Mycobacteria represent a genus of aerobic bacteria. Some of their species cause serious diseases such as leprosy and tuberculosis.
What are mycobacteria?
From Mycobacterium or Mycobacterium is formed a genus of bacteria that includes about 100 species. Mycobacteria belong to the family Mycobacteriaceae, of which they are the only representatives. Mycobacteria also include species that have pathological effects on humans. For example, Mycobacterium leprae is responsible for the development of leprosy, while Mycobacterium tuberculosis causes tuberculosis. Similarly, animals can be affected by the mycobacteria with diseases such as bovine tuberculosis. Gram staining does not adequately identify mycobacteria. However, the structure of their cell wall is similar to that of Gram-positive bacteria. This means that the cell membrane is not equipped with an outer membrane and is composed of multilayer peptidoglycan. About 25 Mycobacterium species are of importance for medicine. In addition to Mycobacterium tuberculosis and Mycobacterium leprae, they include Mycobacterium bovis and several non-tuberculous mycobacteria. Mycobacterium tuberculosis was discovered in 1882 by the German physician Robert Koch (1843-1910), who identified it as the causative agent of bacterial tuberculosis.
Occurrence, distribution, and characteristics
Mycobacteria are commonly found in nature. For example, their habitats include soil and water bodies. They are also found in numerous animal species. Only a few species require a special host, so most of them live in the wild. Most of them are non-tuberculous mycobacteria that do not cause disease. The pathogenic mycobacteria, which belong to the Mycobacterium tuberculosis complex, are found as intracellular parasites within macrophages. The germs are protected from external influences by their special wall structure. This has mycolic acids as well as waxy substances. The lipids within the cell wall also result in the typical acid resistance of mycobacteria. Because the structure of the wall prevents a rapid exchange of oxygen with the environment, the growth and reproduction of the mycobacterium progresses only slowly, which is a typical characteristic of all mycobacteria. Among the general characteristics of mycobacteria is that they always require oxygen. They also need organic matter, which they use to obtain energy. Most bacteria of this species have the shape of a rod. Only in older cultures are branches sometimes formed. These usually decay into rods or spheres (cocci) in the further course. Most of the cell wall components act as antigenic components. Within host organisms, they trigger an immune reaction, which in turn causes a type IV allergy (late type allergy). In addition, a tuberculin reaction may occur. Other typical features of mycobacteria include mycolic acids, the extensive lipid content of the cell wall, and the phthiocerol outer envelope. Mycobacterium bovis and Mycobacterium tuberculosis also possess the so-called cord factor, which enables cord-like or pigtail-like growth in older cultures. The long-chain mycolic acids ensure the pronounced acid resistance of the mycobacteria. Due to their special cell wall structure, the germs achieve a strong resistance, so that they are capable of infection for several months even in the wild, provided that favorable conditions prevail. Apart from a few exceptions, Mycobacterium proves resistant to antibiotics. In addition, there is a pronounced resistance to acids and alkalis. Physiologically, mycobacteria are short to coccoid rods that are immobile. The growth rate of the germs is divided into two groups. Thus, there are slow-growing and fast-growing mycobacteria. The slow-growing specimens have a generation time of 6 to 24 hours in laboratory cultures, while that of fast-growing specimens is 1 to 4 hours. After one week, the fast-growing mycobacteria are macroscopically recognizable as a colony. For the slow-growing pathogens, this process takes up to 8 weeks. Most pathogens are found among the slow-growing mycobacteria.
Diseases and ailments
Some species of Mycobacterium are capable of causing serious illness in humans. People who suffer from a weakened immune system are considered to be particularly at risk. Animals can also be affected by some Mycobacterium species, posing agricultural problems. One of the most serious diseases caused by mycobacteria is tuberculosis, also known as consumption. Together with malaria and AIDS, it is one of the most common infectious diseases. It is estimated that around one third of people worldwide suffer from tuberculosis infections. The World Health Organization (WHO) speaks of approximately 9 million new cases per year. In addition, about 2 million patients die of tuberculosis every year. About 95 percent of all cases occur in developing countries. The resistance of numerous mycobacteria to antibiotics makes the fight against the infectious disease more difficult. In addition, co-infections with the HI virus are not uncommon. Important factors for successful treatment of tuberculosis are early initiation of therapy, efficient treatment with antituberculotics, and prevention of the development of resistance. Leprosy is also one of the most insidious diseases caused by mycobacteria. The pathogen Mycobacterium leprae multiplies only at a slow rate, so that it can take months or even years after infection for the disease to break out. The exact modes of transmission are still unknown. A droplet infection is suspected. However, only about 5 percent of all people are at risk of a leprosy outbreak, since all others have genetic immunity. However, infection and transmission of the germ is possible. Leprosy is noticeable by the formation of ulcers on the face, ears and limbs.
