Ibc proteins as serotype markers of group B streptococci.

A collection of 106 clinical isolates of group B streptococci were type classified by a direct fluorescent antibody test (FAT). In all, 99 of the isolates were typable and belonged to the serotypes Ia (8 strains), Ib (9 strains), Ic (30 strains), II (15 strains), or III (37 strains). The presence of Ibc proteins was demonstrated in 60 of the strains, including 6 of the non-typable isolates. Indirect FAT was performed to test whether the bacteria produced the alpha and beta antigens of the Ibc protein fraction, using specific anti-alpha and anti-beta sera. Both alpha and beta were produced by 22 strains, only alpha by 33 strains, and only beta by 5 strains. The results show that the alpha and beta antigens can be used as markers to detect serovars among group B strains which carry identical serotype-specific carbohydrate antigens, or strains listed as nontypable according to the classification currently used.