Mycobacteria are a genus within the family of Mycobacteriaceae and the order of Actinomycetales. They include pathogens such as Mycobacterium tuberculosis, which can cause tuberculosis in humans.

What are mycobacteria?

Mycobacteria are Gram-positive aerobic bacteria. They are acid-resistant, rod-like and can not move independently. Aerobic means that the bacteria are dependent on oxygen. For energy they also need organic substances.

A striking feature of mycobacteria is its cell wall. Much of the cell wall is effective as an antigen component. As a result, mycobacteria cause a reaction of the immune system in the affected organisms. This leads to an allergy type IV. It is the allergy of late type. The most well-known late-allergic reaction to a mycobacterium is the tuberculin reaction, which is also used in the context of tuberculosis diagnostics in the form of the tuberculin test. Also typical is the high lipid content of the cell wall. Together with the mycolic acids it ensures a high acid resistance of the bacterium.

Due to the specific cell wall structure mycobacteria are very resistant. They may also remain infectious outside their host under favorable conditions for several months. With few exceptions, they are resistant to most antibiotics. Even most alkalis and acids can not harm mycobacteria. Acid resistance can be detected by Ziehl-Neelsen staining. Similar acid resistance is found only in bacterial species such as Nocardia or Corynebacterium.

Occurrence, distribution & characteristics

The group of mycobacteria includes around 100 species. Most of these bacteria are non-pathogenic. This means that they do not cause disease in humans. They occur freely in the environment and are assigned to the non-tuberculous mycobacteria. These bacteria feed on the decomposition of dead organic matter. They can be detected in dust, seawater, freshwater, soil and groundwater. An example of such mycobacteria are the mycobacteria of the Mycobacterium terrae complex and the Mycobacterium fortuitum. The drinking water contains the species Mycobacterium gordnoae or Mycobacterium chelonae. Most pathogenic mycobacteria belong to the obligate pathogenic Mycobacterium tuberculosis complex. They live within phagocytes (macrophages) and thus belong to the parasites.

The mycobacteria that cause tuberculosis are common worldwide. According to the World Health Organization (WHO), one third of the world's population is infected with the bacteria. Every second, a new case of tuberculosis occurs. However, only a small proportion of infections actually lead to an outbreak of the disease. In Germany, there are approximately 4, 000 tuberculosis cases each year. The number of unreported cases is quite high. Especially in Bremen, Berlin and Hamburg, the disease is widespread in Germany.

Tuberculosis is most commonly transmitted through the inhalation of infectious aerosols. This transmission path is also referred to as droplet infection. For an infection with the mycobacteria, the inhalation of a few very small microdroplets containing only one to three pathogens is sufficient. Transmission via the bloodstream is also possible. This transmission path is rather rare. In the past, infection with contaminated food was quite common. Since milk has been pasteurized, the intestinal tuberculosis in humans in Germany is virtually non-existent.

The infection with the tuberculosis pathogens can also happen sexually or by smear infections. Furthermore, the bacteria can be transmitted to the child in infections of the uterus. The same applies to urogenital tuberculosis. Again, a transfer during the birthing process is possible.

Diseases & complaints

The most important cause of tuberculosis is Mycobacterium tuberculosis. Most infections are already blocked by the immune system in the airways. Only one-tenth of all infected people suffer from tuberculosis. Whether the disease breaks out depends above all on the state of the immune system. The nutritional status, the amount of bacteria absorbed and the frequency of contact with the pathogens also play a role. In alveoli, mycobacteria meet laryngeal cells (alveolar macrophages). These can absorb the pathogens, but do not destroy them due to the special cell wall. Therefore, the immune system of the body forms a kind of protective wall around the infection. This protective barrier consists of macrophages, Langhans giant cells, epithelioid cells and lymphocytes. Within this wall, an inflammatory focus develops with central necrosis (tissue descent). The structure of defense cells and inflammation is also referred to as tuberculous granuloma. Within this protective barrier, the mycobacterium is isolated and can do no further damage. It is hampered by the defense cells in the proliferation, but can live within the wall decades. If the immune system is not as strong as it was at the beginning of the infection, the barrier can break and the bacteria are released. It comes to a reinfection and, in a very poor state of the immune system, a miliary tuberculosis.

Basically, the course of tuberculosis can be divided into stages. The first symptoms are called primary tuberculosis. This stage is associated with uncharacteristic symptoms such as loss of appetite, night sweats, fever and fatigue. Other symptoms include coughing without sputum and hoarseness. When the bacteria spread in the blood, a serious clinical picture develops with fever, weight loss, cough and shortness of breath. Also, a tuberculous meningitis, a meningitis, may arise. In case of extreme immune deficiency, sepsis may develop, which usually ends in death.

In ten percent of all infected the disease breaks out later than secondary tuberculosis. Typical symptoms of secondary tuberculosis include persistent coughing with mucous expectoration, fatigue, night sweats, chest pain and dyspnoea.

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