Schematic architecture of the LPS embedded in the outer membrane of Gram-negative bacteria. LPS is a complex glycolipid composed of a highly conserved lipid A anchor, the more variable core oligosaccharide, and the hypervariable antigenic O-polysaccharide (O-antigen) consisting of several copies of oligosaccharide repeating units. Glc indicates glucose; Gal, galactose, GlcNAc, N-acetyl-d-glucosamine; and Hep, heptose.