Streptococcus agalactiae beta gene and gene product variations

Streptococcus agalactiae (group B streptococci; GBS) are serotyped on the basis of the capsular polysaccharide antigens and subtyped on the basis of the strain-variable and surface-localised c proteins c alpha, c beta, and R proteins. This study compared c beta protein detection and the polymerase chain reaction (PCR) for beta gene detection, by examining 50 clinical GBS strains. The c beta protein was detected by antibody-based immunofluorescence in a GBS whole-cell assay and Western blotting by probing with the anti-c beta antibody or human IgA. Absorption experiments were performed to test for surface-anchoring of c beta; and bacterial supernates were examined to test for c beta production. Primers for the PCR target regions resulted in a 620-bp product that included beta gene-encoding IgA-binding domains. The results demonstrated four categories of GBS with respect to the beta gene and the c beta protein: (1) strains (16 of 50) that harboured the beta gene and regularly expressed normal surface-localised c beta with a M(r) of 120 kDa; (2) strains (5 of 50) that harboured the gene but did not express the protein; (3) strains (2 of 50) that harboured the gene but expressed a c beta that was not surface-localised and had reduced M(r); (4) strains (27 of 50) without beta gene and c beta expression. One strain amongst the third group generated a PCR product of 1330 bp. These results demonstrate considerable strain variability of the beta gene of GBS and of its product the c beta protein.