Further details of the chemical structure of lipid A of Salmonella have been evaluated. It was found that pyrophosphate bridges interlink β-1,6-linked glucosamine disaccharide units, which are esterified by lauric, palmitic, and 3-d-myristoxymyristic acid and which are substituted at the amino groups by 3-d-hydroxymyristic acid. Lipid A is the biologically active component of lipopolysaccharides. Free lipid A, solubilized by complexing with bovine serum albumin, exhibits strong endotoxic activity in a number of biological tests. Lipid A exhibits immunogenic properties when suitably exposed on the bacterial surface. Immunization of rabbits with lipid A leads to the production of specific antibodies to lipid A that are capable of cross-reacting with a large variety of lipopolysaccharides.

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