FUNGUS · Atypical fungus (extracellular)
Pneumocystis jirovecii
Pneumocystis jirovecii — Toluidine blue / silver stain (cup-shaped cysts)
Stain: Toluidine blue O (or methenamine silver / GMS) on BAL or lung biopsyMorphology: Cup- or crescent-shaped cysts (~5–8 µm) staining dark blue/black, often in clusters resembling 'crushed ping-pong balls'Yield: HIGHDifficulty: MEDIUM
Image: Wikimedia Commons · CDC/PHIL (Public Domain)
Key facts
**Pathogenesis**: Inhaled or reactivated; cannot be cultured. Causes interstitial pneumonia in T-cell deficiency, especially AIDS with CD4 <200. **Diagnostic clue**: AIDS patient with dry cough, dyspnea, diffuse bilateral ground-glass opacities, hypoxemia worse with exertion, elevated LDH and β-D-glucan. Diagnosis via BAL with silver/GMS or DFA — cup-shaped cysts. **Virulence**: Major surface glycoprotein A binds alveolar epithelium.
Boards buzzwords
- cup-shaped cysts
- ground-glass infiltrates
- AIDS CD4 < 200
- elevated LDH
- diffuse hypoxemia disproportionate to exam
Associated diseases
- Pneumocystis pneumonia (PCP/PJP) — AIDS-defining (CD4 <200)
- PCP in solid organ / stem cell transplant, chronic steroids, biologics
Treatment
TMP-SMX (high-dose) × 21 days first-line; add prednisone if PaO2 <70 or A-a gradient ≥35; alternatives: pentamidine, atovaquone, clindamycin+primaquine, dapsone+TMP; primary prophylaxis when CD4 <200
