Lipid A and anti-lipid A.

Lipid A in free form, in crude antigen preparations, and on Formalin-treated Escherichia coli and Salmonella minnesota R595 was employed in studies of its antigenic composition, immunogenicity, and availability on gram-negative bacteria. Analyses with immunodiffusion and crossed immunoelectrophoresis of isolated lipid A preparations revealed three components. Inhibition experiments with enzyme-linked immunosorbent assay showed that the lipid A structure was not exposed on the tested smooth or rough E. coli strains or on S. minnesota R595. In crude O antigen preparations from some of the strains, however, lipid A was available for reaction with antibodies. The inaccessibility of lipid A on the bacterial surface may explain the poor protective capacity of anti-lipid A antibodies against bacterial infections. An enzyme-linked immunosorbent assay was more sensitive for measuring anti-lipid A antibody activity than indirect hemolysis or indirect hemagglutination. With an enzyme-linked immunosorbent assay it was shown that in rabbits the immunogenicity of lipid A was approximately the same when coated on erythrocytes or, as is more commonly done, when lipid A-coated hydrolyzed bacteria were used. Some antisera from rabbits immunized with E. coli of different serotypes showed activity against lipid A, with a higher frequency for antisera from rabbits immunized with R mutants.