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Mycobacterium tuberculosis complex refers to a group of Mycobacterium species that can cause tuberculosis.


Mycobacterium tuberculosis has been present in the human population since antiquity. There is evidence of tubercular decay found in the skulls and spines of Egyptian mummies 4000 years old. Hippocrates, the ancient Greek physician, noted that "phthisis" (consumption) was the most widespread and fatal disease of his time, known today as tuberculosis. From 1700 to 1900, it was responsible for the deaths of one billion human beings. In 1882, Robert Koch, also known as the father of Bacteriology, discovered a staining technique that enabled him to see Mycobacterium tuberculosis. The annual death rate at time of discovery equaled 7 million people. Koch’s discovery lead to subsequent inventions for curing infected and sick individual and preventative medicine. Mycobacterium tuberculosis (MTB) was the cause of the "White Plague" of the 17th and 18th centuries in Europe. Another important development was provided by the French bacteriologists Calmette and Guerin who created BCG (Bacillus of Calmette and Guerin) vaccine which lowered the virulence of the TB bacterium still used today. In 1943, Streptomycin was purified from a fungus called Streptomyces griseus which inhibited TB. It was administered for the first time in a critically ill patient to be cured in 1944.

Some general characteristics of Mycobacterium tuberculosis is that it is fairly large nonmotile rod-shaped (bacilli) bacterium distantly related to the Actinomycetes, its rods are 2-4 micrometers in length and 0.2-0.5 um in width and it is an obligate aerobe. It is also a facultative intracellular parasite and has a slow generation time of 15-20 hours. In order to culture Mycobacterium tuberculosis there are two types of media that are usually used. One is called, Middlebrook's medium, an agar based medium, and the other, Lowensein-Jensen medium, which is an egg based medium. MTB colonies are small and buff colored when grown on either medium. Both types of media contain inhibitors to keep contaminants from out-growing MTB. It takes 4-6 weeks to get visual colonies on either type of media. MTB is not classified as either Gram-positive  or Gram-negative because it does not have the chemical characteristics of either, although the bacteria do contain peptidoglycan (murein) in their cell wall. If a Gram stain is performed on MTB, it stains very weakly Gram-positive or not at all (cells referred to as "ghosts"). Mycobacterium species, along with members of a related genus, Nocardia, are classified as acid-fast bacteria due to their impermeability by certain dyes and stains. The acid-fast method used is called the Ziehl-Neelsen stain.

A person can become infected with tuberculosis bacteria when he or she inhales minute particles of infected sputum from the air. The bacteria get into the air when someone who has a tuberculosis lung infection coughs, sneezes, shouts, or spits. Tuberculosis is spread (transmitted) primarily from person to person by breathing infected air during close contact.There is a form of atypical tuberculosis, however, that is transmitted by drinking unpasteurized milk. Related bacteria, called Mycobacterium bovis, cause this form of TB. Previously, this type of bacteria was a major cause of TB in children, but it rarely causes TB now since most milk is pasteurized (undergoes a heating process that kills the bacteria. When the inhaled tuberculosis bacteria enter the lungs, they can multiply and cause a local lung infection (pneumonia). The local lymph nodes associated with the lungs may also become involved with the infection and usually become enlarged. In addition, TB can spread to other parts of the body. The body's immune (defense) system, however, can fight off the infection and stop the bacteria from spreading. It can form scar tissue around bacteria (fibrosis) putting it in an inactive state. Initial exposure to the bacteria is often referred to as primary TB and an individual typically has no symptoms and cannot spread TB to other people. Scar tissue and lymph nodes may eventually harden, like stone, due to the process of calcification of the scars (deposition of calcium from the bloodstream in the scar tissue). Scars often appear on x-rays and imaging studies like round marbles and are referred to as a granuloma. If these scars do not show any evidence of calcium on x-ray, they can be difficult to distinguish from cancer.

Effective regimens for the treatment of TB must contain multiple drugs to which the organisms are susceptible because administration of a single drug often leads to the development of a bacterial population resistant to that drug. Following streptomycin and BCG, p-aminosalicylic acid (1949), isoniazid (1952), pyrazinamide (1954), cycloserine (1955), ethambutol (1962) and rifampin (rifampicin; 1963) were introduced as anti-TB agents. The registered number of new cases of TB worldwide roughly correlates with economic conditions: the highest incidences are seen in those countries of Africa, Asia, and Latin America with the lowest gross national products. WHO estimates that eight million people get TB every year, of which 95% live in developing countries. An estimated 3 million people die from TB every year.


Mycobacterium tuberculosis is by far the most well known species. However, Mycobacterium africanum is also in this group.[1][2]

References

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