Exopolysaccharides (EPS) are high-molecular-weight carbohydrate polymers produced by microorganisms such as bacteria, fungi, yeast, and algae. These natural polymers are actively secreted outside the microbial cell wall or remain loosely associated with the cell surface. They function as biological architecture, forming a slimy or gelatinous matrix that surrounds the cells. Their unique structural features translate into a wide range of uses in both biological systems and industrial technology.
What Exopolysaccharides Are and How They Are Made
EPS are carbohydrate polymers composed of numerous simple sugar residues, known as monosaccharides, linked together in long chains. Common sugar components include glucose, galactose, rhamnose, and fructose, sometimes with non-carbohydrate additions like acetate or phosphate. EPS are categorized as homopolysaccharides (HoPS), consisting of a single type of monosaccharide (e.g., dextran), or heteropolysaccharides (HePS), made up of two or more different monosaccharides in repeating units.
The biosynthesis of EPS is an energy-intensive process occurring primarily inside the microbial cell before secretion. For most HePS, precursor sugar molecules are assembled into repeating units within the cytoplasm by specialized enzymes called glycosyltransferases. These units are then transported across the cell membrane and polymerized into the final structure outside the cell.
Some simpler HoPS, such as dextran and levan, are synthesized differently using extracellular enzymes called glycansucrases. These enzymes are secreted by the microbe and build the polymer directly outside the cell, using sucrose as the sugar donor molecule. The final molecular weight of EPS is substantial, often exceeding one million Daltons, and structures range from simple linear chains to highly complex, branched architectures.
Biological Roles in Microbial Survival
Exopolysaccharides provide a survival advantage to the microorganisms that produce them. The primary function of this secreted material is to act as a protective barrier, shielding microbial cells from environmental threats. This slimy layer offers protection against desiccation, preventing cells from drying out under harsh conditions.
EPS also serve as a chemical buffer, protecting microbes from toxic compounds, antibiotics, and predators like protozoa or bacteriophages. The polymers are a primary component of the biofilm matrix, a complex scaffolding that allows microbial communities to adhere firmly to surfaces. This adhesion enables microbes to colonize ecological niches and withstand physical stresses like fluid flow.
Within the biofilm, the EPS matrix creates a favorable microenvironment by sequestering nutrients and facilitating communication. For lactic acid bacteria in the gut, the EPS layer shields them against host defenses, such as stomach acid and bile salts. This protection is necessary for their successful colonization and persistence in challenging habitats.
Applications in Food Science and Gut Health
Food Technology
The unique physical properties of EPS make them valuable in food production as natural texturizers and stabilizers. These polymers interact strongly with water, improving viscosity and gelation even at low concentrations. In fermented dairy products like yogurt and kefir, EPS produced by starter cultures such as Lactobacillus and Streptococcus prevent syneresis (whey separation) by binding water and creating a smoother, thicker texture.
This natural thickening reduces the need for synthetic stabilizers, helping manufacturers achieve desirable consistency. The EPS interact with milk proteins, such as casein micelles, forming a network that stabilizes the product structure. EPS-producing cultures can also be selected to improve the yield and moisture retention of cheeses.
Gut Health
Beyond their technological function, many microbial EPS are recognized for their health benefits, primarily as prebiotics. Prebiotics are non-digestible fibers that selectively feed beneficial gut bacteria, promoting their growth and activity. When consumed, EPS resist digestion in the upper gastrointestinal tract and travel to the colon, where they are fermented by the resident microbiota.
The fermentation of these complex sugars produces short-chain fatty acids, such as butyrate, which nourish colon cells and are associated with systemic health effects. Furthermore, EPS exert immunomodulatory effects by interacting directly with immune cells lining the gut. This interaction influences the production of signaling molecules and may contribute to anti-inflammatory, anti-tumor, and anti-viral activities, making them attractive components for functional foods.
Emerging Uses in Medicine and Industry
The biocompatibility, biodegradability, and structural diversity of EPS are driving their use in medical and industrial applications. In the pharmaceutical field, EPS are being developed as drug delivery systems because they can encapsulate therapeutic compounds. Their ability to form hydrogels and micro- or nanoparticles allows for the design of controlled-release systems that protect a drug until it reaches a specific target.
Specific EPS are also used in tissue engineering and wound healing. Certain EPS accelerate the healing process by promoting cell migration and proliferation, providing a scaffold for tissue regeneration. Their bioactive properties, such as antioxidant and anti-inflammatory characteristics, can be leveraged in wound dressings to create a favorable recovery environment.
Industrial uses are expanding, particularly in environmental remediation. Certain EPS can chelate, or bind to, heavy metals and pollutants in water. This characteristic makes them candidates for use in bioremediation processes to clean up contaminated sites. Other applications include their use as biodegradable components in cosmetics, textiles, and the petroleum industry.