Relationship between cell surface carbohydrates and intrastrain variation on opsonophagocytosis of Streptococcus pneumoniae

Streptococcus pneumoniae undergoes spontaneous phase variation between a tr ansparent and an opaque colony phenotype, the latter being more virulent in a murine model of sepsis, Opaque pneumococci have previously been shown to express lower amounts of C polysaccharide (cell wall teichoic acid) and in this study were shown to have a higher content of capsular polysaccharide by immunoelectron microscopy, This report then examined the relationship be tween expression of these two cell surface carbohydrate structures and thei r relative contribution to the increased virulence of opaque variants. Comp arison of genetically related strains showed that the differential content of capsular polysaccharide did not affect the amount of teichoic acid as me asured by a capture enzyme-linked immunosorbent assay (ELISA), In contrast, when the teichoic acid structure was altered by replacing choline in the g rowth medium with structural analogs, the quantity of capsular polysacchari de as measured by a capture ELISA was decreased, demonstrating a linkage in the expression of the two surface carbohydrate structures. A standardized assay was used to assess the relative contribution of cell surface carbohyd rates to opsonophagocytosis. The opaque variants required 1.2- to 30-fold m ore immune human serum to achieve 50% opsonophagocytic killing than did rel ated transparent variants (types 6B and 9V), The opsonophagocytic titer was proportional to the quantity of capsular polysaccharide rather than teicho ic acid. The major factor in binding of the opsonin, C-reactive protein (CR P), was also the amount of capsular polysaccharide rather than the teichoic acid ligand. Only for the transparent variant (type 6B), which bound more CRP, was there enhanced opsonophagocytic killing in the presence of this se rum protein. Increased expression of capsular polysaccharide, therefore, ap peared to be the major factor in the decreased opsonophagocytic killing of opaque pneumococci.