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Tuberculosis: How Mycobacterium tuberculosis Replicates and Spreads

TB Pathogenesis: From Inhalation to Disease

Mycobacterium tuberculosis (Mtb) is an obligate aerobe, acid-fast bacillus with a unique lipid-rich cell wall. Its slow growth and immune evasion drive TB's chronic nature.

Reproduction (Within Host)

  • Binary fission: Primary mode; generation time 15–20 hours (vs. 20 min for E. coli).
  • No budding: Mtb divides by septum formation; "budding" is not a recognized mechanism. Some mycobacteria form branching filaments, but not true budding.
  • Intracellular niche: Replicates within macrophage phagosomes (arrests phagolysosomal fusion via ESX-1 secretion system).
  • Metabolic adaptation: Utilizes host lipids (cholesterol, fatty acids) via β-oxidation; shifts to dormancy (non-replicating persistence) under stress.

Transmission

  1. Source: Pulmonary/laryngeal TB case coughing, singing, speaking.
  2. Droplet nuclei: 1–5 µm particles remain airborne for hours.
  3. Inhalation: Reaches alveolar spaces.
  4. Early infection: Alveolar macrophages phagocytose; most bacilli killed; some survive → replicate.
  5. Primary complex: Ghon focus (subpleural) + hilar lymphadenopathy.
  6. Latent TB Infection (LTBI): Immune containment (Th1, IFN-γ, granuloma); asymptomatic, non-infectious, positive IGRA/TST.
  7. Reactivation (5–10% lifetime risk): Immunosuppression (HIV, TNF-α inhibitors, diabetes, aging) → granuloma breakdown → active TB.

Active TB Disease

  • Pulmonary (85%): cough ≥2 weeks, hemoptysis, fever, night sweats, weight loss.
  • Extrapulmonary: lymph nodes, pleura, spine (Pott's), meninges, GU, disseminated (miliary).

Diagnosis and Treatment

  • Sputum AFB smear, Xpert MTB/RIF Ultra (NAAT), culture (gold standard, 2–6 wks), DST.
  • Drug-susceptible TB: 2HRZE / 4HR (6 months).
  • MDR/RR-TB: All-oral regimens (bedaquiline, linezolid, etc.) 9–12 months.
  • LTBI treatment: 3HP (weekly INH+rifapentine ×12), 4R, 3HR, 1HP.

Key Corrections to Common Misconceptions

  • Mtb does not reproduce by budding.
  • TB is directly transmitted person-to-person via aerosols (not "not directly").
  • Latent infection is not disease; only active TB is contagious.

Understanding Mtb's slow growth, intracellular lifestyle, and immune interplay is essential for diagnosis, treatment, and elimination efforts.

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