The present invention is directed to methods for producing and selecting novel mutant strains of B. fragilis that constitutively express a particular capsular polysaccharide or only selected capsular polysaccharides; compositions directed to the novel mutant strains of B. fragilis that constitutively express a particular capsular polysaccharide or only selected capsular polysaccharides; improved methods for purification of individual capsular polysaccharides; and compositions directed to novel res02 and inv19 genes and their gene products. Significantly, the present invention provides methods and compositions for overexpressing and purifying immunomodulatory capsular polysaccharide A (PSA) in high yield.