Virulence or pathogenicity of pneumococcus depends upon

The virulence of Streptococcus pneumoniae depends on the capsular polysaccharide. The capsular polysaccharide because of its acidic and hydrophilic propeies, protects the cocci from phagocytosis. Capsulated streptococcus pneumoniae are not phagocytosed efficiently in fluid media or exudates, however, they are susceptible to surface phagocytosis, being engulfed against a firm surface, such as a fibrin clot or epithelium. The enhanced virulence of type 3 Streptococcus pneumoniae is due to an abundance of its capsular material. Non-capsulated strains are avirulent. Reference: Textbook of Microbiology; Anathanarayan and panicker's; 10th edition; Page no: 226