Structure of O-Antigens

The lipopolysaccharide (LPS) is the major constituent of the outer leaflet of the outer membrane of Gram-negative bacteria. Its lipid A moiety is embedded in the membrane and serves as an anchor for the rest of the LPS molecule. The outermost repetitive glycan region of the LPS is linked to the lipid A through a core oligosaccharide (OS), and is designated as the O-specific polysaccharide (O-polysaccharide, OPS) or O-antigen. The O-antigen is the most variable portion of the LPS and provides serological specificity, which is used for bacterial serotyping. The OPS also provides protection to the microorganisms from host defenses such as complement mediated killing and phagocytosis, and is involved in interactions of bacteria with plants and bacteriophages. Studies of the OPSs ranging from the elucidation of their chemical structures and conformations to their biological and physico-chemical properties help improving classification schemes of Gram-negative bacteria. Furthermore, these studies contributed to a better understanding of the mechanisms of pathogenesis of infectious diseases, as well as provided information to develop novel vaccines and diagnostic reagents.