Lipopolysaccharide is the immuno-dominant antigen of most Gram-negative pathogens. This heteropolymer is the main component of the outer leaflet of the bacterial outer membrane. The general structure of LPS is comprised of three regions and is illustrated here. Lipid A is an acylated disaccharide of N-acetylglucosamine that is embedded in the outer membrane. This moiety, also LPS modelknown as endotoxin, is responsible for most of the toxicity of this molecule. A short oligosaccharide, termed the core, is attached to lipid A. The innermost portion of the core is normally composed of one or more 2-keto-3-deoxy-octanates (KDO) and one or more molecules of L-glycerol-D-manno-heptoses (heptose). The outermost portion of the LPS core is more divergent in structure. A variety of neutral, amino, and carboxyl sugars have been reported in the outer core of various microorganisms. In addition, the LPS core is often decorated with phosphates, amino acids, and additional sugars. The O-antigen, the final portion of the LPS, is composed of repeating tri- or tetra-saccharide subunits. The O-antigen is the most variable portion of the LPS structure; a single species may produce several hundred immunologically distinct forms. The mechanism of LPS biosynthesis and a proposed genetic nomenclature have been reviewed (39). Each LPS sugar moiety is added to the growing structure by a specific sugar transferase. The gene products responsible for core biosynthesis are named Waa (formerly Rfa) while the O-antigen gene products are called Wba (formerly Rfb). Bacterial strains defective for any of these enzymes will produce a truncated LPS molecule. The degree of truncation has historically been determined by altered LPS mobility in acrylamide gel electrophoresis. Various LPS-rough phenotypes are classified from Ra to Re – least to most truncated, respectively.Many biological roles have been attributed to LPS (38). As already discussed, LPS is a powerful proinflammatory molecule. However, there are precedents for LPS playing a variety of roles in bacterial virulence. Several bacteria specifically use LPS to adhere to host tissues. The LPS core of Actinobacillus pleuropneumoniae is specifically required for adhesion to porcine tracheal cells (40). Likewise, the LPS core appears to be involved in Pseudomonas aeruginosa adherence to pulmonary epithelial cells (36). It is possible that LPS may also playLPS structure a role in binding of periodontal pathogens to host tissues. It is interesting that LPS from some periodontal pathogens has been shown to bind to oral implant biomaterials (33). Lipopolysaccharide can also protect pathogens from immune clearance. Some pathogens appear to use LPS to mimic host structures (4, 18). Many pathogens have been shown to attach sialic acid to the LPS core or O-antigen. Mucosal pathogens like Campylobacter, Neisseria, and Haemophilus are all known to incorporate sialic acid (12, 16, 20). The presence of this amino sugar helps these bacteria to avoid immune clearance by inhibiting antibody opsonization, complement-mediated killing, and phagocytosis. Finally, the presence of intact LPS also provides an additional physical barrier to many toxic molecules. This barrier function provides increased resistance to non-specific defenses such as bile acids and the complement cascade.The structure, genetic organization, and biological function of Fusobacterium nucleatum LPS is poorly understood. We hope to address these topics by 1) cloning and characterizing the genes involved in LPS biosynthesis is this organism, 2) generating specific mutations in these genes, and 3) comparing the mutants to wild type strains with regard to biofilm formation, immune evasion or survival, stimulation of host cytokine production, and virulence.