For the first time, an O-specific polysaccharide antigen that is a Shigella sonnei, phase I, exopolysaccharide has been produced and characterized, said exopolysaccharide being an authentic natural compound in the form of a bacterial capsular polysaccharide consisting of 1-100 repeating disaccharide units of O-[4-amino-2-(N-acetyl)amino-2,4-dideoxy-β-D-galactopyranosyl]-(1→4)-O-[2-(N-acetyl)amino-2-deoxy-α-L-altrpyranuronic acid] linked by (1→3) bonds to form a polysaccharide chain. The exopolysaccharide has a molecular mass of from 0.4 to 400 kDa, contains a non-toxic lipid component, namely non-hydroxylated fatty acids, and exhibits low pyrogenicity and high immunogenicity, generating mucosal protection against Shigella sonnei shigellosis by inducing the synthesis of specific antibodies against S. sonnei in mammals, including humans.Without using lipopolysaccharides as the source of production, an exopolysaccharide with a high degree of purity is produced from a liquid phase culture of S. sonnei bacteria by means of a workable industrial method with a high yield.Effective, highly specific and safe vaccines for the prophylaxis and/or treatment of Shigella sonnei shigellosis are developed on the basis of the above-mentioned exopolysaccharide, as well as pharmaceutical compositions with a broad spectrum of action, in particular, in modulating immune response.
