Identification of Bacteroides fragilis by indirect immunofluorescence.

Rabbit antiserum against live or heat-killed Bacteroides fragilis NCTC 9343 bacteria was titrated against hot phenol water-extracted polysaccharide antigens from five different species of the 'B. fragilis group' of bacteria using an enzyme immunoassay and shown to be specific for the B. fragilis NCTC 9343 polysaccharide. When the antiserum was used in indirect immunofluorescence, 97.1% of 244 B. fragilis strains were correctly identified. Only 8 of the other 312 Bacteroides strains were stained by the anti-B fragilis serum. The 'cross-reactive' Bacteriodes strains all belonged to the "B. fragilis group" of bacteria (i.e., B; distasonis, B. ovatus, B. thetaiotaomicron, B. uniformis, and B. vulgatus). None of the 425 aerobic enteric bacteria representing Salmonella, E. coli, Proteus, Yersinia, Shigella, Klebsiella, Enterobacter, Citrobacter, and Pseudomonas were positive using the anti-B. fragilis serum. Likewise, all the 59 gram-positive strains representing Streptococcus, Bacillus, Peptostreptococcus, Peptococcus, Propionibacterium, Lactobacillus, Eubacterium, and Clostridium did not stain. Our data shows in accordance with other findings [11], that B. fragilis strains possess a species-specific cell envelope antigen(s) which promises to be important for production of antisera, making a rapid identification of the species possible.