Group B streptococcal Ibc protein antigen: distribution of two determinants in wild-type strains of common serotypes

Studies were carried out on the distribution of the Ibc protein antigenic marker in wild-type strains of group B streptococci of diverse serotypes isolated from epidemiological studies. Rabbits were immunized with group B streptococcal strain H36B, a prototype Ib strain, to produce antibody to the Ibc protein antigens. One antiserum (no. 970) contained antibody only against the trypsin-sensitive (TS) portion of the Ibc antigen. A second antiserum (no. 973), however, contained antibody to both the TS and the trypsin-resistant (TR) determinants or components of the antigen. A total of 785 wild-type strains of group B streptococci were serotyped by using antiserum 973 as well as antisera to the polysaccharide types Ia, Ib, II, III, and IV. Remarkably, 59% of all the strains tested (462 of 785) reacted positively with the Ibc antiserum, although not all carried both components of the Ibc antigen. Of the 99 Ib strains examined, 84% carried both TS and TR components. In contrast, 96% of the 202 Ic strains carried only the TR component of the Ibc antigen. Antiserum 970 failed to identify these strains. Routine typing with an antiserum which contains antibodies to only one portion of the Ibc antigen could result in significant serotype misidentification. Differentiation of group B streptococcal strains by the presence or absence of individual TS or TR components of the Ibc antigen could prove to be a useful additional epidemiological and serological marker. It is noteworthy that wild-type Ic strains carry, ordinarily, only the TR component, in contrast to the prototype Ic strain, which possesses the complete Ibc protein antigen. The possible contribution of the Ibc antigen to group B streptococcal virulence is of interest and requires further study.