What are they?
Mycobacteria are a diverse group of rod-shaped bacteria that include more than 70 different species. Except for Mycobacterium tuberculosis (which causes the disease tuberculosis (TB)), and Mycobacterium leprae (which causes leprosy), most mycobacteria live in the soil and water in both rural and urban settings throughout the world. They can be found in aerosols, rivers and swamps, in treated city water, public swimming pools, hot spas, humidifiers, aquariums, garden soils, food, and many other places. Because they are protected by their waxy lipid-rich cell wall, mycobacteria are resistant to disinfectants and water treatment measures.
There is not a standard naming convention for this group of microorganisms. They may be referred to as nontuberculous mycobacteria (NTM), mycobacteria other than tuberculosis (MOTT), atypical mycobacteria, and/or environmental mycobacteria. The term MOTT is still often used but is an older designation. “Nontuberculous mycobacteria” has come into use more recently so, for the purpose of this article, this group will be referred to as NTM.
Almost half of the NTM species identified are associated with opportunistic infections in animals and humans, and several have caused sporadic outbreaks. NTM are acquired through environmental exposure to water, aerosols, soil, and dust – through inhalation, ingestion, and through breaks in the skin due to injuries, surgical procedures, or IV catheters. Unlike M. tuberculosis, they are not passed from person-to-person with the exception of M. leprae, which requires extended close contact (such as an infected family member). NTM can cause lung infections that mimic tuberculosis, lymph node infections, bone infections, abscesses, and skin and soft tissue infections, which may be localized or disseminated throughout the body. M. leprae can cause peripheral nerve damage and skin lesions. Most NTM reproduce slowly, which allows the infection to emerge weeks, months, or even years after the initial exposure.
Anyone can become infected, but people with suppressed immune systems (such as AIDS patients and transplant recipients), people with pre-existing lung damage (such as from smoking and previous tuberculosis) and those with lung diseases (such as emphysema or cystic fibrosis) are most likely to be affected. NTM infections can be challenging and time-consuming to treat since the organisms may be resistant to commonly prescribed antibiotics.
The table below identifies different NTM species and provides a brief description of each.
Examples – Many Overlap
M. avium- intracellular complex (MAC)MAC has become one of the most common infections in patients with AIDS; often in the lungs and disseminated throughout the body; found widelyM. kansasiiMost frequently causes lung infections; increased prevalence in the South – Texas and FloridaM. abscessusMay infect implants, such as pacemakersM. chelonaeMay cause post-surgical infections in artificial heart valves and prosthetic implantsM. fortuitumMay cause post-surgical infectionsM. xenopiFound in hot water systemsM. scrofulaceumMay cause cervical adenitis, especially in childrenM. marinumFound in fresh and salt water, aquariums and swimming pools; infects through breaks in the skin and may cause persistent soresM. ulceransEndemic in the tropics; causes Buruli ulcer, large lesions; the 3rd most common mycobacterial infection in healthy peopleM. lepraeInfects mucous membranes and cool areas such as skin; causes nerve damage and numbness and skin nodules; can lead to skin damage and infection
Signs and Symptoms
The symptoms associated with nontuberculous mycobacteria (NTM) infections depend on which part(s) of the body are involved. Pulmonary infections may cause TB-like symptoms, including:
- Chronic cough, sometimes with bloody sputum
- Weight loss
Skin-related NTM infections may cause persistent sores, boils, ulcers, and granulomas. Those affecting lymph nodes may cause inflammation in the node.
All of these symptoms may also be seen in a variety of other conditions. The diagnosis of most NTM infections depends on the positive identification of mycobacteria in body fluids or tissues.
The goals of testing are to detect nontuberculous mycobacteria (NTM) infections and to distinguish between mycobacteria species. It is not possible to distinguish between TB and NTM infections without testing. The sample(s) collected for analysis depend on the part(s) of the body that the doctor suspects are infected. For pulmonary infections, 3 to 5 sputum specimens are collected first thing in the morning on different days when they are most likely to contain the most mycobacteria. For other parts of the body, washings/aspirates, swabs of the infected area, fluids and/or tissue samples may be collected.
Because of their unique cell wall, all species of mycobacteria will appear as acid fast bacteria (AFB) when a smear of the patient's specimen is treated with a special stain and examined under the microscope. Positive AFB smears are presumed to be potential TB infections until more laboratory data is available.
AFB cultures are performed on samples that have been decontaminated of other bacteria, digested of mucus, and concentrated to increase the ability to detect them in the culture. Nutrients and incubation at appropriate temperature provide a supportive environment for the slow growing mycobacteria. The results of cultures are definitive: they can tell your doctor what organisms are present and what drugs are likely to kill them, but they take time - days to several weeks for positive samples. Cultures are held for six to eight weeks before being reported as negative. M. leprae cannot be detected with this method. It is diagnosed primarily through clinical signs as this species will not grow on culture media.
Once the mycobacteria species has been identified and treatment has begun, AFB smears and cultures are used to monitor the effectiveness of treatment.
AFB smears and cultures are the primary methods used to detect NTM infections. Other more rapid methods, such as the molecular detection of the organism's genetic material (DNA/RNA), may be performed on the primary specimen and also used as a means to identify the species of mycobacteria once the bacteria are grown in culture. Characterization of the mycolic acids (cells wall constituents) may also be used to identify the species of mycobacteria.
X-rays may be ordered to look for changes caused by a mycobacterial infection. NTM infections (and TB infections) can cause a number of characteristic findings on x-rays, including cavities (holes) and calcification in organs such as the lungs and kidneys.
The goals with treatment are to resolve the nontuberculous mycobacteria (NTM) infection in the affected patient and prevent further damage to tissues and organs. If there is evidence of widespread infection due to a common exposure, the medical community investigates the outbreak to find and eliminate the source of the infection(s). With M. leprae, treatment is also necessary to prevent the spread of the infection.
The treatment of NTM infections usually involves more than one antibiotic for a prolonged period of time. The length of treatment depends on the results of the AFB smears and cultures used to monitor the effectiveness of treatment. A few of the NTM infections, such as those caused by M. ulcerans, are best treated through surgical debridement of the skin ulcers (removal of damaged infected skin) to prevent further spread of the infection. In cases where the infection is localized, such as an infected lymph node, the infected tissue may be surgically removed.
Although symptoms often resolve after several weeks, it is crucial that affected patients continue to take their drugs for the time period recommended by their doctor. There are often a large number of mycobacteria to kill and it may take several months or longer to make sure that all of them have been eradicated. Patients should follow their doctor's recommendations for the best treatment for their specific condition.
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