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Germs that infect humans


Mycobacterium tuberculosis

Mycobacterium tuberculosis

© Dennis Kunkel Microscopy, Inc.

Microorganism: the bacterium Mycobacterium tuberculosis

Disease: tuberculosis

Occurrence of the disease

History: Tuberculosis is a very old disease. In fact, lesions caused by tuberculosis have been found on human skeletons dating from 3000 BC. Until the 1800s, people believed that tuberculosis was a divine punishment. In Europe, between 1780 and 1831, large migrations of rural populations into the cities caused a major increase in the incidence of tuberculosis. At that time, one death in seven was caused by tuberculosis. Deplorable sanitation, malnutrition, and child labor were also factors favoring the spread of the disease. In 1878, the bacteriologist Robert Koch discovered the bacterium responsible for tuberculosis; he received the Nobel Prize for this work. In 1924, Calmette and Guérin produced BCG, the first vaccine against this terrible disease.

Current situation: Large-scale vaccination against tuberculosis has not been done in Quebec since 1975, since the incidence of the disease is low. However, although the incidence of the disease was very low during the 1970s, there were 1,798 cases reported in Canada during 1998. Worldwide, the number of cases of tuberculosis reported has grown throughout the 1980s to reach eight million persons per year; three million of these will die.

Forecast: we must watch this disease very carefully; if not controlled, it could once again cause major devastation.

Mechanism of action of the microorganism: bacteria enter a person’s body when she or he breathes contaminated air. Once in the lungs, the bacteria multiply, which causes an immune reaction. If the cells of the immune system are incapable of eliminating the bacteria, there will be formation of tubercles. These tubercles in the lungs are very dangerous, as they can liquefy and create holes, or harden and form masses.

The bacterium responsible for tuberculosis can also cause foci of infection at other places in the body, causing “miliary tuberculosis.” Tuberculosis can also evolve into meningitis or into scrofula (infection of the lymph nodes).

Symptoms of the disease: cough, weight loss, spitting up of blood

Incubation period: two to six weeks

Contagious period: the contagious period lasts as long as bacteria are present in the lungs.

Hosts: humans and, occasionally, other mammals

Transmission: tuberculosis spreads in the same way as influenza, that is, through droplets containing microorganisms that are present in the air when an infected person coughs.

Discoverer of the microorganism: Robert Koch in 1882.

Treatment: certain antimicrobial drugs such as isoniazid and pyrazinamide. It is also possible to use the antibiotic streptomycin if the bacteria are resistant to isoniazid.

Geographical distribution of the microorganism: worldwide; however, industrialized countries are much less affected by this disease than are developing countries.

Prevention: isolation of patients so that they do not infect others, as well as regular hand washing. There is also a vaccine that can be administered as prevention.

Vaccine: BCG (Bacille Calmette-Guérin) is a live vaccine with limited effectiveness. It is made from a bacterium similar to that which causes tuberculosis, but which normally infects cattle.