Bacterial structure and functional relation to abscess formation.

The capsular polysaccharide complex (CPC) of Bacteroides fragilis exhibits unusual biologic properties. This polysaccharide complex promotes the formation of abscesses and prevents abscess induction in a rat model of intra-abdominal sepsis. Each of these biologic properties is mediated by a T cell-dependent immune mechanism. The CPC consists of two distinct polysaccharides, PS A and PS B, each with repeating units that have positively charged amino groups and negatively charged carboxyl or phosphate groups. Analysis of these polysaccharides as well as other charged carbohydrates before and after chemical modification revealed that these oppositely charged groups are required for promotion of intra-abdominal abscesses as well as for protection against abscess induction. These studies provide a structural rationale for the distinct properties associated with the B. fragilis CPC, and delineate one mechanism by which this host response occurs.