Biochemical basis of the immunogenicity of the common enterobacterial antigen

Of the numerous members of the family Enterobacteriaceae only a few strains, notably Escherichia coli O14 and R mutants of the E. coli R1-core type, engender antibodies against the common enterobacterial antigen (CA) following immunization of rabbits with heated suspensions or culture supernatants; other members produce nonimmunogenic CA of identical serological specificity. The biochemical basis of the immunogenic properties of CA of the former strains was investigated by determining the relationship between the CA determinant and the lipopolysaccharide molecule. Lipopolysaccharides extracted from R mutants of the E. coli R1-core type or of E. coli O14 by the phenol-chloroform-petrol ether method contain the CA determinant, in contrast to extracts of other CA-producing R mutants. This is evident from the observation that only the former absorb CA antibodies, utilizing erythrocytes coated with alkali-treated lipopolysaccharide preparations. Based on the findings that CA of R mutants of E. coli R1-core type follows lipopolysaccharide during all purification steps and that alkali treatment increases its affinity for erythrocytes parallel to that of the lipopolysaccharide, it is concluded that the CA determinant either is part of the lipopolysaccharide molecule or is strongly complexed with it. It is suggested that this association between CA and the lipopolysaccharide of E. coli R1-core type and E. coli O14 accounts for the heat stability of the immunogenicity of CA of these unusual strains.