Exopolysaccharides

Exopolysaccharides (EPS) are high-molecular-weight polysaccharides secreted by microorganisms into their external environment. They accumulate as a gelatinous matrix around cells or within biofilms and can be homopolysaccharides (consisting of a single sugar) or heteropolysaccharides (containing two or more types of monosaccharides). EPS often carry chemical substituents such as acetyl, pyruvyl, or sulfate groups that influence solubility, charge, and biological interactions.

Most EPS are produced by bacteria and fungi, with notable producers including Xanthomonas, Pseudomonas, Bacillus, Sphingomonas, and various yeast and molds. Biosynthesis typically occurs intracellularly or at the inner cell membrane and is followed by export across the cell envelope through diverse mechanisms, including Wzx/Wzy-dependent assembly, ABC transporter–dependent secretion, or synthase-dependent pathways. Production is commonly upregulated by surface attachment, nutrient limitation, osmotic or desiccation stress, and quorum sensing.

Functions of EPS include forming the extracellular biofilm matrix that mediates adherence and cohesion, protecting cells from desiccation, osmotic stress, and antimicrobial agents, trapping nutrients, and modulating interactions with hosts or other microbes. EPS can bind metals and influence soil structure, sediment stability, and microbial ecology in aquatic and terrestrial environments.

Commercially important EPS such as xanthan gum, gellan, and alginate are used as thickeners, stabilizers, and rheology modifiers in food, cosmetics, and pharmaceuticals. Other examples include dextran, levan, and pullulan. Biosynthesis is subject to complex regulation and remains an area of active biotechnological investigation for sustainable production and novel applications.