Serological studies of Bacteroides fragilis

Using direct agglutination methods, a simple serological scheme for the classification of Bacteroides fragilis is described. Twenty strains of B. fragilis were selected by a process of sucessive screening from 151 strains obtained from various sources. O-antigens were prepared from the 20 strains, and used to raise antisera in rabbits. Each of the 20 antisera reacted with its homologous antigen and eight antisera cross-reacted with other subspecies. These cross-reactions were successfully removed after absorption of the antisera with the cross-reacting antigens, resulting in 19 type-specific antisera, titres ranging from 40 to 320, and 19 distinct serotypes of B. fragilis. There was no correlation between the antigenic and the biochemical characteristics of these strains and no cross-reactions occurred with other gram-negative anaerobes, B. melaninogenicus, Sphaerophorus necrophorus and Fusobacterium necrogenes.