Bacterial cells produce a cell wall, and all bacterial cell walls contain peptidoglycan. Gram-negative bacteria contain an extra layer in the cell wall structure called the outer membrane, which is outside the peptidoglycan layer.   Endotoxin is a component of this outer membrane. Un0purified endotoxin contains lipids, proteins, and carbohydrate, but highly purified endotoxin does not contain proteins and is therefore referred to as lipopolysaccharide (LPS).

  LPS consists of two parts: a hydrophilic polysaccharide and a hydrophobic lipid, lipid A. The polysaccharide can be further subdivided into O-specific polysaccharide and core oligosaccaride.

  The chemical structure of the O-specific polysaccharide differs greatly in fifferent serotypes. The core is more conserved in its structure and composition. Lipid A is the most conserved part of the LPS molecule. Lipid A consists of a disaccharide of D-glucosamine with a linkage and substituted with phosphates and lipid chains.