Oantigen

O-antigen, or O-polysaccharide, is the variable outermost part of the lipopolysaccharide (LPS) found on the outer membrane of most Gram-negative bacteria. It consists of repeating oligosaccharide units and extends outward from the core polysaccharide and lipid A. The specific monosaccharide composition and linkage pattern define the serogroup and form the basis for many serotyping schemes used in epidemiology and clinical microbiology.

Structure and biosynthesis

O-antigen repeats are synthesized as separate units in the cytoplasm and transported across the inner membrane by the Wzx flippase. They are then polymerized by Wzy into a long-chain polysaccharide, with chain length regulated by Wzz proteins. The genes required for O-antigen biosynthesis are typically organized in a dedicated gene cluster on the chromosome, with organization and content varying among species. The completed LPS molecule can be smooth, containing O-antigen, or rough if the O-antigen is absent or truncated.

Function and significance

The O-antigen contributes to the permeability barrier of the outer membrane and mediates interactions with the host immune system. Its high variability between strains enables immune evasion and influences virulence and serum resistance. Because of this diversity, O-antigen typing is a central tool for differentiating and tracking strains in clinical and public health settings. O-antigen structure also affects recognition by innate immune receptors such as Toll-like receptor 4, shaping inflammatory responses. The O-antigen is a common target in diagnostics and, in some cases, vaccine development for pathogenic Gram-negative bacteria.