Which of the following factor is mainly responsible for virulence in Streptococcus

M proteins: The most impoant protein used for typing as well as for virulence. It acts as a virulence factor by inhibiting phagocytosis. It is antigenic. The antibody to the M protein promotes phagocytosis of the coccus and is therefore protective. M protein is heat and acid stable but susceptible to tryptic digestion. It can be extracted by Lancefield's acid extraction method and typing is done with type-specific sera. About 80 M protein types have been recognized. Reference: Textbook of Microbiology; Anathanarayan and paniker's; 10th edition; Page no: 213