Mycobacterial Infections, Atypical

Mycobacterial Infections, Atypical



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

Although there are more than a dozen species of atypical mycobacteria, the two most common are Mycobacterium kansasii and M. avium-intracellulare. These microbes are found in many places in the environment: tap water, fresh and ocean water, milk, bird droppings, soil, and house dust. The manner in which these bacteria are transmitted is not completely understood. There is no evidence that they are transmitted from person to person.
M. avium-intracellulare (MAC or MAI) is a rare cause of lung disease in otherwise healthy humans but a frequent cause of infection among those whose resistance has been lowered by another disorder (opportunistic infection). According to some experts, MAC infection is an almost inevitable complication of HIV. The infection is caused by one of two similar organisms, M. avium and M. intracellulare.
AIDS patients are almost always attacked by these mycobacteria. Once inside the body, the atypical mycobacterial organisms colonize and grow in the lungs like tuberculosis. Because AIDS patients have a poorly functioning immune system, the microbes multiply because they aren't stopped by the body's normal response to infection. Once they have colonized the lungs, the organisms enter the bloodstream and spread throughout the body, affecting almost every organ. These devastating infections can invade the lymph nodes, liver, spleen, bone marrow, gastrointestinal tract, skin, and brain.
Symptoms include shortness of breath, fever, night sweats, weight loss, appetite loss, fatigue, and progressively severe diarrhea, stomach pain, nausea and vomiting. If the infection spreads to the brain, the patient may experience weakness, headaches, vision problems, and loss of balance.
MAC and M. kansasii sometimes cause lung infections in middle-aged and elderly people with chronic lung conditions. MAC, M. kansasii, and M. scrofulaceum may cause inflammation of the lymph nodes in otherwise healthy young children. M. fortuitum and M. chelonae cause skin and wound infections and abscesses after trauma or surgical procedures. M. marinum causes a nodular inflammation, usually on the arms and legs. This infection is called "swimming pool granuloma" because it is associated with swimming pools, fish tanks, and other bodies of water. M. ulcerans infection causes chronic skin ulcerations, usually on an arm or leg. Atypical mycobacteria infections can also occur without causing any symptoms. In such cases, a tuberculin skin test may be positive.


The diagnosis is made from the patient's symptoms and organisms grown in culture from the site of infection. In cases of lung infection, a diagnostic workup will include a chest x ray and tests on discharges from the respiratory passages (sputum).


These nontypical mycobacteria are not easy to treat in any patient and the problem is complicated when the person has AIDS. Antibiotics are not particularly effective, although rifabutin (a cousin of the anti-tuberculosis drug rifampin) and clofazimine (an anti-leprosy drug) have helped some patients. It is also possible to contain the infection to some degree by combining different drugs, including ethionamide, cycloserine, ethambutol, and streptomycin.


Because drug therapy is not easily effective, the overwhelming infections caused by these mycobacteria in AIDS patients can be fatal.


People with HIV infection can prevent or delay the onset of MAC by taking disease-preventing drugs such as rifabutin.
AIDS patients and persons with tissue damage, such as skin wounds or pulmonary disease, can make a number of lifestyle changes to help prevent MAC infection. Since these mycobacteria are found in most city water systems, in hospital water supplies, and in bottled water, at-risk persons should boil drinking water. Persons at risk should also avoid raw foods, especially salads, root vegetables, and unpasteurized milk or cheese. Fruits and vegetables should be peeled and rinsed thoroughly. Conventional cooking (baking, boiling or steaming) destroys mycobacteria, which are killed at 176°F (80°C).

Key terms

Culture — A test in which a sample of body fluid, such as prostatic fluid, is placed on materials specially formulated to grow microorganisms. A culture is used to learn what type of bacterium is causing infection.
Human immunodeficiency virus (HIV) — The virus that causes AIDS.
Finally, at-risk patients should avoid contact with animals, especially birds and bird droppings. Pigeons in particular can transmit MAC.



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