Biosynthetic capacity of type-specific antigen synthesis determines the virulence of serotype III strains of group B streptococci

The level of type-specific antigen (that covalently associated with the cell wall peptidoglycan and that released extracellularly) synthesized by virulent strains of type III group B streptococci was quantitated and compared. Additionally, the effect of the physiological age of the cells and the influence of the exogenous phosphate ion concentration on the level of antigen synthesis by these organisms were also examined. Approximately 4% of the total antigen synthesized by the organism is noncovalently bound to the cell surface, and the difference in level of the noncovalently associated type-specific antigen between virulent and avirulent strains was negligible. In contrast, when the cell-associated covalently bound type antigens were evaluated, virulent strains were demonstrated to have two- to threefold higher levels than those of avirulent strains during the exponential and stationary phases of growth under various growth conditions. Furthermore, virulent strains that had high levels of cell-associated type antigen also secreted more extracellular type antigen than did avirulent strains. Thus, the data were consistent with the hypothesis that an overall production of type-specific antigen correlated with virulence in mice. However, the cell-associated type-specific antigen probably represented a better indicator for virulence potential since the addition of purified extracellular type-specific antigen to a mutant strain that lacks cell surface type antigen did not alter the 50% lethality value of the organism. To account for variation in the level of type-specific antigen produced by these strains, the kinetics of both the group- and type-specific antigens synthesis was investigated at the cell membrane level by utilizing an intact protoplast system. The data obtained from these studies strongly suggested that variation in virulence is due to differences in the biosynthetic capacity for synthesis of the type-specific, but not the group-specific, antigen.