Functionality of exopolysaccharides produced by lactic acid bacteria

Abstract

Many strains of lactic acid bacteria (LAB) are able to produce exopolysaccharides (EPS). Depending on their chemical composition, EPS can be classified as homopolysaccharides (HoPS), which contain a single type of monosaccharide, and heteropolysaccharides (HePS), which are formed of several repeating units of different monosaccharides. Most HoPS are synthesized by extracellular glycansucrases that use sucrose as the glycosyl (fructose or glucose) donor. In the HePS the precursor repeatingunits are formed intracellularly and assembled extracellularly. The different genes participating in the production of HePS, and the regulation of the chain length, are usually organized in clusters. HoPS are generally longer in size and are produced in higher amounts than HePS. This chapter reviews the current knowledge about the physiological role of EPS in the producing LAB strains, as well as their possible benefits for human health and their technological application to the manufacture of dairy products. The physiological functions of EPS in the producing bacteria have not yet been clearly determined although they might be related to better adaptation to, and recognition of, the environment. It has been suggested that EPS produced by LAB act as protective agents against desiccation, bacteriophage attack, antimicrobial compounds, and phagocytosis. They can also be involved in the adhesion to surfaces such as the human intestinal mucosa. EPS may exert beneficial effects on human health, although more in vivo studies are needed to demonstrate their efficacy. Among these beneficial effects, their possible role as prebiotics, serum lowering cholesterol ability, and immunomodulatory and anticarcinogenic activities have been suggested. Some LAB strains which produce EPS are being used in the industry because they confer beneficial physical properties to dairy products, such as the improvement of rheological properties and reduction of syneresis in yoghurts, and better consistency of curd in low-fat ripened cheeses. In spite of these interesting technological properties, the use of EPS from LAB as food additives in the industry is still limited because of the low production levels compared to the xanthans produced by Gram-negative microorganisms.