15. Classification, pathogenesis and diagnosis of tuberculosis

Essay 15- Classification, pathogenesis and diagnosis of tuberculosis

What is tuberculosis?
Tuberculosis (TB) is a bacterial infection that primarily affects the lungs but can also impact other parts of the body
Etiology
Mycobacterium tuberculosis
A closely related organism isolated from cases in West, Central, and East Africa is M. africanum.
The complex includes some zoonotic members, such as M. bovis (the bovine tubercle bacillus)- characteristically resistant to pyrazinamide, once an important cause of TB transmitted by unpasteurized milk
M. tuberculosis is a rod-shaped, non-spore-forming, thin aerobic bacterium measuring 0.5 μm by 3 μm. Mycobacteria, including M. tuberculosis, are often neutral on Gram’s staining.
However, once stained, the bacilli cannot be decolorized by acid alcohol; this characteristic justifies their classification as acid-fast bacilli
Acid fastness is due mainly to the organisms’ high content of mycolic acids, long-chain cross-linked fatty acids, and other cell-wall lipids
One molecule in the mycobacterial cell wall, lipoarabinomannan, is involved in the pathogen–host interaction and facilitates the survival of M. tuberculosis within macrophages
Risk factors
Exposure and recent infection with TB within 2 years
HIV +ve and immunosuppression
Drug abuse
Children younger than 5

Infection occurs when person inhales droplet nuclei containing tubercle bacilli that reach the alveoli of the lungs
The first contact of the Mycobacterium organism with a host leads to manifestations known as primary tuberculosis.
This primary TB is usually localized to the middle portion of the lungs, and this is known as the Ghon focus of primary TB.
In most infected individuals, the Ghon focus enters a state of latency. This state is known as latent tuberculosis
Tuberculosis is a classic example of a cell-mediated delayed type IV hypersensitivity reaction:
By stimulating the immune cells (the helper T-Lymphocyte, CD4+ cells), Mycobacterium tuberculosis induces the recruitment and activation of tissue macrophages. This process is enhanced and sustained by the production of cytokines, especially interferon-gamma.
Two main changes involving macrophages occur during this process, namely, the formation of multinucleated giant cells and the formation of epithelioid cells
The aggregation of giant cells surrounding the Mycobacterium particle and the surrounding lymphocytes and other cells is known as a granuloma
In addition to interferon-gamma (IFN-gamma), the following cytokines play important roles in the formation of a tuberculosis granuloma, Interleukin-4 (IL-4), Interleukin-6 (IL-6), and tumor necrotic factor-alpha (TNF-alpha).