Study Notes: Tuberculosis (TB) – Microbiology and Clinical Aspects

Tuberculosis: Microbiology and Clinical Aspects

Overview of Tuberculosis

Tuberculosis (TB) is a significant infectious disease caused by bacteria of the Mycobacterium tuberculosis complex. It primarily affects the lungs but can involve other organs. Understanding TB involves microbiology, immunology, epidemiology, and pharmacology.

Microbiology of Tuberculosis

Causative Organisms

• Mycobacterium tuberculosis is the primary causative agent of TB in humans.

• Other species include Mycobacterium bovis, M. africanum, and M. microti.

Sites of Infection

• TB most commonly infects the lungs (pulmonary TB).

• Extrapulmonary TB can affect lymph nodes, pleura, bones, joints, genitourinary tract, and the central nervous system.

Inhalation and Infection

• Infection occurs when infectious droplet nuclei containing TB bacilli are inhaled into the alveoli of the lungs.

• The infectious dose is low; even a single droplet nucleus can initiate infection.

Military Tuberculosis

• Miliary TB is a form of disseminated TB resulting from massive lymphohematogenous dissemination of bacilli.

• It is characterized by numerous tiny lesions resembling millet seeds throughout the body.

Risk Factors and Epidemiology

Global Risk Factors

• The greatest risk factor for developing TB globally is HIV infection, due to immunosuppression.

• Other risk factors include malnutrition, diabetes, smoking, and close contact with active TB cases.

Reactivation TB

• Reactivation TB occurs when latent TB infection becomes active, often due to weakened immunity.

• Risk factors for reactivation include HIV, immunosuppressive therapy, and aging.

Pathogenesis and Immune Response

Mycolic Acids

• Mycolic acids are long-chain fatty acids found in the cell wall of mycobacteria.

• They contribute to the organism's acid-fastness and resistance to desiccation and disinfectants.

Immune Response

• The immune system responds to TB infection through both innate and adaptive mechanisms.

• Granuloma formation is a hallmark of TB, involving macrophages, T cells, and other immune cells.

• Granulomas help contain the infection but may also serve as a reservoir for latent bacteria.

Diagnosis and Laboratory Identification

Laboratory Diagnosis

• Diagnosis involves sputum smear microscopy, culture, nucleic acid amplification tests, and chest radiography.

• Acid-fast staining (Ziehl-Neelsen stain) is used to identify mycobacteria in clinical specimens.

Treatment and Pharmacology

First-Line Anti-TB Drugs

• Standard therapy includes isoniazid, rifampicin, pyrazinamide, and ethambutol.

• Treatment is prolonged (usually 6 months) due to the slow growth of mycobacteria and the risk of resistance.

Mechanisms of Action

• Isoniazid: Inhibits mycolic acid synthesis.

• Rifampicin: Inhibits bacterial RNA polymerase.

• Pyrazinamide: Disrupts membrane energetics and transport functions.

• Ethambutol: Inhibits cell wall arabinogalactan synthesis.

Multi-Drug Resistant TB (MDR-TB)

• MDR-TB is defined as TB resistant to at least isoniazid and rifampicin.

• Treatment requires second-line drugs, which are less effective, more toxic, and require longer duration.

Summary Table: First-Line Anti-TB Drugs

Drug	Mechanism of Action	Main Side Effects
Isoniazid	Inhibits mycolic acid synthesis	Hepatotoxicity, peripheral neuropathy
Rifampicin	Inhibits DNA-dependent RNA polymerase	Hepatotoxicity, orange discoloration of body fluids
Pyrazinamide	Disrupts mycobacterial cell membrane metabolism	Hepatotoxicity, hyperuricemia
Ethambutol	Inhibits arabinogalactan synthesis in cell wall	Optic neuritis (visual disturbances)

Key Equations and Concepts

• Acid-Fastness: Due to high lipid content (mycolic acids) in the cell wall, mycobacteria retain carbol fuchsin dye after acid-alcohol decolorization.

• Granuloma Formation: Immune-mediated structure that walls off TB bacilli, preventing spread but not necessarily eradicating the infection.

Additional info:

• TB remains a leading cause of death from infectious disease worldwide, especially in low- and middle-income countries.

• BCG vaccine provides variable protection against TB, especially severe forms in children.