## Virulence Factors of S. pneumoniae **Key Point:** S. pneumoniae possesses multiple virulence factors that enable invasion, evasion of immunity, and tissue damage. Understanding which factors are actually present is critical for pathogenesis and vaccine design. ### Correct Virulence Factors Present in S. pneumoniae | Virulence Factor | Mechanism | Clinical Significance | |---|---|---| | **Polysaccharide capsule** | Antiphagocytic; inhibits complement deposition | Most important virulence factor; basis for pneumococcal vaccines (PCV, PPSV23) | | **Pneumolysin** | Cholesterol-dependent cytolysin; creates pores in host cell membranes | Causes tissue damage, inflammation, bacteremia | | **Hyaluronate-like capsule** | Molecular mimicry of host hyaluronic acid | Evades immune recognition; poor immunogenicity | | **Neuraminidase** | Cleaves sialic acid from host cells | Facilitates adherence and invasion | | **IgA protease** | Cleaves IgA at hinge region | Disables mucosal immunity | ### Why M Protein is NOT a Virulence Factor of S. pneumoniae **High-Yield:** M protein is the **hallmark virulence factor of Group A Streptococcus (GAS)**, NOT S. pneumoniae. M protein provides antiphagocytic properties in GAS through molecular mimicry of host myosin and tropomyosin, but S. pneumoniae does not produce M protein. This is a classic exam trap that conflates two different streptococcal species. **Clinical Pearl:** The distinction matters clinically: - **GAS (M protein)** → rheumatic fever, post-streptococcal glomerulonephritis - **S. pneumoniae (capsule + pneumolysin)** → invasive pneumonia, meningitis, bacteremia **Mnemonic: PEN-CAP** — S. pneumoniae virulence = **PEN**eumolysin, **CAP**sule (polysaccharide), and other factors (neuraminidase, IgA protease), but NOT M protein.
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