Secreted antigens as virulence associated markers in Streptococcus bovis strains from pigeons

SDS-PAGE and Western blot analysis were performed on the culture supernatant of 7 pigeon S. bovis reference strains belonging to the serotypes 1, 2, 3 and 5. The culture supernatant of highly virulent serotype 1, 2 and 5 strains contained four antigens that were absent in low virulent serotype 3 strains, notably a 185 kDa minor protein band (A) and a tripler (T1) of 70 kDa. The less virulent serotype 3 strains on the other hand contained protein triplets, that had a molecular mass of either 68 kDa (T2) or 74 kDa (T3). The prevalence of A, T1, T2 and T3 was examined in 68 S. bovis strains isolated from healthy pigeons and in 68 S. bovis strains isolated from pigeons that died of S. bovis septicaemia. Six supernatant phenotypes were identified: A(-) T1 (32 strains), A(-) T2 (10 strains), A(-) T3 (7 strains), A(+) T1 (84 strains), A(+) T2 (1 strain) and A(+) T3 (2 strains). Ninety-four percent of the strains lacking the A and T1 proteins were isolated from healthy pigeons, and only 6% were isolated from septicaemia. Strains expressing A and/or T1, however, were isolated from septicaemia in 57% of the cases. These observations may indicate that the A and/or T1 proteins are associated with virulence.