High Expression of a C Protein β Antigen Gene among Invasive Strains from Certain Clonally Related Groups of Type Ia and Ib Group B Streptococci

Serotyped strains of group B streptococci can be divided into subtypes based on restriction endonuclease digestion patterns (RDP). Profiles of cell-bound proteins were compared among RDP types. Proteins that showed a remarkable difference in the degree of expression were found among strains of RDP Ia-3, which has been considered potentially virulent, as well as of RDP Ib-1. For RDP Ia-3 strains, the protein was predominant in strains from cerebrospinal fluid (CSF) but was mostly a minor component in other strains. For RDP Ib-1 strains, the protein was predominant in strains from CSF, showed diversity in strains from blood, and was mostly a minor component in other strains. By N-terminal sequencing analysis, the protein was identified as a C protein β antigen. The level of bound immunoglobulin A (IgA) or anti-β antigen monoclonal antibody correlated with the level of expressed β antigen, and invasive strains showed remarkably high levels of binding; the exception was a CSF-derived strain of RDP Ib-1 which produced a large amount of β antigen and showed a high level of binding of anti-β antigen monoclonal antibody but no IgA binding. PCR-based amplification revealed that the β antigen gene was detected in all RDP Ia-3 and Ib-1 strains but was not found in any strains of other RDP types. Competitive reverse transcriptase PCR demonstrated that the difference in the amount of protein produced was due to the difference in the level of expression of the β antigen mRNA. Our findings imply that differences in gene expression for a protein may contribute to the invasiveness of RDP Ia-3 and Ib-1 strains for the host.