The present invention relates to purification and use of a novel emulsion stabilizing polysaccharide. In particular, a polyelectrolyte exopolysaccharide with high molecular weight comprising a high molecular weight polymer with a tri-saccharide repeating unit is disclosed. In one aspect of the invention, methods are directed to isolating and purifying a high molecular weight exopolysaccharide (EPS) from a cell supernatant. In another aspect, methods are disclosed for isolating a lipopolysaccharide (LPS) and a high molecular weight Acinetobacter polyelectrolyte exopolysaccharide (APE) from Acinetobacter bacteria. Compositions are also directed to lipid nanoparticles comprising a therapeutic agent encapsulated by a high molecular weight polysaccharide and nanoparticles comprising a therapeutic agent bound to a cationic polysaccharide cross-linked with a polyanion.