Production by Aeromonas of common enterobacterial antigen and its possible taxonomic significance

In a study of production of the common enterobacterial antigen (CA) by members of the Aeromonas group, ten strains of A. shigelloides, nine strains of A. hydrophila, and nine strains of A. salmonicida were used. Passive hemagglutination and hemolysis tests as well as the hemagglutination-inhibition procedure revealed that all strains of A. shigelloides, in contrast to the strains of the other two species, produce this antigenic determinant. The antigen of A. shigelloides was demonstrated even when the supernatant fluids of agar-grown cultures were used in a dilution of 1:1,000, whereas 10-times concentrated supernatant fluids obtained from the other two species were negative. Supernatant liquids of cultures of A. shigelloides failed to induce a significant CA immune response in rabbits; nonetheless, the animals were primed immunologically and responded with prompt production of CA antibodies in significant titers to a booster injection of a subeffective dose of CA obtained from Salmonella typhimurium. Strains of A. hydrophila and A. salmonicida neither induced CA antibody formation nor primed the animals. It is concluded that of the three species of the Aeromonas group only A. shigelloides, which may produce O antigen cross-reacting with Shigella sonnei and which has been isolated from patients with dysentery or gastroenteritis, produces CA. Production of this antigen, therefore, may help to characterize microorganisms belonging or related to the family Enterobacteriaceae.