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Mycobacterium tuberculosis

Micro > Bacteriology > Mycobacteria
Pathogen:  Mycobacterium tuberculosis
Description: M. tuberculosis is an obligate aerobe. Its cell surface contains mycolic acid, which makes it impermeable to the gram stain therefore need to use an acid-fast stain.
    
Clinical diseases: Primary TB is transmitted by aerosol droplets and then deposited in the lower lobes of the lungs. The bacteria is then ingested by macrophages (and the myolic acid cell wall of M. Tuberculosis allows the bacteria to survive and divide). The bacteria forms caseous granulomas (which leave fibrotic, calcified scars called tubercles that contain a few dormant organisms).
It is important to note that this primary infection can spread to other sites by blood or lymphatics and form extrapulmonary tubercles. Secondary TB can occur when there is a reactivation of the bacteria and macrophages cause further damage to lung tissues. The bacteria can spread though blood and lymphatics to other sites - kidneys, lymph nodes, CNS, and GI; this is considered to be Miliary Tb.
Other sites of TB include: genitourinary, spine (Pott's disease), lymphadenitis, peritoneal, meningitis, and others

Diagnosis:
The Mantoux tuberculin skin test, more commonly known as the PPD test can be used to detect latent infection. A small amount of M. bovis (a closely related organism) protein is injected into arm and than the reaction determines the result. The PPD test however does not confirm an active infection, as those with latent Tb or those who have had the BCG vaccine may also have a positive PPD test.

Rapid PCR of sputum samples is available in some settings, and can also detect the gene responsible for Rifampin resistance.

To confirm an active infection a chest x-ray and sputum sample needs to be taken to evaluate for pulmonary disease. An acid-fast stain is used to detect the infection in sputum samples.  For other sites, demonstration of the organism in tissue is required for definitive diagnosis.

Treatment:  Treat with multiple drugs to avoid resistance: Typically 4 drugs are used for initial therapy. Rifampin, isoniazid (INH), pyrazinamide, ethambutol, streptomycin are considered "first line", with a standard regimen consisting of INH/RIF/PZA/EMB.
For pulmonary disease: active treatment: Rifampin, Isoniazid (with B6), Pyrazinamide, Ethambutol for 2 months, followed by Isoniazid and Rifampin for 4 months.
Extrapulmonary disease may require longer course of therapy.
Latent Treatment: Isoniazid (with B6) for 9 months.
Other regimens exist for latent TB that may shorten the duration of treatment required. INH may be given with a daily or three-times-weekly dosing regimen.
Epidemiology:  According to the World Health Organization about 13 million individuals have TB worldwide and 1.5 million deaths occur due to this disease.

Prevention and control:  M. tuberculosis is largely preventable and in the United States patients are sought out and care for before they develop active disease. Isolation and proper ventilation systems are a good way of preventing spread of the disease. In developing countries, children can be given the BCG vaccine, which reduces the overall incidence and the incidence of more serious manifestations (although it is not fully protective). However, this vaccine is not effective in adults.
Related concepts
1.       M. tuberculosis
2.       Acid-fast
3.       PPD
4.       BCG vaccine
Video about the pathogenesis of M. tuberculosis: https://www.youtube.com/watch?v=PMugeb0LdQI
 
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