Group A Streptococcus (GAS) grows in the throat or skin and can lead to streptococcal illnesses, such as scarlet fever. Therefore, identifying the specific expressed proteins as antigens is urgent for the development of an efficient vaccine. The aim of this study was to identify and evaluate the GAS antigen that be recognized in adult sera. Using western blotting and epidemic statistical analysis, we identified a 55kDa GAS protein that is immunoreactive with the sera of adults but not the sera of children. The 55kDa GAS protein was further confirmed as M12 protein by MALDI-TOF MS. Moreover, we constructed and expressed the NH_2-terminal and COOH-terminal of M12 recombinant proteins to determine the crucial epitope of M12. Opsonization and indirect bactericidal experiments showed that the recombinant M12 proteins can interfere the adult serum GAS opsonization and be recognized by adult antibody. Therefore, M12 antigen expressed in GAS could possibly serve as a potential target used in production of a vaccine that might prevent diseases by GAS in Taiwan.
