Enterobacteriaceae share a common antigen (CA). This antigen exists as a powerful immunogen, when produced by Escherichia coli O14, and as a minimally effective immunogen, when present in cultures of most other smooth strains. The present study was directed toward the assessment of immunogenicity of this antigen produced by various rough mutants and their parent strains as well as that of viable and nonviable bacterial suspensions. Rabbits were immunized intravenously, and the passive hemagglutination test was used for the quantitation of CA antibodies. The following results were obtained. (i) Immunization of rabbits with viable or heat-killed (1 hr, 100 C) suspensions of smooth parent strains resulted in only slight or insignificant antibody production. Of seven additional smooth strains of Enterobacteriaceae tested, none induced CA antibodies upon immunization with killed (1 hr, 100 C) suspensions, and only two engendered antibodies in moderate titers when viable suspensions were used, possibly due to the presence of undetected R mutant cells. (ii) In contrast, administration of viable suspensions of the corresponding mutant strains, regardless of core types, including E. coli R1, R2, R3, and Salmonella Ra types, caused a significant antibody response. (iii) The immunogenicity of R mutants differed strikingly, inasmuch as mutants with the R1 core [E. coli O8(-):K27(-)(R) and Shigella boydii type 3(-)(R)] engendered CA antibodies when heated suspensions were administered, in contrast to heated suspensions of mutants with the R2, R3, and Ra cores. (iv) Viability per se is not an abolute requirement for immunogenicity of R2 mutants, since CA antibodies were engendered by suspensions killed at 60 C or by merthiolate or formaldehyde. (v) Ethanol fractionation revealed that immunogenic CA of the R1 mutant is ethanol-insoluble and that of the parental smooth strain is ethanol-soluble; thus, CA of R1 mutants resembles that of E. coli O14 in both immunogenicity and ethanol insolubility.

Animals↗

PubMed1972

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.

Adsorption↗

PubMed1972