A lipopolysaccharide (LPS) is a large molecule that contains both lipid and a carbohydrate. They are a major suprastructure of gram-negative bacteria which contributes greatly to the structural integrity of the bacteria, and protects them from host immune defenses.

It comprises three parts: polysaccharide (O) side chains; core polysaccharides; and lipid A. Lipid A contains unusual fatty acids (e.g. hydroxy-mysteric acids) and is inserted into the outer membrane while the rest of the LPS projects from the surface. Core polysaccharide contains unusual sugars (e.g. KDO, keto-deoxyoctulonate and heptulose). It contains two glucosamine sugar derivatives each containing three fatty acids with phosphate or pyrophosphate attached.

LPS can also act as an endotoxin, because it can bind to the same CD14 receptor on macrophages, triggering the whole cascade for macrophage/endothelial cells to secrete pro-inflammatory cytokines. Lipid A can also be toxic, the core polysaccharide is attached to lipid A.

The polysaccharide side chain is referred as the O-antigen of the bacteria. O side chain (O antigen) is a also a polysaccharide chain that extends from the core polysaccharide. The composition of the O side chain varies between different gram-negative bacterial strains. O side chains are easily recognized by the antibodies of the host, however, the nature of the chain can easily be modified by gram-negative bacteria to avoid detection. LPS also increases the negative charge of the cell wall and helps stabilize the overall membrane structure.