Tuberculosis - 2nd Year MBBS Microbiology

(Based on Jawetz, Sherris & Ryan, and Medical Microbiology 9e - equivalent to Apurba Sastri coverage)

1. DEFINITION

• Jawetz Melnick & Adelbergs Medical Microbiology, p. 329: Organisms are thin, straight rods measuring about 0.4 × 3 μm; they do not stain with common aniline dyes and appear as "Gram-invisible" or "ghosts."

2. MORPHOLOGY AND BIOLOGY

• Shape: Thin, straight rod (bacillus), ~0.4 x 3 µm
• Staining: Acid-fast (Ziehl-Neelsen), due to mycolic acids in cell wall; also stained by fluorochrome stains (auramine-rhodamine) - yellow-orange fluorescence
• Growth: Obligate aerobe; slow growing (doubling time ~15-20 hrs); grows on Lowenstein-Jensen (LJ) medium - buff-colored, rough, dry, eugonic colonies ("breadcrumb-like") after 4-8 weeks
• Cord factor: Trehalose-6,6'-dimycolate - virulent strains form "serpentine cords" in culture; inhibits leukocyte migration, causes chronic granulomas, acts as immunologic adjuvant
• Cell wall components: Mycolic acids, lipoarabinomannan (LAM), wax-D, PPD (purified protein derivative) - responsible for acid-fastness, resistance to disinfectants, intracellular survival

3. PATHOGENESIS

Step-by-step mechanism:

• Droplet nuclei (<5 µm diameter) containing 1-3 bacilli are inhaled when an infected person coughs, sneezes, or speaks
• Droplets evaporate; remaining organisms deposit in peripheral alveoli (middle and lower lung zones initially)

• Bacilli are phagocytosed by alveolar macrophages
• Stage 1 (Early non-immune phase): MTB blocks phagosome-lysosome fusion and inhibits acidification of phagosome - bacilli survive and multiply within AMs
• Infected AMs migrate to hilar lymph nodes; from there, bacteremia disseminates organisms throughout the body (bone, kidney, meninges, upper lung apices - all seeded at this stage)

• Th1 cellular immunity is activated: CD4+ T cells secrete IFN-γ → activates macrophages → enhanced killing of bacilli
• Delayed-type hypersensitivity (DTH) reaction also develops simultaneously (tuberculin test becomes positive)

1. Exudative lesion: Acute inflammatory reaction - edema, PMNs, later monocytes. Resembles bacterial pneumonia. Can resolve, become necrotic, or progress to productive type. Tuberculin test turns positive during this phase.
2. Productive (Proliferative/Granulomatous) lesion: Chronic granuloma (TUBERCLE) with three zones:
   • Central zone: Multinucleated Langhans giant cells containing bacilli
   • Middle zone: Pale epithelioid cells (activated macrophages), radially arranged
   • Peripheral zone: Fibroblasts, lymphocytes, monocytes (later - fibrous tissue)
   • Central area undergoes caseation necrosis (cheese-like, acellular)
   • A caseous tubercle may break into a bronchus → empty contents → form a CAVITY

• Healing: Fibrosis → calcification (Ghon focus)
• Progression: Cavity formation, spread via bronchi, lymphatics, blood
• Latency: Bacilli remain dormant within granulomas for years/decades (latent TB)
• Reactivation: Triggered by immunosuppression (HIV, malnutrition, diabetes, steroids, old age)

Ghon focus = primary focus of infection in lung parenchyma Ghon complex = Ghon focus + enlarged hilar/mediastinal lymph nodes (seen on X-ray as calcified lesions)

4. CLINICAL MANIFESTATIONS

A. Primary Tuberculosis

• Usually asymptomatic or mild fever and malaise
• X-ray may show mid-zone infiltrates (Ghon focus) + enlarged hilar lymph nodes
• Tuberculin test converts from negative to positive
• In <5% of cases: disease progresses to active TB or disseminates

B. Reactivation (Post-Primary / Adult) Tuberculosis

• Fever (low-grade, usually afternoon), night sweats
• Fatigue, malaise, weakness
• Anorexia, progressive weight loss (historically called "consumption")

• Cough (universal symptom) - initially dry, later productive
• Hemoptysis (blood in sputum) - as disease progresses
• Chest pain, breathlessness
• Radiograph: Apical infiltrates → cavity formation (upper lobe preference due to high O2 tension)

