Mycobacterial Infections, Atypical (Encyclopedia of Medicine)
Atypical mycobacterial infections are infections caused by several types of mycobacteria similar to the germ that causes tuberculosis. These atypical mycobacterial infections are a frequent complication in patients with human immunodeficiency virus (HIV) infection or AIDS.
Mycobacteria are a group of rod-shaped bacteria that cause several diseases, among them leprosy and tuberculosis. For some time, scientists have known of bacteria that are similar to Mycobacterium tuberculosis, the cause of tuberculosis, but that grow and act differently. When tuberculosis was a much more widespread problem and microbiology was much less able to tell the difference between similar microbes, these atypical mycobacteria were ignored. Today, they have been classified more precisely as members of the same species and called atypical (or nontuberculosis) mycobacteria.
Although the medical profession has known about these atypical infections for a long time, they were not considered a serious problem until the early 1980s. It was then that many of these atypical infections were noticed among homosexuals and intravenous drug users in New York City. These bacteria rarely cause infection in humans other than those with HIV or AIDS.
Causes and symptoms...
(The entire section is 949 words.)
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Mycobacterial Infections, Atypical (World of Microbiology and Immunology)
Atypical mycobacteria are species of mycobacteria that are similar to the mycobacteria that are the cause of tuberculosis. Like other mycobacteria, they are rod-like in shape and they are stained for observation by light microscopy using a specialized staining method called acid-fast staining. The need for this staining method reflects the unusual cell wall chemistry of mycobacteria, relative to other bacteria. In contrast to other mycobacteria, atypical mycobacteria do not cause tuberculosis. Accordingly, the group of bacteria is also described as nonpneumoniae mycobacteria. This group of bacteria is also designated as MOTT (mycobacteria other than tuberculosis). Examples of atypical mycobacteria include Mycobacterium kansasii, Mycobacterium avium, Mycobacterium intracellulare, Mycobacterium marinum, and Mycobacterium ulcerans.
The atypical mycobacteria are widely present in the environment. They inhabit fresh and salt water, milk, soil, and the feces of birds. Other environmental niches, which so far have not been determined, are possible. The nature of their habitats suggests that transmission to people via soiled or dirty hands, and the ingestion of contaminated water or milk would be typical. Yet, little is still known about how people become contaminated. One species, known as Mycobacterium marinum, is found in swimming pool water, and can cause a skin infection in fingers or toes upon contact with the skin of a swimmer. Additionally, some evidence supports the transmission of atypical mycobacteria in aerosols (that is, as part of tiny droplets that can drift through the air and become inhaled).
Contamination with atypical mycobacteria may be a natural part of life. For the majority of people, whose immune systems are functioning efficiently, the microbe does not establish an infection. However, for those who immune system is not operating well, the presence of the atypical mycobacteria is a problem. Indeed, for those afflicted with acquired immunodeficiency syndrome (AIDS), infection with atypical mycobacteria (typically with Mycobacterium avium and Mycobacterium intracellulare) is almost universal.
Atypical mycobacteria tend to first establish a foothold in the lungs. From there the bacteria can spread, via the bloodstream, throughout the body. Infections in almost every organ of the body can ensue. Examples of sites of infection include the brain, lymph nodes, spleen, liver, bone marrow, and gastrointestinal tract. The overwhelming nature of the infections can be fatal, especially to people already weakened by AIDS.
The spectrum of infection sites produces a wide range of symptoms, which include a feeling of malaise, nausea, worsening diarrhea, and, if the brain is affected, headaches, blurred vision, and loss of balance.
Infrequently, those with healthy immune systems can acquire an atypical mycobacterial infection. The result can be a bone infection (osteomyelitis), a form of arthritis known as septic arthritis, and localized infections known as abscesses.
The diagnosis of infection caused by atypical mycobacteria is complicated by the fact that the growth of the microorganisms on conventional laboratory agar is very difficult. Specialized growth medium is required, which may not be available or in stock in every clinical laboratory. The delay in diagnosis can result in the explosive development of multi-organ infections that are extremely difficult to treat.
Treatment of atypical mycobacteria is complicated by the unusual cell wall possessed by the bacterium, relative to other bacteria. The cell wall is made predominantly of lipids. Partially as a result of their wall construction, atypical mycobacteria are not particularly susceptible to antibiotic therapy. As well, aggressive therapy is often not possible, given the physical state of the AIDS patient being treated. A prudent strategy for AIDS is the use of certain drugs as a means of preventing infection, and to try to avoid those factors that place the individual at risk for acquiring atypical mycobacterial infections. Some risk factors that have been identified include the avoidance of unwashed raw fruit and vegetables. As well, contact with pigeons should be limited, since these birds are known to harbor atypical mycobacteria in their intestinal tracts.
See also Bacteria and bacterial infections; Immunodeficiency diseases