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