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Participation of Lipopolysaccharide Genes in the Determination of the Enterobacterial Common Antigen: Analysis in Salmonella Groups B and C₁

Central Public Health Laboratory (State Serum Institute), Helsinki, Finland, and Max-Planck-Institut für Immunbiologie, Freiburg, Germany

ABSTRACT

The enterobacterial common antigen (CA) is present in salmonellae of groups B (S. typhimurium) and C₁ (S. montevideo). Mutation at the rfe gene(s), which is required for the biosynthesis of O side chains of the lipopolysaccharide in group C₁ (S-6, 7) but not in group B (S-4, 12), destroys the capacity of the bacteria to synthesize CA. When such mutated group C₁rfe genes (C-rfe^–) were introduced into group B strains, the hybrids also became CA^– and could be restored to CA⁺ by introduction of either C-rfe⁺ or B-rfe⁺ (corresponding genetic region in group B). This indicated the presence of genes for CA synthesis at the rfe site in both groups B and C₁. In rfe^– mutants of group C₁, which were rough and CA^–, the CA⁺ phenotype could be restored by replacing the rfe^– gene(s) with C-rfe⁺. In contrast, B-rfe⁺ was able to support the synthesis of trace amounts of CA only, although it was sufficient to restore their ability to synthesize the S-6, 7 side chain of the lipopolysaccharide. Corresponding hybrids (B-rfe⁺, C-rfb⁺ or C-rfb^–) were constructed by introducing the C-rfb genes into a group B strain; they also showed only a trace of CA reactivity.