O-specific antigen polysaccharide, also known as O-specific polysaccharide chain, is located at the outermost layer of lipopolysaccharide molecule and consists of several to dozens (up to 40) oligosaccharide repeats. Each oligosaccharide unit is an oligosaccharide composed of 3 to 5 monosaccharides. These monosaccharides are usually neutral sugars and amino sugars. The structure of the O antigen chain is the most variable part of lipopolysaccharide composition, and its diversity determines the antigen characteristics of different gram-negative strains.

The O antigen of gram-negative bacteria has species specificity, which is caused by the different types, distribution positions, arrangement directions, and spatial configurations of monosaccharides in its polysaccharide chain. O antigen can react specifically with corresponding antibodies. Different kinds of Escherichia coli have different types and arrangements of monosaccharides of the O antigen chain, forming specific antigenicity. Therefore, the O antigen chain is the structural basis of the serological specificity of different strains. This structure is also the main antigenic determinant of gram-negative strains, and most of them have the specificity of gram-negative strains, which can cause the specific reaction of strains. According to the colony morphology, when the O-antigen polysaccharide chain is missing, it is rough bacteria, and the complete O-antigen polysaccharide chain is smooth bacteria.