BACTERIA · Acid-fast bacillus (mycolic acid cell wall)
Mycobacterium tuberculosis
Mycobacterium tuberculosis — Ziehl-Neelsen acid-fast stain
Stain: Ziehl-Neelsen acid-fast stainMorphology: Slender slightly curved/serpentine red-pink rods on blue background, often in cordsYield: HIGHDifficulty: MEDIUM
Image: Wikimedia Commons (CC BY-SA 4.0)
Key facts
**Pathogenesis**: Cord factor (trehalose dimycolate) inhibits macrophage activation and forms serpentine cords; sulfatides prevent phagolysosome fusion. Triggers Th1/IFN-γ-driven caseating granulomas. **Diagnostic clue**: Acid-fast on Ziehl-Neelsen / auramine-rhodamine; slow grower on Löwenstein-Jensen; PPD ≥5/10/15 mm (risk-stratified) or IGRA positive. **Virulence**: Mycolic acid waxy cell wall, cord factor, sulfatides, mycobactin (iron-scavenging).
Boards buzzwords
- acid-fast bacilli
- cord factor
- caseating granuloma
- Ghon complex
- Langhans giant cells
- PPD/IGRA positive
Associated diseases
- Primary TB (Ghon focus, Ghon complex)
- Reactivation/secondary TB (apical cavitary)
- Miliary/disseminated TB
- Pott disease (vertebral)
- TB meningitis
- Scrofula
Treatment
RIPE × 2 months then RI × 4 months (Rifampin, Isoniazid + B6, Pyrazinamide, Ethambutol); latent TB → INH + B6 × 9 months OR rifampin × 4 months OR 3HP weekly × 12
