Tuberculosis: Bacterial Infection

Tuberculosis

• Chronic infective granulomatous inflammation caused by tubercle bacilli.
• Etiology: low standard of nutrition and housing, low natural resistance.

Mycobacterium Tuberculosis

• There are two types: human and bovine.
• Aerobic, non-motile organism which can be killed by pasteurization.
• Weak gram positive, stained with Ziehl-Neelson stain.
• Structure: central core of protein (tuberculoprotein), polysaccharide fraction, and lipid capsule.

Methods of Infection

• Infection occurs through macrophages by endocytosis, replication within the phagosome, and blocking fusion of phagosome to lysosome.
• Bacteraemia and seeding of multiple sites occur, and most people are asymptomatic.

Pathogenesis

• About 3 weeks after infection, tubercle bacilli are drained to lymph nodes and presented to T cells, which differentiate to T-helper 1 cells that activate macrophages to kill bacteria.
• Type IV hypersensitivity reaction is developed, leading to the formation of granuloma and caseous necrosis.
• Caseation necrosis is due to hypersensitivity reaction and endarteritis obliterans.

TB Caseating Granuloma

• Morphology: cellular reaction (tubercle formation) or exudative reaction (serous membranes).
• Immunity: high immunity results in solid organs, lungs, and cellular tissue reaction; low immunity results in serous sacs, lungs, and exudative tissue reaction.

Fate of the Tubercle

• High immunity: small lesion, totally replaced by fibrosis, and bacteria may remain as a latent infection.
• Low immunity: large caseous lesion, surrounded by fibrosis, and bacteria may spread through blood, lymphatic, or direct spread.

Reactivation and Spread

• Reactivation leads to secondary pulmonary tuberculosis due to lowered immunity.
• Spread can occur through blood, lymphatic, or direct spread, leading to miliary tuberculosis or isolated organ tuberculosis.

Primary Pulmonary TB

• Mode of transmission: primary pulmonary complex (Ghon’s complex).
• Ghon’s focus: small area (1 cm), subpleural, lower part of upper lobe or upper part of lower lobe, formed of coalescent caseating tubercles.

Secondary Pulmonary T.B

• Etiology: reactivation or re-infection.
• Pathogenesis: preexisting hypersensitivity induces rapid and marked tissue response, leading to rapid localization of the disease with less lymph node involvement.
• Course: marked caseation and destruction with extrusion of the caseous material into bronchial tree lead to cavitations and dissemination of infection.

Diagnosis of Tuberculosis

• History of contact with tuberculous patient.
• Toxic manifestation.
• Productive cough, pleuritic pain, hemoptysis.
• Tuberculin test.
• Biopsy: characteristic but not pathognomonic granuloma.
• Identification of TB bacilli (sputum culture, PCR) is the most accurate test for diagnosis.