The primary cause of TB, ''Mycobacterium tuberculosis'', is a small aerobic non-motile bacillus. High lipid content of this pathogen accounts for many of its unique clinical characteristics. It divides every 16 to 20 hours, an extremely slow rate compared with other bacteria, which usually divide in less than an hour. (For example, one of the fastest-growing bacteria is a strain of ''E. coli'' that can divide roughly every 20 minutes.) Since MTB has a cell wall but lacks a phospholipid outer membrane, it is classified as a Gram-positive bacterium. However, if a Gram stain is performed, MTB either stains very weakly Gram-positive or does not retain dye due to the high lipid & mycolic acid content of its cell wall. MTB can withstand weak disinfectants and survive in a dry state for weeks. In nature, the bacterium can grow only within the cells of a host organism, but ''M. tuberculosis'' can be cultured ''in vitro''.
Using histological stains on expectorate samples from phlegm (also called sputum), scientists can identify MTB under a regular microscope. Since MTB retains certain stains after being treated with acidic solution, it is classified as an acid-fast bacillus (AFB). The most common acid-fast staining technique, the Ziehl-Neelsen stain, dyes AFBs a bright red that stands out clearly against a blue background. Other ways to visualize AFBs include an auramine-rhodamine stain and fluorescent microscopy.
The ''M. tuberculosis'' complex includes four other TB-causing mycobacteria: ''M. bovis'', ''M. africanum'', ''M. canetti'' and ''M. microti''. ''M. africanum'' is not widespread, but in parts of Africa it is a significant cause of tuberculosis. ''M. bovis'' was once a common cause of tuberculosis, but the introduction of pasteurized milk has largely eliminated this as a public health problem in developed countries. ''M. canetti'' is rare and seems to be limited to Africa, although a few cases have been seen in African emigrants. ''M. microti'' is mostly seen in immunodeficient people, although it is possible that the prevalence of this pathogen has been underestimated.
Other known pathogenic mycobacteria include ''Mycobacterium leprae'', ''Mycobacterium avium'' and ''M. kansasii''. The last two are part of the nontuberculous mycobacteria (NTM) group. Nontuberculous mycobacteria cause neither TB nor leprosy, but they ''do'' cause pulmonary diseases resembling TB.
Risk factors
Persons with silicosis have an approximately ''30-fold'' greater risk for developing TB. Persons with chronic renal failure who are on hemodialysis also have an increased risk: 10—25 times greater than the general population. Persons with diabetes mellitus have a risk for developing active TB that is two to four times greater than persons without diabetes mellitus, and this risk is likely greater in persons with insulin-dependent or poorly controlled diabetes. Other clinical conditions that have been associated with active TB include gastrectomy with attendant weight loss and malabsorption, jejunoileal bypass, renal and cardiac transplantation, carcinoma of the head or neck, and other neoplasms (e.g., lung cancer, lymphoma, and leukemia) that silicosis greatly increases the risk of tuberculosis, more research about the effect of various (indoor) air pollutants on the disease would be necessary. Some possible indoor source of silica includes paint, concrete and Portland cement.
Low body weight is associated with risk of tuberculosis as well. A body mass index (BMI) below 18.5 increases the risk by 2—3 times. On the other hand, an increase in body weight lowers the risk [http://archinte.ama-assn.org/cgi/content-nw/full/167/12/1297/IOI70054T5], Patients with diabetes mellitus are at increased risk of contracting tuberculosis, and they have a poorer response to treatment, possibly due to poorer drug absorption
Other conditions that increase risk include IV drug abuse; recent TB infection or a history of inadequately treated TB; chest X-ray suggestive of previous TB, showing fibrotic lesions and nodules; prolonged corticosteroid therapy and other immunosuppressive therapy;Immunocompromised patients (30-40% of AIDS patients in the world also have TB) hematologic and reticuloendothelial diseases, such as leukemia and Hodgkin's disease; end-stage kidney disease; intestinal bypass; chronic malabsorption syndromes; vitamin D deficiency; and low body weight. These findings were more recently confirmed by a series of studies in South Africa. Specific gene polymorphisms in ''IL12B'' have been linked to tuberculosis susceptibility.
Some drugs, including rheumatoid arthritis drugs that work by blocking tumor necrosis factor-alpha (an inflammation-causing cytokine), raise the risk of activating a latent infection due to the importance of this cytokine in the immune defense against TB.
Further Reading
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