Mycobacteria

Mycobacterium tuberculosis (Koch bacillus):
Important properties :
• It is acid-fast bacillus(AFB) elongated.
• It is non-capsulated.
• It is non-motile.
• Strict aerobe.
• Facultative intracellular.
Source and Transmission:
• Humans are only natural host of M.tuberculosis (strict human pathogen).
• M.tuberculosis transmitted direct from infected person to person by respiratory aerosol(droplet nuclei) during coughing and sneezing, or by saliva exchange during mouth kissing. The organism also can be transmitted by fomite of patients.
Pathogenesis and immunity:
• The pathogenicity is dependent on presence of organism and immune response of host. TB has virulence factors: toxic substances in its cell wall as complex lipid, polysaccharides, lipoarabinomannon, also produces urease and cord factor.
• When the mycobacteria inhale and establish themselves in tissues, they engulfed by alveolar macrophage. The organisms can be survive and proliferate inside macrophages (intracellular survive) especially in monocytes, reticuloendothelial cells, and giant cells. This phagocytic cell may act as vehicle transporting the infection to other organs.
• Cellular immunity: Two major mechanisms of host cellular response against T.B infection :.
1. The organism recognized by alveolar macrophage. The stimulation of T-cells to release cytokines (IFN-?). These cytokines lead to activation of alveolar macrophage to release TNF-? and kill T.B by phagocytosis, or TB killed by activated macrophages that aggregated to form granuloma, which has anoxic and acidic center, leading to tissue necrosis. This environment does not favour for growth of the organism, most T.B die, but some are survive. N.B: TNF-? cause fat loss that result in weight loss of human body in Tb cases. TNF creates malnutrition condition, emaciation and cachexia(body weight loss).
2. DTH functions to kill intracellular T.B by lysing infected macrophage in several ways: cytotoxic T-cells have been identified that infected macrophage and lysing. Natural killer cell or lymphokine-activated T-cell can be stimulated by IL-2 to destroy T.B-containing macrophage.
• Humoral immunity: Abs are formed but not play important role in resistance of host.