A study of the antigenic composition of the fragilis group of Bacteroides.

Representative strains of 22 serotypes of the fragilis group of Bacteroides and four non-fragilis control strains of B. melaninogenicus, B. disiens, B. bivius and Fusobacterium nucleatum were tested by SDS-PAGE and immunoblotting with hyperimmune rabbit sera. SDS-PAGE showed 25 polypeptide bands but, after immunoblotting, 24 antigenic bands were observed in various combinations in all the strains. Three of these were detected only in the control strains, whereas six others were present in different combinations in all strains of the fragilis group but were not present in the controls. Cluster analysis of the antigenic bands showed that the controls were antigenically different from the fragilis group strains. Strains of the fragilis group from the same geographic localities grouped in single clusters; most faecal isolates and NCTC strains appeared separate. There was no correlation between the species of Bacteroides and their antigenic structure. SDS-PAGE with immunoblotting is a superior technique for typing the fragilis group of Bacteroides. Specific antigens have been identified which may be used in the serodiagnosis of infection with these organisms.