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BACTERIA · Gram-positive cocci

Streptococcus pneumoniae

Streptococcus pneumoniae — Gram stain

Stain: Gram stainMorphology: Lancet-shaped gram-positive diplococci, often with surrounding capsule haloYield: HIGHDifficulty: EASY
Streptococcus pneumoniae microscopic image — Gram-positive cocci, Gram stain
Image: Wikimedia Commons · File:Gram_stain_of_Streptococcus_pneumoniae.jpg · CDC/PHIL (Public Domain)

Key facts

**Pathogenesis**: Polysaccharide capsule is the primary virulence factor — it inhibits phagocytosis; IgA protease cleaves mucosal IgA enabling nasopharyngeal colonization. **Diagnostic clue**: Gram-positive lancet-shaped diplococci in sputum, alpha-hemolytic on blood agar, optochin-sensitive, bile-soluble, positive quellung reaction. **Virulence**: Pneumolysin (cholesterol-binding cytolysin), capsule (90+ serotypes), teichoic acid/peptidoglycan activate complement and TLR2.

Boards buzzwords

  • rusty sputum
  • lancet diplococci
  • optochin-sensitive
  • bile-soluble
  • quellung-positive

Associated diseases

  • Community-acquired pneumonia (lobar)
  • Adult bacterial meningitis
  • Otitis media (#1 cause in children)
  • Sinusitis
  • Sepsis in asplenic / sickle cell patients

Treatment

Penicillin G or ceftriaxone first-line; vancomycin + ceftriaxone for suspected meningitis until susceptibility known; PCV13/PCV15/PCV20 and PPSV23 vaccines

Related organisms

Listeria monocytogenes
Listeria monocytogenes
Gram-positive rod (facultative intracellular)
Mycobacterium tuberculosis
Mycobacterium tuberculosis
Acid-fast bacillus (mycolic acid cell wall)
Nocardia spp.
Nocardia spp.
Gram-positive filamentous, partially acid-fast
Treponema pallidum
Treponema pallidum
Spirochete
Mycobacterium leprae
Mycobacterium leprae
Acid-fast bacillus (obligate intracellular)
Neisseria meningitidis
Neisseria meningitidis
Gram-negative diplococci