Tuberculosis and Mycobacterial Diseases

TB Microbiology & Pathogenesis - Tiny Terror Tactics

• Agent: Mycobacterium tuberculosis (MTB); obligate aerobe, acid-fast bacillus (AFB).
• Cell Wall: High lipid (mycolic acid); resists decolorization (Ziehl-Neelsen stain).
• Transmission: Airborne droplet nuclei.
• Pathogenesis:
  • Inhaled MTB phagocytosed by alveolar macrophages.
  • Tactics: Inhibits phagolysosome fusion, replicates intracellularly.
  • CMI (CD4+ T-cells, IFN-γ) contains infection via granulomas (caseating necrosis).
  • Ghon focus (lung lesion) → Ranke complex (calcified lesion + hilar nodes).
  • Outcome: Latent TB (LTBI) or Active Disease.

⭐ Cord factor is a key virulence factor of M. tuberculosis, associated with serpentine cording on microscopy.

Pulmonary TB - Coughs & Cavities

• Symptoms: Chronic cough > 2 weeks (often with sputum), low-grade fever (evening rise), night sweats, weight loss, anorexia.
• Hemoptysis: can occur, especially with cavitary disease or Rasmussen's aneurysm.
• Radiological Signs:
  • Cavitation: classic feature, typically in apical/posterior upper lobes or superior segments of lower lobes.
  • Infiltrates, consolidation, pleural effusion.
  • Hilar lymphadenopathy.

⭐ Post-primary TB typically affects the apical and posterior segments of the upper lobes or superior segments of the lower lobes.

Extrapulmonary TB (EPTB) - Beyond Breath

• Most common: Lymph node (scrofula). Other sites: Pleura, CNS (meningitis), bone/joint (Pott's spine), genitourinary, gastrointestinal, pericardial, miliary (hematogenous spread).
• Diagnosis: Site-dependent. Often requires biopsy/fluid aspiration for AFB smear, culture (gold standard), NAAT (e.g., GeneXpert), and histopathology (caseating granulomas).
• Miliary TB: Diffuse, millet-seed sized lesions on imaging (e.g., CXR).

⭐ Tuberculous meningitis often presents with CSF findings of high protein, low glucose, and lymphocytic pleocytosis (typically <500 cells/µL).

TB Diagnosis & Management - Detect, Defeat, DOTS

• Diagnosis:

  • Sputum microscopy (AFB), CBNAAT (detects Rifampicin resistance), Culture (Gold Std: LJ/MGIT).
  • TST (Mantoux): Induration ≥10mm general; ≥5mm (HIV+, contacts). IGRA.
  • CXR: Apical infiltrates, cavities.

• Management (ATT): 📌 RIPE (Rifampicin, Isoniazid, Pyrazinamide, Ethambutol)

  • New Pulmonary TB: 2 months (HRZE) + 4 months (HR).
  • DOTS: Directly Observed Treatment, Short-course. Crucial for adherence & preventing resistance.

• Drug-Resistant TB (DR-TB):

  • MDR-TB: Resistant to Isoniazid & Rifampicin.
  • XDR-TB: MDR-TB + Fluoroquinolone & Second-Line Injectable resistance.

⭐ Rifampicin is a potent enzyme inducer, affecting the metabolism of many co-administered drugs like OCPs and antiretrovirals.

TB Prevention & NTM - Shield & Stragglers

• Prevention (Shield):
  • BCG Vaccine: Live attenuated M. bovis. Dose: 0.05ml (birth), 0.1ml (>1 month).

  ⭐ BCG vaccine primarily protects against severe childhood TB (meningitis, miliary TB), with variable efficacy against adult pulmonary TB.

  • Isoniazid Preventive Therapy (IPT): 6 months for eligible contacts, HIV+ individuals.
  • Airborne infection control; National TB Elimination Program (NTEP) India.
• NTM (Stragglers - Nontuberculous Mycobacteria):
  • Atypical mycobacteria; e.g., M. avium complex (MAC), M. kansasii.
  • Manifestations: Pulmonary, lymphadenitis, skin/soft tissue, disseminated (immunocompromised).
  • Treatment: Species-specific multi-drug regimens, often prolonged.

High‑Yield Points - ⚡ Biggest Takeaways

• Ghon complex (calcified lung focus + hilar node) is typical of Primary TB.
• Miliary TB presents as disseminated disease with "millet seed" appearance on CXR.
• MDR-TB (Isoniazid & Rifampicin resistance) and XDR-TB are major public health threats.
• DOTS strategy is essential for effective TB treatment and control.
• Isoniazid: peripheral neuropathy (give B6), hepatitis. Rifampicin: orange fluids, enzyme inducer.
• Mycobacterium avium complex (MAC) infection is common in AIDS patients with low CD4 counts.