Immune response to Bacteroides fragilis in experimentally infected rabbits

Experimental Bacteroides fragilis infections in rabbits were produced by injecting 10(9)-10(10) organisms into subcutaneously implanted steel-net tissue cages. Four B. fragilis type strains with divergent antigenic set-up were used. Viable counts in the cage contents reached a steady state at 10(7)-10(9) within 2-3 weeks. Bacteria coated with immunoglobulin were demonstrated in the pus. The antibody response in serum, followed with indirect immunofluorescence and detected within 7-10 days, was dominated by IgG; IgM antibodies were also involved but to a lesser degree. Serum antibody levels were not significantly influenced by infecting dose or strain, variations in viable counts or antibiotic therapy. Antibody response was directed against heat-stable as well as heat-labile antigens. Non-homogeneous staining, i.e. positive fluorescence of some of the cells in the preparation with the remaining cells negative, was found with heterologous and homologous cells and was found to be related to heat-labile antigen.