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Mycobacterium tuberculosis

"Captain among these Men of Death" (John Bunyon 1660)
White Death
White Plague

Where did TB come from?

Mycobacterium tuberculosisMycobacteria are Gram-resistant (waxy cell walls), non-motile, pleomorphic rods, related to the Actinomyces. Most Mycobacteria are found in habitats such as water or soil. However, a few are intracellular pathogens of animals and humans. Mycobacterium tuberculosis, along with M. bovis, M. africanum, and M. microti all cause the disease known as tuberculosis (TB) and are members of the tuberculosis species complex. Each member of the TB complex is pathogenic, but M. tuberculosis is pathogenic for humans while M. bovis is usually pathogenic for animals.

M. bovis was causing TB in the animal kingdom long before invading humans. However, after the domestication of cattle between 8000-4000 BC, there is archaeological evidence of human infection by M. bovis probably through milk consumption. M. tuberculosis is probably a human-specialized form of M. bovis developed among milk-drinking Indo-Europeans who then spread the disease during their migration into western Europe and Eurasia. By 1000 BC, M. tuberculosis and pulmonary TB had spread throughout the known world.

What are Mycobacteria?

Tuberculosis complex organisms are:

The video shows M. tuberculosis cells subjected to the Ziehl-Neelsen acid-fast staining procedure, a commonly used diagnostic method for Mycobacteria:

How is TB diagnosed?

Symptoms of tuberculosis include:

Diagnosis of tuberculosis is made by a positive tuberculin skin test, an immune reaction to a small quantity of tuberculosis antigens. It can be confirmed by X rays of the chest and microscopic examination of sputum. Detection of significant numbers of acid-fast bacilli (using the Ziehl-Neelsen stain) in sputum or tissue samples is considered a positive diagnosis, although disease may confirmed by laboratory culture of the bacterium (difficult, dangerous and slow - takes at least 4 weeks).

Ziehl-Neelsen acid-fast staining procedure:
  1. Heat fix cells on glass microscope slide.
  2. Flood the slide with carbol fuchsin stain.
  3. Heat the slide gently until it steams (5 min).
  4. Pour off the carbol fuchsin.
  5. Wash slide thoroughly with water.
  6. Decolourize with acid-alcohol (5 min).
  7. Wash slide thoroughly with water.
  8. Flood slide with methylene blue counterstain for 1 min.
  9. Wash with water.
  10. Blot excess water and dry in hand over bunsen flame.
Ziehl-Neelsen staining

How does TB kill people?

Tuberculosis kills 3,000,000 people in the world every year, more than AIDS, malaria, and other tropical disease combined. One third of the world's population is infected with tuberculosis. Tuberculosis is the leading infectious disease cause of death and represents more than a quarter of the world's preventable deaths.

Transmission of TB occurs primarily by the aerosol route but can also occur through the gastrointestinal tract. Coughing by people with active TB produces droplet nuclei containing infectious organisms which can remain suspended in the air for several hours. Infection occurs if inhalation of these droplets results in the organism reaching the alveoli of the lungs. Only 10% of immunocompetent people infected with M. tuberculosis develop active disease in their lifetime - the other 90% do not become ill and cannot transmit the organism. However, in some groups such as infants or the immunodeficient (e.g. those with AIDS or malnutrition), the proportion who develop clinical TB is much higher.

In the lung, the organism is taken up by alveolar macrophages and carried to lymph nodes, from where it may spread to multiple organs. Two to eight weeks after infection, cell mediated immunity (CMI) and hypersensitivity (DTH) develop leading to the characteristic reaction to the tuberculin test and, in immunocompetent individuals, containment of infection. Inflammatory immune responses eventually result in lung damage.

How is TB treated?

In many countries, vaccination against TB is routinely practised. The Bacillus Calmette-Guerin (BCG) vaccine is a live, attenuated strain of Mycobacterium bovis which was introduced in 1922. However, the true efficacy of BCG is unknown. Early clinical trials in Europe showed up to 80% protection, but more recent trials in India and Africa showed little value.

The first effective treatment for TB was developed in the 1940s - streptomycin.
Online tutorial: Domagk, Fleming, Waksman & the Third Man

TB is currently treated by means of combination therapy, using cocktails of 3-4 drugs with different properties:

If you or a member of your family are concerned about tuberculosis, consult your doctor.


So that's how TB was defeated?

A couple of decades ago, we thought so. Sadly, that was not the case:

CoverTimebomb: The Global Epidemic of Multi-Drug Resistant Tuberculosis
by Lee B. Reichman, Janice Hopkins Tanne
Tuberculosis has returned, reaching epidemic proportions worldwide. Over one-third of the world's population has latent tuberculosis; 15 million Americans are infected with this highly contagious, airborne respiratory disease. TB chooses hosts indiscriminately; the middle-class is not immune...


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