzymes and proteins involved in biosynthesis and secretion are not necessarily unique to HePS formation. These processes involve a genetic organization that includes specific eps genes and "housekeeping" genes involved in sugar nucleotide biosynthesis. The intentional and controlled use of HePS from LAB or use of strains producing HePS in situ is important in the food industry. However, instability in production and variability of HePS yields are well documented. Therefore, a well understood optimized carbon flux and supply of sugar nucleotides, knowledge of the GTF, and the functional expression of combinations of genes from different origin into stable, industrial strains open interesting ways to polysaccharide engineering.