Quantitative determination of the antibody response to the capsular polysaccharide of Bacteroides fragilis in an animal model of intraabdominal abscess formation.

The humoral antibody response to the capsular polysaccharide of Bacteroides fragilis was quantitated in an animal model of intraabdominal abscess formation using a sensitive quantitative radioactive antigen-binding assay. Antibody detected by this technique correlated highly with antibody measured by quantitative precipitin analysis (r = 0.943). Animals infected with encapsulated B. fragilis develop high levels of circulating serum antibody to the capsular polysaccharide. This antibody can be induced by implantation of live organisms, heat-killed organisms, heterologous strains of B. fragilis, and various outer membrane components that contain the capsular antigen. The immunogenicity of the capsular polysaccharide could be enhanced when administered as part of the outer membrane or when not separated from outer membrane proteins. Evidence of an antibody response to this capsular polysaccharide offers support for the demonstrated pathogenic potential of encapsulated B. fragilis.