In biochemistry, lipopolysaccharide (LPS) is 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 of three parts: polysaccharide side chains, core polysaccharides and lipid A. Lipid A contains unusual fatty acids (e.g. hydroxy-mysteric acids), Core polysaccharide contains unusual sugars (e.g. KDO, keto-deoxyoctulonate and heptulose). The polysaccharide side chain is referred as the O-antigen of the bacteria.
LPS is also referred to as endotoxin, because it can bind to the CD14 receptor of macrophage, triggering the whole cascade for macrophage/endothelial cells to secrete pro-inflammatory cytokines.