What molecule is unique to the outer membrane of Gram-negative bacteria?

The molecule unique to the outer membrane of Gram-negative bacteria is lipopolysaccharide, or LPS. The outer membrane is an extra lipid bilayer found in Gram-negative bacteria, outside the thin peptidoglycan layer and the inner cytoplasmic membrane. LPS forms much of the outer leaflet of that membrane.

LPS has three main regions: lipid A, a core oligosaccharide, and the O-antigen polysaccharide. Lipid A anchors the molecule in the membrane and is chiefly responsible for its endotoxin activity. When LPS enters the bloodstream, it can provoke a strong immune response, including fever, inflammation, low blood pressure, and—in severe cases—septic shock.

The O-antigen projects outward and varies between bacterial strains, helping determine serological types. Escherichia coli and Salmonella are familiar Gram-negative examples carrying LPS. The term endotoxin is often used as a synonym for LPS, although the two terms are not perfectly identical in every historical usage.

Peptidoglycan is found in bacterial cell walls, mycolic acids are characteristic of Mycobacterium envelopes, and teichoic acids are associated with Gram-positive cell walls.