Lipopolysaccharide (LPS) is a molecule that is a constituent of the outer membrane of Gram-negative bacteria. The molecule can also be referred to as endotoxin. LPS can help protect the bacterium from host defenses and can contribute to illness in the host.

The LPS comprises much of the portion of the outer membrane that is oriented towards the outside of the bacterium. There are fewer phospholipid molecules in this outer "leaflet" of the membrane than there are on the inner side of the membrane. Thus, because of the presence of lipopolysaccahride, the construction of the outer membrane is asymmetric. In contrast, the inner membrane of Gram-negative bacteria and the single membrane of Gram-positive bacteria are more symmetric, with both leaflets of the membrane comprised of much the same molecules.

A complete LPS consists of a lipid portion and a chain of sugar...