Lipopolysaccharide (LPS) is the major component of the outer membrane of Gram-negative bacteria consisting of three domains: a hydrophobic anchor called lipid A, a non-repeating core oligosaccharide (OS) and a repetitive glycan polymer called O-antigen or O-polysaccharide (O-PS). The enzymes for the biosynthesis of lipid A are well conserved among Gram-negative bacterial species [MD:M00060 M00866
]. The core OS connecting lipid A with O-antigen is divided into inner and outer parts. Five core types, K-12 and R1-R4, are identified in Escherichia coli, with the common inner part composed of Kdo (ketodeoxyoctulosonic acid) and Hep (heptose) residues and the outer part made of various sugar residues. O-antigen is the most variable part used for serotyping of pathogenic Escherichia coli, where about 170 types are known. All three domains are involved in toxicity, pathogenicity, antimicrobial resistance and other activities.