Immunodiffusion techniques were used on trichloroacetic acid extracts from 10 strains of Bacteroides fragilis in detecting precipitating antibodies against this species in immune rabbit sera. Species and even strain specificities were observed in these precipitin reactions. Multiple antigens were detected in the extracts from some strains, whereas only one precipitin band per extract developed during agar-gel diffusion tests of others. The antigen extracts were found to be both heat stable and resistant to hydrolysis by alpha-chymotrypsin. Four serological patterns were demonstrated in homologous and heterologous reactions with the B. fragilis. antigen-antibody systems used. The results showed that some strains were serologically distinct from others, indicating that the strains tested are of more than one serotype.
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