⭐ THE 20% THAT GIVES 80% MARKS

(If you learn ONLY this box, you can answer almost all TB exam questions.)

1. Organism: Mycobacterium tuberculosis, acid-fast bacillus (AFB).
2. Cell wall: Mycolic acids + lipoarabinomannan → waxy wall → acid-fast, slow growth.
3. Transmission: Airborne droplets; infects macrophages.
4. Virulence:
• Cord factor → serpentine cords → inhibits phagolysosome fusion.
• Sulfolipids → prevent lysosome fusion.
5. Granuloma formation (caseating):
• Th1 → IFN-γ → macrophage activation → granulomas.
6. Primary TB: Ghon focus + hilar LN = Ghon complex.
7. Reactivation TB: apex of lung (high O₂); hemoptysis, night sweats, weight loss.
8. Diagnosis:
• AFB stain (Ziehl-Neelsen), Auramine-rhodamine (fluorescent).
• Culture: Lowenstein–Jensen (LJ), but slow (3–6 weeks).
• IGRA (Quantiferon) or Mantoux (PPD) for latent TB.
9. Treatment: RIPE = Rifampicin + Isoniazid + Pyrazinamide + Ethambutol.
10. Drug resistance: MDR-TB (INH + RIF resistance); XDR-TB (INH+RIF + fluoroquinolone + injectable resistance).

This is your 80%-marks block.

🔬 FULL MICROBIOLOGY — SIMPLE & COMPLETE

1️⃣ The Organism: Mycobacterium tuberculosis

• Slender, rod-shaped bacillus
• Acid-fast (due to mycolic acid)
• Strict aerobe (prefers apex of lung)
• Slow growing (divides every 18–24 hrs)
• Non-motile, non-spore-forming

Why acid-fast?

Because of mycolic acids and waxy wall that resists decolorisation by acid.

Stains

• Ziehl–Neelsen (red AFB)
• Auramine–rhodamine (fluorescent, more sensitive)

2️⃣ Cell Wall Structure (VERY EXAM IMPORTANT)

Unique TB cell wall contains:

• Mycolic acids (long fatty acids)
• Lipoarabinomannan (LAM) → similar to LPS; inhibits macrophages
• Cord factor (trehalose dimycolate) → serpentine cords
• Arabinogalactan
• Peptidoglycan

Function of TB cell wall

• Prevents phagolysosome fusion
• Resistant to drying, disinfectants
• Explains slow growth
• Stimulates granuloma formation

📌 Memory:

“Mycolic acid = Myco-LONG acid → long survival.”

3️⃣ Transmission & Initial Infection

• Spread by airborne droplets
• Bacteria reach alveoli → taken up by alveolar macrophages

TB loves high oxygen → apex in reactivation.

4️⃣ Pathogenesis — How TB Causes Disease

1. Macrophage ingestion of bacilli
2. TB survives inside macrophages
3. Th1 cells activated → IFN-γ release
4. IFN-γ activates macrophages → killing + granuloma formation
5. Granuloma = epithelioid cells + Langhans giant cells + caseous necrosis

5️⃣ Types of TB

A. Primary TB

• First infection
• Occurs in children
• Usually lower lobe (mid-zone)
• Forms Ghon focus + hilar nodes → Ghon complex
• Most recover → latent TB

B. Latent TB

• Bacteria dormant in granulomas
• Non-infectious
• Detected by PPD or IGRA

C. Reactivation TB

• Due to ↓ immunity (HIV, diabetes, steroids)
• Upper lobes (high O₂)
• Symptoms:
• chronic cough
• hemoptysis
• fever
• night sweats
• weight loss (“consumption”)

D. Miliary TB

• Dissemination via bloodstream
• Millet-seed pattern
• Affects liver, bone marrow, spleen, meninges

E. Extrapulmonary TB

• Meningitis
• Pott disease (spine)
• Renal TB
• Lymphadenitis (scrofula)
• Pericarditis
• GI TB

6️⃣ Diagnosis

A. Smear microscopy

• Ziehl–Neelsen (AFB stain)
• Auramine (fluorescent)

B. Culture

• Lowenstein–Jensen medium
• Takes 3–6 weeks

C. Molecular tests

• GeneXpert / CBNAAT
• Detects TB + rifampicin resistance
• Rapid (2 hours)

D. Immunological tests

● Mantoux (PPD skin test)

• Positive if ≥10 mm (or ≥5 mm in high-risk groups)

● IGRA (Quantiferon)

• Detects latent TB
• Unaffected by BCG vaccination

7️⃣ Treatment – RIPE Regimen

Intensive phase:

• Rifampicin
• Isoniazid
• Pyrazinamide
• Ethambutol

Duration: 2 months

Continuation phase:

• Rifampicin + Isoniazid

Duration: 4 months

8️⃣ Drug Mechanisms (VERY HIGH-YIELD)

• Rifampicin: inhibits RNA polymerase
• Isoniazid (INH): inhibits mycolic acid synthesis
• Pyrazinamide: works in acidic phagolysosomes
• Ethambutol: inhibits arabinogalactan synthesis

9️⃣ Drug Resistance

MDR-TB:

• Resistant to INH + Rifampicin

XDR-TB:

• Resistant to INH + RIF + fluoroquinolone + injectable (amikacin/kanamycin)

🔟 Prevention

• BCG vaccine (live attenuated M. bovis)
• Good for meningitis in children
• Not fully protective for pulmonary TB
• Masks, screening contacts, treating latent TB