Possible virulence marker for Streptococcus agalactiae (Lancefield Group B).

Mouse passage of a stock strain of each of the serotypes Ia, Ib, Ia/c, II and III of Streptococcus agalactiae was followed by increased virulence. The change was associated with an increased content of sialic acid and also with the appearance of an antigen common to all passaged strains. This antigen was subsequently detected in all but one of 12 strains isolated from infected babies or diabetic adults, but in none of 12 organisms recovered from carriers.