Relationships between serological groups and deoxyribonucleic acid homology groups in Bacteroides fragilis and related species.

The serological properties of antigens extracted from strains of Bacteroides fragilis and related species belonging to several different deoxyribonucleic acid homology groups were investigated. Antisera prepared against Formalin-treated whole cell suspensions of representative strains were tested against cell suspensions, cell wall preparations, and extracts of homologous and heterologous strains by using agglutination, immunodiffusion, and hemagglutination techniques. Serological results indicated that the species were antigenically distinct, although minor cross-reactions were observed. Homology groups, including the two B. fragilis subgroups, were relatively homogeneous, although the presence of serotypes within each homology group was suggested. Immunodiffusion tests demonstrated, however, that each possessed a mosaic antigen composition; at least 6 antigenic determinants could be demonstrated in B. fragilis 2553, and up to 10 were found in B. fragilis 2393. Hemagglutination tests using antigen extracts also indicate a mosaic of antigens in each strain.