Ultrastructure of surface components of Streptococcus gallolyticus (S-bovis) strains of differing virulence isolated from pigeons

Virulence of Streptococcus gallolyticus (S. bovis) strains isolated from pi geons is associated with the presence of the extracellular proteins A, T1, T2 and T3. Based on the presence or absence of these proteins, six supernat ant-phenotypes are distinguished. Experimental infection studies have indic ated that strains belonging to the A(-)T1, A(+)T1, A(+)T2 and A(+)T3 groups are highly virulent for pigeons, strains belonging to the A(-)T3 groups ar e moderately virulent and A(-)T2 strains are of low virulence. In this stud y the surface structure of 15 pigeon 5. gallolyticus strains representing h igh, moderate and low virulence supernatant-phenotypes was examined by elec tron microscopy. The presence of capsular material was determined by transm ission electron microscopy after polycationic ferritin labelling and immuno stabilization. Capsules from cells labelled with polycationic ferritin were usually thicker than those from cells exposed to antiserum, The capsule of the virulent strains had a regular, continuous appearance whilst irregular ity of the capsule was a characteristic of the low virulence A(-)T2 strains . Negative staining revealed the presence of fimbriae in all strains belong ing to the high virulence A(-)T1, A(+)T1, A(+)T2 and A(+)T3 supernatant gro ups and in one strain of the moderately virulent A(-)T3 group. The fimbriae were thin, flexible structures with a diameter of approximately 3-4 nm and a length of up to 700 nm. Fimbriae as described above were absent in two o ther A(-)T3 strains examined and in the low virulence A(-)T2 strains. Resul ts from this study indicate that morphological differences in surface struc ture exist among virulent and low virulence pigeon 5. gallolyticus strains, and that the capsule and/or fimbriae are possibly involved in virulence.