Molecular analysis of clinical group B streptococcal strains by use of alpha and beta gene probes

Group B streptococci (GBS) are a leading cause of neonatal sepsis and meningitis. Besides the type-specific capsule, which is considered to be a major virulence factor of the species, some proteins are believed also to be virulence determinants and have been found to elicit protective immunity. In the present work, the genes for two surface proteins, the alpha and beta antigens, were detected in hybridization tests with chromosomal DNA of clinical GBS isolates. Using as a probe a PCR-generated 1.5 kb part of the beta gene, hybridization was found for 4/19 type Ia, 8/11 type Ib, 5/6 type II but for 0/8 type III strains. Positive outcome of hybridization coincided with an ability of the strains to bind IgA. A 200 bp alpha gene probe hybridized with all tested strains of serotypes Ia, Ib or II but only with 4/17 type III strains. By Southern blot, it was found that the size of the EcoRI chromosomal gene fragments hybridizing with the alpha gene probe correlated with the genomic presence or absence of the beta gene, possibly reflecting evolutionary relationship between the two genes. This assumption was further supported by pulsed field gel hybridization analysis which, however, showed the chromosomal positions of these two genes not to be adjacent.