Causes of tuberculosis
Reading File
Finding Sources
Searching PubMed
"tuberculosis"[MeSH Terms] AND "etiology"[MeSH Terms]
Finding Sources
Reading File
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
Reading File
Finding Sources
Finding Sources
Reading File
Reading File
I now have comprehensive information from multiple authoritative textbooks. Let me compile a thorough answer.
Causes of Tuberculosis
1. The Causative Organism
Tuberculosis (TB) is a granulomatous disease caused by the Mycobacterium tuberculosis complex (MTBC) — a group of closely related, slow-growing, acid-fast bacilli. The key species include:
| Species | Primary Host |
|---|---|
| M. tuberculosis | Humans (main cause) |
| M. africanum | Humans (Horn of Africa) |
| M. bovis | Cattle; can infect humans via unpasteurized milk |
| M. caprae | Goats/sheep |
| M. pinnipedii | Sea lions |
Humans are the only significant reservoir for M. tuberculosis. Human-to-animal-to-human transmission is extremely rare.
Bacterial Characteristics
- Rod-shaped, ~2–4 µm long; non-motile, non-spore-forming, facultative anaerobe
- Doubles every 18–24 hours (extremely slow growth)
- Possesses a uniquely "waxy" cell envelope composed of mycolic acids, arabinogalactan, and lipids (phthiocerat dimycocerosate, phenolic glycolipids, trehalose dimycolates, sulfolipids)
- This waxy coat confers acid-fastness (retains carbol fuchsin after acid-alcohol washing) and creates a formidable antibiotic barrier
- Encodes a type VII secretion system (ESX loci), especially ESX-1, which secretes virulence proteins ESAT-6 and CFP-10
— Murray & Nadel's Textbook of Respiratory Medicine; Goldman-Cecil Medicine
2. Transmission — How It Spreads
TB is transmitted primarily by airborne aerosol droplet nuclei (1–5 µm particles) produced when a person with active pulmonary or laryngeal TB coughs, sneezes, speaks, or sings. These droplet nuclei remain suspended in air and, when inhaled, reach the terminal alveoli.
Key points about transmission:
- Requires close, prolonged contact with an infectious source case
- Smear-positive (sputum AFB-positive) cases are most infectious
- Laryngeal TB is highly contagious
- Extrapulmonary TB (except laryngeal) is generally not infectious
- M. bovis can also be acquired via ingestion of unpasteurized dairy products
— Textbook of Family Medicine 9e; Goldman-Cecil Medicine
3. Pathogenesis — What Happens After Infection
Once inhaled, the bacilli are phagocytosed by alveolar macrophages. Rather than being destroyed, M. tuberculosis survives intracellularly by:
- Inhibiting phagosome maturation (via ESX-3/EsxH, preventing fusion with lysosomes)
- Evading reactive oxygen/nitrogen killing
- Using complex lipids (PDIM) to permeabilize the phagosome
- Triggering type I IFN secretion (via cGAS-STING pathway), which paradoxically supports bacterial survival
The host mounts a granulomatous response, forming caseating granulomas — hallmark lesions of TB — composed largely of macrophages and lymphocytes. Key immune mediators include:
| Mediator | Role |
|---|---|
| TNF-α | Essential for macrophage activation and granuloma integrity; TNF blockade raises TB risk 25-fold |
| IFN-γ | Activates macrophage microbicidal activity; deficiency leads to rapidly progressive disease |
| IL-12 | Drives CD4+ Th1 differentiation, promoting IFN-γ production |
| IL-1β | Critical for control of intracellular bacterial replication |
— Murray & Nadel's Textbook of Respiratory Medicine
4. Latent TB vs. Active TB
Most people (~90–95%) who are infected develop latent TB infection (LTBI) — the bacteria persist in a dormant state contained by immune responses. Only 5–10% progress to active, transmissible disease.
Risk Factors for Progression from Latent to Active TB
| Risk Factor | Relative Risk |
|---|---|
| Advanced, untreated HIV infection | 9.5–9.9× |
| Close contact with an infectious TB case | 6.1× |
| Untreated old TB on chest X-ray | 5.2× |
| Corticosteroids (≥15 mg prednisone/day) | 2.8× |
| Chronic renal failure | 2.4× |
| TNF-α inhibitor therapy | 2.0× |
| Poorly controlled diabetes mellitus | 1.7× |
| Weight ≥10% below ideal | 1.6× |
| Smoking | 1.5× |
Other factors increasing risk: malignancy, malnutrition, very young or elderly age, immunosuppressive therapy, silicosis, and living in high-endemic areas.
— Fishman's Pulmonary Diseases and Disorders; Murray & Nadel's Textbook of Respiratory Medicine
Summary
| Cause/Factor | Detail |
|---|---|
| Primary causative agent | Mycobacterium tuberculosis (obligate human pathogen) |
| Route of infection | Airborne droplet nuclei (respiratory) |
| Intracellular survival | Evades phagosomal killing in macrophages |
| Disease activation | Immune dysregulation (HIV, TNF blockade, steroids, diabetes, etc.) |
| Tissue injury | Caseating granulomas via TNF, IFN-γ, macrophage-mediated inflammation |
This is a shared conversation. Sign in to Orris to start your own chat.