Microbiology @ Leicester: Microbiology Video Library: Mycobacterium tuberculosis
"Captain among these Men of Death" (John Bunyon 1660)
Where did TB come from?
Mycobacteria are Gram-positive (no outer cell membrane), 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:
Obligate aerobes growing most successfully in tissues with a high oxygen content, such as the lungs.
Slow-growing with a generation time of 12 to 18 hours (c.f. 20-30 minutes for Escherichia coli).
Hydrophobic with a high lipid content in the cell wall. Because the cells are hydrophobic and tend to clump together, they are impermeable to the usual stains, e.g. Gram's stain.
Known as "acid-fast bacilli" because of their lipid-rich cell walls, which are relatively impermeable to various basic dyes unless the dyes are combined with phenol. Once stained, the cells resist decolorization with acidified organic solvents and are therefore called "acid-fast". (Other bacteria which also contain mycolic acids, such as Nocardia, can also exhibit this feature.)
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).
Flood slide with methylene blue counterstain for 1 min.
Wash with water.
Blot excess water and dry in hand over bunsen flame.
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:
Antibacterial activity: e.g. isoniazid, rifampin, streptomycin
Inhibiting the development of resistance: e.g. isoniazid, rifampin, ethambutol
A couple of decades ago, we thought so. Sadly, that was not the case:
Timebomb: 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...
The White Death: A History of Tuberculosis by Thomas Dormandy.
An engrossing account of the amazingly complex social, artistic & natural history of this ubiquitous disease, as well as a telling chronicle of the medical profession at its worst & best. Make no mistake, TB is with us still, mutating through the new vectors of HIV, prisons, orphanages & multidrug resistance. An impressive & eminently readable history!