Isolation and characterization of a cross-reacting antigen in strains of Bacteroidaceae

A Bacteroidaceae cross-reacting antigen (BCA) was isolated from several strains belonging to species in the genera Fusobacterium and Bacteroides. We showed that each of the 28 strains examined synthesized either BCA or the O-specific lipopolysaccharide (LPS). The structural difference between the two antigens was demonstrated by passive hemagglutination. BCA coated erythrocytes spontaneously and reacted with equal intensity to all bacterial antisera of BCA+ strains, whereas LPS was species specific and also coated erythrocytes after the antigen was treated with NaOH. BCA is an acid polysaccharide as proven by immunoelectrophoresis and by its capacity to form a salt linkage with lysozyme. Chemical analysis demonstrated the presence of galactosamine, glucosamine, galactose, glucose, mannose, and N-acetyl. BCA was nontoxic and did not contain lipid A in its molecule. The small particle size of BCA determined its hapten character. When attached to acid-treated Salmonella minnesota Re cells, it acted as an immunogen. By immunizing rabbits with this hapten-bacterial cell suspension, we obtained a highly potent antiserum that agglutinated all BCA+ strains.