Primary lipid A structures. E. coli has a bis-phosphorylated diglucosamine backbone with six amide and ester linked fatty acyl chains. The non-endotoxic (and LPS-antagonist) R. sphaeroides lipid A has an identical disaccharide backbone but has five acyl residues, shorter ester-linked primary acyl chains and an unsaturated acyl group. B. fragilis, which expresses low endotoxic potential, has a mono-phosphorylated backbone and five acyl chains with longer chain lengths than those seen in E. coli. Lipid A from Chromobacterium violaceum, like R. sphaeroides, is an LPS antagonist. Figures adapted from: Weintraub et al , Rietschel et al  and Takayama & Qureshi .