Type classification of group B streptococci by the fluorescent antibody test

Fluorescein-isothiocyanate conjugates were prepared from rabbit anti-type Ia, Ib, II and III sera, and used to classify 90 strains of group B streptococci isolated from the genital tract of puerperal women. Of the 84 strains that were typeable, the numbers belonging to serotypes Ia, Ib, Ic, II and III, were 7, 14, 6, 37 and 20, respectively. The anti-type Ib conjugate showed cross-reactivity with 21 of the type II strains and one of the type III strains. Absorption of the anti-type Ib conjugate with bacteria of a type Ic strain and testing of enzyme-digested streptococci indicated cross-reactivity due to antibody to the Ibc protein antigen of group B streptococci. The potential usefulness of this antigen in the classification of type II and type III strains is discussed.