Properties of a Streptococcus suis isolate of serotype 2 and two capsular mutants.

Encapsulation is thought to be a critical virulence factor of Streptococcus suis. In the present study two capsular type 2 mutants of S. suis (M42 and M2) and their S. suis parent strain (89-1591) were further characterized. All three cultures reacted with group D specific antiserum whereas parent strain 89-1591 and mutant M42 but not mutant M2 reacted with specific antiserum against capsular type 2. Both mutants had higher surface hydrophobicity and showed an increased adherence to human epithelial cells and to lung macrophages of rabbits as compared to the parent strain. In phagocytosis experiments with polymorphonuclear leucocytes the encapsulated parent strain was more resistant to phagocytosis than both mutant strains. These findings might help to understand the role of encapsulation of S. suis in the process of infection.