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