C. Miliary Tuberculosis

• Hematogenous dissemination → tiny "millet-seed" sized foci in many organs
• High fever, hepatosplenomegaly, lymphadenopathy
• Chest X-ray: "snowflake" pattern (2-3 mm discrete lesions throughout both lungs)
• High mortality if untreated
• Especially in immunocompromised (HIV+), infants, elderly

5. EXTRAPULMONARY TUBERCULOSIS

6. SYMPTOMS SUMMARY - PULMONARY TB

• Low-grade fever (evening rise), night sweats
• Fatigue, malaise, anorexia
• Weight loss

• Persistent cough (>3 weeks) - cardinal symptom
• Mucopurulent sputum
• Hemoptysis (massive in erosion of blood vessels in cavity walls - Rasmussen's aneurysm)
• Dyspnea (late)
• Pleuritic chest pain

7. SUPPURATIVE COMPLICATIONS

1. Cavity formation - Liquefied caseous material drains into bronchus; air enters → cavity (seen on X-ray as thick-walled cavities in upper lobes)
2. Lung abscess - Secondary bacterial superinfection of TB cavity
3. Bronchiectasis - Permanent dilation and destruction of bronchi from repeated infection
4. Empyema thoracis - TB cavity ruptures into pleural space → pyopneumothorax
5. Pyopneumothorax - Air + pus in pleural cavity (from rupture of cavity)
6. Cold abscess - In TB lymphadenitis, spinal TB; called "cold" because there are NO signs of acute inflammation (no heat, redness, tenderness) - due to chronic nature
7. Psoas abscess - Cold abscess from lumbar vertebral TB tracking along psoas muscle sheath

8. NON-SUPPURATIVE COMPLICATIONS

1. Fibrosis and calcification - Healing by fibrous scarring; calcified Ghon focus
2. Pericarditis → Constrictive pericarditis (fibrous obliteration of pericardial space)
3. Pleuritis → Pleural effusion → Fibrothorax (if not drained) → Respiratory failure
4. Pott's disease → Gibbous deformity (kyphosis), paraplegia (non-suppurative spinal complication)
5. Addison's disease - Fibrotic destruction of adrenal cortex
6. Amyloidosis (AA amyloid) - Secondary amyloidosis from chronic TB inflammation; deposits in kidney, liver, spleen
7. Miliary TB - Hematogenous spread causing non-suppurative lesions in multiple organs
8. Cor pulmonale - Right heart failure from severe pulmonary fibrosis/hypertension
9. Renal failure - From amyloidosis or direct renal TB scarring
10. Aspergilloma ("fungus ball") - Aspergillus colonizes an old TB cavity; hemoptysis

9. LABORATORY DIAGNOSIS

A. Specimens

• Sputum (early morning, 3 consecutive days), gastric washings (children), urine, pleural fluid, CSF, joint fluid, biopsy, blood, BAL

B. Direct Smear Microscopy (most important rapid test)

• Primary stain: Carbol-fuchsin (hot staining)
• Decolorizer: Acid-alcohol (1% H₂SO₄ + ethanol)
• Counterstain: Methylene blue
• Result: MTB appears as bright pink/red rods against blue background
• Sensitivity: ~40-60% (requires 5,000-10,000 bacilli/mL)
• Grading: Scanty (1-9 AFB/100 fields), 1+ (10-99/100 fields), 2+ (1-10/field), 3+ (>10/field)

• AFB appear as yellow-orange fluorescent rods - more sensitive than ZN
• Used for screening; positive cases confirmed by ZN

C. Culture (Gold Standard)

1. Lowenstein-Jensen (LJ) medium (egg-based, coagulated): Growth in 4-8 weeks; eugonic, buff-colored, rough, dry colonies
2. Middlebrook 7H10/7H11 (agar-based): Better for susceptibility testing
3. BACTEC MGIT 960 (liquid broth with O₂ sensor): Results in 1-3 weeks - currently preferred
4. Lowenstein-Jensen slope should always include:
   • Non-selective: LJ without antibiotics
   • Selective: LJ with antibiotics (pyruvate medium for M. bovis)

• Niacin production: Positive (only MTB among common Mycobacteria produces niacin)
• Nitrate reduction: Positive
• Catalase: Positive (heat-sensitive, inactivated at 68°C)
• Pyrazinamidase: Positive

D. Tuberculin Skin Test (TST / Mantoux Test)

• 0.1 mL of PPD-S (5 TU) injected intradermally on volar forearm
• Read at 48-72 hours - measure induration (NOT erythema)
• Interpretation:
  • ≥5 mm = positive in HIV+, recent TB contact, fibrotic changes on CXR
  • ≥10 mm = positive in immigrants, healthcare workers, high-risk groups
  • ≥15 mm = positive in low-risk individuals
• A positive test indicates infection, NOT active disease and NOT immunity
• False positive: BCG vaccination, NTM infection
• False negative: Immunosuppression (HIV, malnutrition, miliary TB), sarcoidosis, improper technique, early disease

