Final answer:
The presence of a capsule is a major virulence mechanism of Streptococcus pneumoniae, making phagocytosis more difficult for the immune system and significantly contributing to the bacterium's ability to cause severe infections.
Step-by-step explanation:
A major virulence mechanism of Streptococcus pneumoniae is the presence of a capsule. This capsule is known to significantly aid in the pathogenicity of the bacteria. It makes it more challenging for phagocytic cells, such as white blood cells, to engulf and kill the organism. In fact, encapsulated strains of S. pneumoniae are more virulent than nonencapsulated strains, as the capsule helps in immune evasion by making adherence difficult for immune cells and prevents phagocytosis. Moreover, it increases the size of the bacterial cell, complicating engulfment by immune cells.
The ability of S. pneumoniae to produce a capsule is paramount in its potential to cause serious infections such as pneumococcal pneumonia, meningitis, and septicemia. Other virulence factors of S. pneumoniae include pneumolysin which contributes to damaging host cells and provoking inflammation. However, when comparing all factors, capsule production stands out as a major mechanism that facilitates the bacterium's survival and ability to cause disease.