Pathogenesis if tuberculosis in microbiology apurba shastry

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Note: Apurba Satpathi's (Apurba Shastri/Satpathi) textbook on microbiology is not in our library, but the content below is drawn from the standard microbiology textbooks (Sherris & Ryan's Medical Microbiology and Jawetz Melnick & Adelberg's Medical Microbiology) that cover tuberculosis pathogenesis in essentially the same framework as Apurba Shastri's text.

Pathogenesis of Tuberculosis

The Organism

Mycobacterium tuberculosis (MTB) is a thin, straight, acid-fast rod (~0.4 x 3 µm). Its cell wall is rich in mycolic acids, peptidoglycolipids, and waxes - properties responsible for:

Acid-fastness (Ziehl-Neelsen stain)
Slow growth (~18-24 hour doubling time)
Resistance to desiccation, detergents, and common antibiotics
Ability to survive intracellularly in macrophages

Transmission

MTB is transmitted almost exclusively via inhalation of droplet nuclei from an active case. Key points:

A single cough generates ~3,000 infected droplet nuclei
Dried droplet nuclei remain airborne for prolonged periods
A room previously occupied by an active TB patient may remain infectious for 30 minutes or more
Outdoor infection is uncommon due to UV light susceptibility
Rare routes: ingestion (unpasteurized milk), abraded skin

Stage 1 - Primary Tuberculosis

Figure: Primary TB - inhalation, alveolar infection, lymph node spread

Primary tuberculosis - droplet inhalation, lymph node drainage, and hematogenous spread

Step-by-step:

1. Inhalation and alveolar deposition Inhaled droplet nuclei are deposited in the peripheral alveoli of the mid/lower lung zones (best-ventilated areas). Surface proteins of MTB may facilitate binding to laminin in the basement membrane of alveolar epithelial cells.

2. Phagocytosis by alveolar macrophages (AMs) AMs recognize and phagocytose the bacilli. This begins a "two-stage battle":

Stage A - Intracellular survival (MTB's evasion strategy):

MTB blocks phagosome-lysosome fusion
MTB blocks acidification of the phagosome (prevents maximal efficiency of lysosomal enzymes)
Result: bacilli multiply freely inside the non-activated macrophage phagosome
Intracellular location also makes chemotherapy more difficult

3. Dendritic cell trafficking and spread

Bacteria escaping AMs are trafficked by dendritic cells via a specialized secretion system (EXS-1) to regional hilar lymph nodes
From lymph nodes, low-level bacteremia disseminates organisms to: liver, spleen, kidneys, bone, brain, meninges, and lung apices
Clinically this is silent; reactivation at these distant sites may occur years later
Hilar lymph node enlargement + lung focus = Ghon complex (Ghon lesion after calcification)

Stage 2 - Immune Response: Th1 + DTH

Figure: Macrophage battle - IFN-γ activation vs. DTH destruction, granuloma and cavity formation

Th1 immune response, DTH, granuloma and cavitation in tuberculosis

Th1 Cellular Immunity (Weeks 3-9):

AMs and dendritic cells release IL-12 and IFN-γ, attracting T cells
CD4+ Th1 cells are recruited; IFN-γ is the primary macrophage activator
CD8+ cytotoxic T cells recognize and destroy MTB-infected macrophages
If Th1 activation is successful, mycobacterial multiplication is arrested and disease resolves

Delayed-Type Hypersensitivity (DTH):

MTB proteins simultaneously trigger a DTH (Type IV hypersensitivity) response
DTH causes fluid exudate, phagocyte recruitment, and release of digestive enzymes
DTH is the sole source of tissue injury in tuberculosis
Magnitude of DTH is proportional to the MTB population at the site
This is the basis of the tuberculin skin test (TST/Mantoux) - positive after 3-8 weeks of infection

Granuloma Formation

The combined Th1 + DTH response forms a tubercle (granuloma):

Zone	Composition
Central zone	Large multinucleated giant cells (Langhans type) containing bacilli
Middle zone	Pale epithelioid cells (activated macrophages), arranged radially
Peripheral zone	Fibroblasts, lymphocytes, monocytes

With time, caseous necrosis develops in the center - a cheesy, semisolid material from DTH-mediated destruction
Foamy macrophages in the granuloma interlayers contain lipid droplets providing nutrition
The lesion is called a tubercle
A caseous tubercle may calcify and heal, or liquefy, rupture into a bronchus, and form a cavity

Latent Tuberculosis

After primary infection, in ~90% of immunocompetent individuals:

Th1 immune response halts MTB multiplication
Lesions heal by fibrosis and calcification
MTB is NOT completely killed; instead, when faced with hypoxia and nutrient deprivation, MTB deploys nitrogen metabolism and hypoxia regulators to enter dormancy (latency)
Latent bacilli reside in healed granulomas and elsewhere in the body
They survive for months to years (or decades), waiting for reactivation
The waxy MTB cell wall aids environmental survival

Reactivation Tuberculosis

In ~10% of infected individuals (much higher in HIV co-infection: 200-300× increased risk):

Triggered by: immunosuppression (HIV, diabetes, malnutrition, corticosteroids, TNF-α inhibitors), aging, malignancy
Location: Almost always begins at the apex of the lung where O₂ tension (PO₂) is highest - favors aerobic MTB growth
Characterized by: chronic tissue lesions, tubercle formation, caseation, fibrosis
Regional lymph nodes minimally involved (unlike primary infection)
Liquefaction of caseous material → cavity formation → organisms coughed up → airborne transmission cycle

Spread of Organisms in the Host

Tubercle bacilli spread by 4 routes:

Direct extension within tissue
Lymphatic channels → regional lymph nodes → bloodstream (miliary distribution)
Bronchi → aspiration to other lung parts or swallowed
Gastrointestinal tract → stomach, intestines

Miliary TB occurs when a caseating lesion erodes a vein, releasing bacilli into the bloodstream, seeding all organs.

Immunity Summary

Feature	First Infection	Reactivation
Location	Mid/lower lung	Apex of lung
Lymph nodes	Massively enlarged, caseating	Slightly involved, no caseation
Ghon complex	Present	Absent
Tuberculin test	Converts positive	Positive
Mechanism	No prior immunity	Pre-existing CMI + DTH

Key Virulence Mechanisms of MTB

Mechanism	Effect
Mycolic acids / cord factor	Inhibit phagosome-lysosome fusion; toxic to mitochondria
Block phagosome acidification	Survive in non-activated macrophages
Waxy cell wall	Resistance to desiccation and killing
EXS-1 secretion system	Trafficking via dendritic cells to lymph nodes
Latency regulators	Survive dormancy for decades

Sources: Sherris & Ryan's Medical Microbiology, 8th Edition, pp. 1093-1099; Jawetz, Melnick & Adelberg's Medical Microbiology, 28th Edition, pp. 329-334

Note: Apurba Satpathi's Review of Microbiology and Immunology is not available in our textbook library, but the pathogenesis framework above matches the standard content covered in Indian medical microbiology curricula and aligns with topics from that text. The content is drawn from Sherris & Ryan's and Jawetz's, which are among the most detailed references for this topic.

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