E. Interferon-Gamma Release Assays (IGRAs)

• Detect IFN-γ released by sensitized CD4+ T cells when stimulated by MTB-specific antigens: ESAT-6, CFP-10, TB7.7 (absent in BCG strains and most NTM)
• QuantiFERON-TB Gold In-Tube (QFT-GIT): ELISA on whole blood
• T-SPOT.TB: ELISPOT assay on PBMCs
• Advantages over TST: Not affected by BCG vaccination, more specific for MTB
• Not reliable in severe immunocompromise or very young children (<5 years)
• Used to diagnose latent TB infection (LTBI)

F. Nucleic Acid Amplification Tests (NAAT)

• GeneXpert MTB/RIF (Xpert MTB/RIF): Most important rapid molecular test
  • Detects MTB DNA AND rifampicin resistance (rpoB gene mutation) simultaneously in ~2 hours
  • WHO recommended as initial diagnostic test
  • Can be used on sputum, CSF, lymph node aspirates
• PCR-based methods: Detect even 10 bacilli/mL
• Line probe assays (LPA/Hain test): Detect both MTB and resistance to INH + RMP rapidly

G. Drug Susceptibility Testing (DST)

• Proportion method on LJ medium (gold standard but slow)
• BACTEC MGIT 960 (liquid, faster)
• Molecular methods (GeneXpert, LPA) for rapid detection of MDR-TB

H. Other Tests

• Histopathology: Biopsy of lymph node/tissue → caseating granuloma with Langhans giant cells, epithelioid cells
• Adenosine deaminase (ADA): Elevated in TB pleuritis (>40 U/L), TB meningitis
• Chest X-ray: Upper lobe infiltrates, cavitation, calcification, miliary pattern
• CBNAAT (Cartridge-Based NAA Test): GeneXpert platform widely used in India (RNTCP)

10. TREATMENT

First-Line Drugs (RIPE Regimen)

Standard Treatment Regimen (WHO / RNTCP / DOTS)

• Intensive phase: 2 months of HRZE (Isoniazid + Rifampicin + Pyrazinamide + Ethambutol)
• Continuation phase: 4 months of HR (Isoniazid + Rifampicin)
• Total duration: 6 months

• Intensive: 2 months HRZE
• Continuation: 10 months HR (total 12 months)
• Corticosteroids added for TB meningitis and pericarditis

• Isoniazid alone for 6-9 months
• OR Isoniazid + Rifapentine once weekly for 12 weeks (3HP regimen)
• OR Rifampicin daily for 4 months

Second-Line Drugs (for MDR-TB)

• Fluoroquinolones: Levofloxacin, moxifloxacin (DNA gyrase inhibitors)
• Injectable aminoglycosides: Amikacin, kanamycin, capreomycin
• Others: Ethionamide, cycloserine, linezolid, bedaquiline (new), delamanid (new)

MDR-TB (Multi-Drug Resistant)

• Resistant to at least INH + RMP
• Treatment: Minimum 18-24 months with 4-5 second-line drugs

XDR-TB (Extensively Drug Resistant)

• Resistant to INH + RMP + any fluoroquinolone + at least one injectable (amikacin/kanamycin/capreomycin)
• Very poor prognosis; newer drugs: bedaquiline, delamanid

Directly Observed Therapy Short-Course (DOTS)

• Healthcare worker directly observes each dose being swallowed
• WHO strategy for TB control; India's RNTCP (now NTP) is based on DOTS
• Prevents non-compliance and drug resistance

Pyridoxine (Vitamin B6)

• Given with INH to prevent peripheral neuropathy (INH competitively inhibits pyridoxal phosphate)

11. PREVENTION

1. BCG (Bacille Calmette-Guérin) vaccine: Live attenuated M. bovis; given intradermally at birth; protects infants and children from miliary TB and TB meningitis; less effective against adult pulmonary TB; contraindicated in HIV/immunocompromised
2. DOTS strategy: For early case detection and completion of treatment
3. Chemoprophylaxis: INH for contacts with TST conversion
4. Isolation: Negative pressure rooms for infectious TB patients
5. Notification: TB is a notifiable disease

KEY FACTS TO REMEMBER (Exam Points)