A biologically active substance produced by bacteria and consisting of lipopolysaccharide, a complex macromolecule containing a polysaccharide covalently linked to a unique lipid structure, termed lipid A. All gram-negative bacteria synthesize lipopolysaccharide, which is a major constituent of their outer cell membrane. One major function of lipopolysaccharide is to serve as a selectively permeable barrier for organic molecules in the external environment. Different types of gram-negative bacteria synthesize lipopolysaccharide with very different polysaccharide structures.