Exopolysaccharides (EPS) are complex carbohydrate polymers found across various environments. These substances are secreted by a wide range of organisms into their surroundings. They influence microbial communities and the physical properties of their habitats. The diverse structures and unique properties of exopolysaccharides make them valuable for understanding biological interactions and exploring human applications.
What Are Exopolysaccharides?
Exopolysaccharides are complex polysaccharides that organisms produce and release outside their cells. They are high-molecular-weight polymers, generally ranging from 10 to 10,000 kilodaltons (kDa) in size. These biopolymers are composed of repeating units of monosaccharides, such as glucose, galactose, rhamnose, fucose, N-acetyl-glucosamine, and N-acetyl-galactosamine.
The composition and arrangement of these monosaccharide units create diverse exopolysaccharide structures. Some are homopolysaccharides, consisting of one type of monosaccharide, like dextran (made solely of glucose units). Others are heteropolysaccharides, built from two or more different monosaccharides, often arranged in repeating units of up to eight sugars. These structures can be linear or highly branched, and sometimes include non-carbohydrate components like acetate, pyruvate, succinate, or phosphate.
Sources of Exopolysaccharides
Exopolysaccharides are produced by a wide variety of organisms, with microbial sources being significant. Bacteria, fungi, yeasts, and algae are producers of these extracellular polymers. Among bacteria, lactic acid bacteria (LAB) found in fermented foods like yogurt and kombucha are well-known for their production.
Other bacterial genera include Pseudomonas, Bacillus, Xanthomonas, and Acetobacter species. For instance, Xanthomonas campestris is the source of xanthan gum, a widely used exopolysaccharide. Certain marine bacteria also secrete exopolysaccharides, which aid their survival in diverse aquatic environments, including deep-sea hydrothermal vents and Antarctic ecosystems.
Biological Functions of Exopolysaccharides
Exopolysaccharides serve many functions for the organisms that produce them, aiding their survival in diverse environments. A primary role is protection against various environmental stressors. For example, exopolysaccharides can shield microbial cells from desiccation, UV radiation, extreme temperatures, unfavorable pH levels, and osmotic stress. They also offer defense against toxic compounds, antibiotics, and the immune responses of host organisms or predators like bacteriophages.
Exopolysaccharides also facilitate adhesion to surfaces and the formation of biofilms. Biofilms are complex communities of microorganisms encased in a self-produced matrix, largely composed of exopolysaccharides, along with proteins and nucleic acids. This matrix provides a structural scaffold, enabling cells to attach to each other and to various surfaces for colonization. The exopolysaccharide matrix also helps in nutrient sequestration and cell-to-cell communication within the biofilm.
Applications of Exopolysaccharides
Exopolysaccharides are used across various industries due to their properties, including high molecular weight, water solubility, and biocompatibility. In the food industry, they act as thickeners, stabilizers, gelling agents, and emulsifiers, improving product texture, consistency, and shelf life. Xanthan gum, produced by Xanthomonas campestris, is a common example used in beverages, dairy products, and sauces for its high viscosity. Sodium alginate, extracted from seaweed but also produced by bacteria like Azotobacter and Pseudomonas, is another example used in dairy products and ice cream for its gelling and stabilizing properties.
Beyond food, exopolysaccharides find applications in pharmaceuticals, cosmetics, and environmental remediation. In pharmaceuticals, their non-toxic properties and biocompatibility make them suitable for drug delivery systems, vaccine adjuvants, and tissue engineering. Dextran, for instance, is approved for medical applications. For cosmetics, exopolysaccharides like gellan gum and levan offer moisturizing, film-forming, and emulsion-stabilizing properties. Their ability to bind heavy metals or hydrocarbons also makes them useful in environmental applications such as bioremediation.