Monoclonal antibodies to detect capsular diversity among Bacteroides fragilis isolates

The capsular polysaccharide complex (CPC) of Bacteroides fragilis is composed of two distinct polysaccharides, designated PS A and PS B, and is a major virulence factor of this microorganism. In order to investigate the antigenic diversity of the CPCs of B. fragilis strains, we generated and characterized 10 monoclonal antibodies (MAbs) directed to the CPCs of three reference strains. The specificities of the MAbs were determined by enzyme-linked immunosorbent assay and dot-immunobinding assay. At least one MAb was specific for each PS A and PS B of the three strains. The MAbs were used to detect capsular antigens on the surface of 231 B. fragilis isolates from different geographical areas by a whole-cell dot-immunobinding assay. Over half of the strains, regardless of the country of origin, reacted with at least one MAb. Clinical extraintestinal infection isolates were significantly more reactive than fecal isolates, suggesting an association between capsular composition and the propensity to cause clinical infections. The patterns of reactivity of the isolates with the 10 MAbs were very different and sometimes extremely complex and indicated a sharing of epitopes among different capsular polysaccharides. The reactive strains could be grouped according to 32 different patterns; some patterns were relatively common, while others were rarer and were shown by only one or two strains. These results show that B. fragilis capsular polysaccharides are antigenically extremely diverse. This complexity and the large number of nonreactive strains indicate that a typing system based on B. fragilis capsular antigens will be difficult to establish.