CLASSIFICATION

M. tuberculosis is a member of the closely related group of species, known as the M. tuberculosis complex. Apart from M. tuberculosis this complex includes M. bovis, M. africanum and M. microti. Apart from M. microti, which affects voles and is of limited human pathogenicity, the other three can all cause human tuberculosis. M. tuberculosis is a typical slow growing Mycobacterium, forming acid fast rods. They grow at 33-39°C, reduce nitrate, are aerobic, produce catalase and are non-pigmented.

BIOCHEMISTRY

M. tuberculosis produces rough non-pigmented colonies after about a week. It is an aerobe. It produces niacin in culture can reduce nitrate. It is inhibited by NAP (p-nitro-a-acetylyamino-b-hydroxy-propiophenone), pyrazinamide and cycloserine. The most commonly used biochemical test for identification now is on the basis of a specific DNA gene probe.

The mycobacterial genome has been estimated to be around 3-5.5 x 109 daltons. The presence of extrachromosomal DNA has been demonstrated. Unlike the fast growing mycobacteria and organisms like Escherichia coli, M. tuberculosis has only one rRNA operon. It was shown that genetic exchange could occur in this genus in the 1970's.

The mycobacterial antigens are complexes of proteins, lipids and polysaccharides. Generally inconsistent results have been obtained when humoral responses to M. tuberculosis infection have been studied. Investigations into the use of responses to more seletive antigens are in progress but have been plagued with poor sensitivity and specificity. More reliable results have been obtained with competitive immunoassays. Following infection with M. tuberculosis a delayed-type hypersensitivity to tuberculin develops. Tuberculin is a standard preparation of a purified protein derivative derived from a culture filtrate of M. tuberculosis.

VIRULENCE FACTORS

M. tuberculosis organisms can survivive for some time in droplet-nuclei of 1-5µm, produced durinmg coughing, sneezing, singing and similar activity by affected individuals. On reaching the lung alveoli on being inhaled by another individual, the M. tuberculosis cells are phagocytosed, but are not killed but able to multiply within the neutophils. Although an inflammatory response develops, this does not affect the continued growth of M. tuberculosis. The bacteria are transported through the lymphatic system and via the blood stream to other organs and also back to the lung. In fully immunocompetent individuals, small lesions remain in a controlled way as a calcified nodule. The only indication of infection is hypersensitivity to tuberculin. Also the intracellular multiplication of M. tuberculosis in macrophages is inhibited. The development of active tuberculosis involves a major contribution of the immune reaction as the disease evolves. Thus the formation of the necrotic lesions associated with active tuberculosis result from the hypersensitivity to tuberculin, which developed following the primary infection. There is a continuous fluctuation between delayed-type hypersensitivity and cell-mediated immunity.

NORMAL FLORA

M. tuberculosis is not part of the normal flora of healthy humans or animals. It can be present in an inactive form in calcified nodules for many years.

PATHOGENS

Intestinal Infections: M. tuberculosis is not a specific intestinal pathogen but all organs can be affected during active tuberculosis. Extraintestinal Infections: The primary focus of infection of M. tuberculosis are the lungs, where it cause pulmonary tuberculosis, but all organs can be affected. Animal Infections: M. tuberculosis is predominately a human pathogen, although farm animals and household pets can be infected.

LABORATORY ID

Specimens cultured for M. tuberculosis are predominantly sputum specimens from suspected cases of lung infections. Gastric washings, bronchial washings, lavage fluid and any other pecimen may be appropriate for testing. The organisms can be most successfully cultured for M. tuberculosis if inoculated into special liquid media at the bedside and then transported to the laboratory in the dark at 35-37°C with 10%CO2 in the atmosphere. Cultivation on special solid egg or agar-based media should be observed for up to six weeks, with weekly examination for acid-fast bacilli. Original specimens should also examined for the presence of acid-fast bacilli.

ENVIROMENTAL

Although predominantly an airborne pathogen transmitted by droplet infection, M. tuberculosis has also been isolated from waste waters.

INDUSTRIAL USES

The main industrial uses are involved in preparation of materials involved in the diagnosis of tuberculosis. A killed preparation of M. tuberculosis is part of Freund's complete adjuvant, used to enhance the immune response of animals in the raising of antibodies. However, due to the side effects produced by this agent, it is generally being replaced by artificial substances.

VACCINES

A vaccine prepared with the BCG strain, which is an attenuated strain of M. bovis is given as preventive measure to individuals, who demonstrate a lack of tuberculin hypersensitivity. This tuberculin hypersensitivity is also used as the measure of the effectiveness of the vaccination, rather than level of humoral immunity.

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