Streptococcus pneumoniae: Pathogenesis from Colonization to Invasive Disease

Streptococcus pneumoniae (pneumococcus) is a Gram-positive encapsulated bacterium causing pneumonia, meningitis, bacteremia, and otitis media. It is a leading cause of vaccine-preventable death globally.

Pathogenic Sequence

1. Adhesion: Surface proteins (pili, choline-binding proteins) bind nasopharyngeal epithelial receptors.
2. Colonization: Asymptomatic carriage in nasopharynx — common in children; reservoir for transmission.
3. Virulence Factor Deployment:
   • Capsule: Polysaccharide layer inhibits phagocytosis (primary virulence factor; >90 serotypes).
   • Pneumolysin: Pore-forming toxin damages cells, triggers inflammation.
   • Autolysin: Promotes lysis, releasing inflammatory cell wall components.
   • Hydrogen peroxide: Oxidative damage to host tissues.
   • Neuraminidase: Exposes receptors, enhances spread.
4. Immune Evasion: Capsule prevents opsonization; IgA protease degrades mucosal antibodies.
5. Invasion: Breach of epithelial barrier → bloodstream (bacteremia) → meninges (meningitis) or lungs (pneumonia).

Host and Strain Factors

Outcome depends on serotype virulence, host immunity (age, comorbidities, vaccination status), and timely antibiotic therapy. PCV13/PPSV23 vaccines target common invasive serotypes.