The invention relates to the field of diagnosis of and vaccination against Streptococcal infections and to the detection of virulence markers of Streptococci. The invention discloses a method for modulating virulence of a Streptococcus, the method comprising modifying a genomic fragment of Streptococcus wherein the genomic fragment comprises at least a functional part of a fragment identifiable by hybridization in Streptococcus suis to a nucleic acid or fragment thereof as shown in FIG. 5.