A schematic diagram of a lipopolysaccharide molecule. For most purposes the terms endotoxin and lipopolysaccharide or LPS are synonymous.
Schematic structures of R-LPS (rough LPS) chemotypes
Rough-mutant bacteria are serum-sensitive and are lysed by complement. Only smooth strains, protected by their O-LPS polysaccharide chains, can survive in the host. These smooth strains express a heterogeneous mixture of unsubstituted complete core (Ra) LPS and O-LPS with different chain lengths (different numbers of O-polysaccharide subunits), giving the typical ladder pattern on electrophoresis. Since only O-LPS and Ra-LPS (complete unsubstituted core) occur in the host, these LPS are responsible for the natural immunity detected when normal or clinical sera are studied with the various R-LPS and S-LPS as antigens in immunoassay. When the incomplete-core R-LPS are used as immunogens, they can elicit novel antibody responses to the "break points" in the core oligosaccharide chains normally occupied by the missing outer sugars. Such antibodies tend to be chemotype-specific, and may dominate following such vaccinations. Immunoblotting on more complete LPS can often reveal their lack of reactivity, especially for monoclonals.