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

https://doi.org/10.1016/S0958-6946(01)00114-5Get rights and content

Abstract

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.

Section snippets

Microbial exopolysaccharides

Polymers from plant, animal, and microbial origin play an important role in food formulations (Tombs & Harding, 1998). Food polymers are long-chain, high-molecular-mass molecules that dissolve or disperse in water to give texturizing properties. Most of the biopolymers used by the food industry are polysaccharides from crop plants (e.g. starch) or seaweeds (e.g. carrageenan) and animal proteins like caseinate and gelatin. The functional properties of plant carbohydrates in foods are determined

Classification of exopolysaccharides from lactic acid bacteria

EPS from LAB can be subdivided into two groups, namely homopolysaccharides (HoPS) and heteropolysaccharides (HePS). HoPS are composed of one type of constituting monosaccharides (d-glucopyranose or d-fructofuranose) (Monsan et al., 2001). HePS are composed of a backbone of repeated subunits, that are branched (at positions C2, C3, C4, or C6) or unbranched, and that consist of three to eight monosaccharides, derivatives of monosaccharides or substituted monosaccharides (Table 1). The

Sugar catabolism in lactic acid bacteria

Although the dairy LAB encounter lactose as the major energy source, they have the capacity to use a number of other mono- and disaccharides (de Vos, 1996). Three different systems for sugar uptake are known in LAB (de Vos & Vaughan, 1994): (i) primary transport systems or a direct coupling of sugar translocation with ATP hydrolysis via a transport-specific ATPase; (ii) secondary sugar transport systems or a coupling of sugar transport with transport of ions or other solutes, both as symport

Biosynthesis of heteropolysaccharides by lactic acid bacteria

HePS are made by the polymerization of repeating unit precursors formed in the cytoplasm (Cerning (1990), Cerning (1995); De Vuyst & Degeest, 1999). These are assembled at the membrane by the sequential addition of activated sugars (sugar nucleotides or nucleoside diphosphate sugars) to the growing repeating unit that is most probably anchored on a lipid carrier. After completion of an HePS repeating unit, it would seem to be exported through the cell membrane, becoming polymerized into a final

Applications of exopolysaccharides from lactic acid bacteria

EPS from LAB are not yet intentionally exploited by industrial manufacturers. A few exceptions exist among the HoPS produced by LAB (Sutherland (1986), Sutherland (1990); Roller & Dea, 1992; Tombs & Harding, 1998). Dextran derivatives and activated dextrans find several commercial uses. Industrial dextrans are used in the manufacture of gel filtration products and as blood volume extenders and blood flow improvers. Further possible uses of dextrans are in paper and metal-plating processes, in

Conclusion

Increasing knowledge on HePS structures, on the molecular organization of eps gene clusters, on the enzymes directing repeating unit biosynthesis and those involved in supply of sugar nucleotides and polymerization of HePS repeating units, and on the factors regulating expression of HePS, will make it possible to enhance their production under defined fermentation conditions. Indeed, a well-understood, optimal carbon flux and supply of sugar nucleotides, the large pool of unknown GTF, as well

Acknowledgements

The authors’ research on EPS production by LAB was financially supported by the Institute Danone by means of a “Navorsingskrediet voor Fundamenteel Voedingsonderzoek”, the Institut Yoplait International, the European Commission (EU) (grants FAIR-CT98-4267 and IC15-CT98-0905), the Flemish Institute for Encouragement of Scientific and Technological Research in Industry (IWT), the Fund for Scientific Research (FWO-Flanders), the Link 2000 Action of the Brussels Capital Region, and the Research

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