Mycobacterium avium complex (NORD)

Definitions & Key takeaways

Mycobacterium avium complex or just MAC, is a group of bacteria that includes Mycobacterium avium, Mycobacterium intracellulare, and Mycobacterium Chimaera. These are acid-fast bacilli, non-motile, facultative intracellular, aerobic bacteria found in soil and water, and are known to cause respiratory infections. MAC infections often attack people with weakened immune systems, such as those with AIDS, and lymphadenitis in children. Symptoms of MAC include fever, night sweats, weight loss, and a persistent cough.

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Mycobacterium avium complex, or MAC, includes three species; Mycobacterium avium, Mycobacterium Chimaera, and Mycobacterium intracellulare.
MAC is the most common nontuberculous mycobacteria, or NTM.The three different species of MAC are difficult to differentiate and cause the same spectrum of diseases, so they are often grouped together.
MAC is very different from Mycobacterium tuberculosis. MAC lung infection is a chronic disease, often present for several years prior to diagnosis.
It typically has a more indolent course, and can be difficult to eradicate, with frequent recurrences after completing long courses of treatment.
In addition, in immunocompromised individuals, MAC can become disseminated, affecting organs other than the lungs, and can cause lymphadenitis.MAC are non-motile, facultative intracellular bacteria which means they can survive both inside and outside the cell.
They’re also aerobic which means they need oxygen to survive. They have a high content of mycolic acid in their cell wall, which makes them waxy, hydrophobic and impermeable to routine stain such as Gram stain.
So, they need special staining methods to be visualized such as Ziehl-Neelsen staining which is able to penetrate the waxy mycobacterial cell wall.
So, the stain binds to the mycolic acid in the mycobacterial cell wall and after staining, an acidic decolorizing solution is applied which removes the red dye from the background cells, tissue fibres, and any organisms in the smear except Mycobacteria, which retain the dye; and this is why they are called “acid-fast,” and they appear bright red on a blue background.
Finally, MAC are slow growing bacteria, and it typically takes them 10 to 21 days to grow on a medium called Lowenstein Jensen.
MAC are ubiquitous in soil and water and they are thought to enter the body through inhalation. Once inside the body, they adhere to mucosal epithelial cells in the respiratory tract, and infect the macrophages that try to destroy them.
Macrophages try to break down the MAC organisms inside an intracellular organelle called a lysosome, which contain digestive enzymes.
However, MAC has unique, antigenic lipids called glycopeptidolipids, or GLPs, which are found on the surface of the cell.
These GLPs act as a protective barrier against lysosomal enzymes, allowing the bacteria to survive and multiply inside macrophages.
When these mycobacteria multiply they can cause inflammation, and this attracts more macrophages to the area. They try to quarantine the invaders by forming granulomas, and as these grow bigger, they form nodules.
Eventually, this leads to nodular bronchiectasis and the inflammation causes damage to the bronchi, which become scarred and dilated.
The damaged bronchi have diminished mucociliary activity as well as decreased local immune function, and thereby lose their ability to move foreign material out of the lungs, so mucus can collect in these dilated areas making it the perfect environment for bacterial growth.
Nodular bronchiectasis tends to develop slowly, but in some cases the more aggressive and severe fibrocavitary disease occurs.
Here the mycobacteria invades the lung tissue, causing damage, severe fibrosis, and formation of cavities, much like tuberculosis.
Since MAC are aerobic organisms, these cavities tend to be in the air-rich upper lobes of the lungs. In rare cases, and almost universally in individuals with compromised immune systems, the infected macrophages can move into the lymphatics and spread the bacteria to other parts of the body, especially to the spleen, liver and bone marrow.
And from these organs, they can spread to the bloodstream, causing disseminated disease. Now, MAC usually cause chronic lung infection in people who already have an underlying lung disease, such as chronic obstructive pulmonary disease, cystic fibrosis or bronchiectasis.
They can also affect people without a prior history of lung disease, especially in thin, middle aged, or elderly females.
People who are immunocompromised, like those with AIDS who have a CD4 cell count below 50 cells per microliter, are more likely to develop disseminated infection.
In young children, MAC can cause lymphadenitis, or swollen lymph nodes. Symptoms of pulmonary MAC infections include cough, fever, fatigue, weight loss, night sweats, shortness of breath and recurrent respiratory infections.
With disseminated MAC infections, there may be nonspecific symptoms such as fever, sweats, fatigue, and weight loss. On a physical exam, there can be hepatomegaly, or an enlarged liver, jaundice, or yellow pigmentation of the skin and mucous membranes, and lymphadenopathy, which means swollen lymph nodes.
Finally, with MAC lymphadenitis, there are usually swollen lymph nodes mainly on one side of the neck.Diagnosis is established by identifying MAC in the sputum using staining methods and culture.
If disseminated infection is suspected, culture specimens should also include blood and urine. Also, with disseminated infection, a complete blood count shows anemia and occasionally pancytopenia, due to bone marrow suppression, and liver function tests may show high transaminase and alkaline phosphatase levels.
Now, to assess pulmonary involvement, a CT scan should be done, which may show evidence of bronchiectasis, tree-in-bud like nodular infiltrates, ground glass opacities, or cavities.
Finally, in cases of MAC lymphadenitis, a lymph node biopsy should be done.For treatment of pulmonary and disseminated MAC infections, a combination of a macrolide, such as azithromycin or clarithromycin, plus ethambutol and a rifamycin, such as rifampin or rifabutin, is used.
In more severe disease, an intravenous aminoglycoside may also be used for a limited duration. For those with refractory disease, or whose organisms have developed macrolide resistance, the addition of inhaled amikacin or clofazimine can also be considered.
For those with pulmonary disease, treatment should be continued for 12 months after establishing negative sputum cultures.
In addition, attention should be placed on trying to limit environmental exposure to MAC from aerosolized water and soil, in order to try to prevent disease recurrence, which can occur in up to 20-50% of individuals.
Also, people with AIDS who have a CD4 count below 50 cells per microliter should receive prophylaxis with a macrolide. Treatment of MAC lymphadenitis is done by surgically excising the affected lymph nodes.##SummaryAlright, as a quick recap.
The Mycobacterium avium complex includes M. avium M.
intracellulare, and M. chimera.
They are acid-fast bacilli, non-motile, facultative intracellular, aerobic bacteria. They infect lung macrophages and survive inside of them with the help of glycopeptidolipids, or GLPs, causing three types of infections, pulmonary infection in people with or without underlying lung diseases, disseminated infection in people with AIDS, and lymphadenitis in children.
Diagnosis is established by identifying the mycobacteria in sputum, blood or urine samples, as well as through compatible imaging studies, such as a chest CT scan to assess for pulmonary involvement.
Treatment regimens typically include a combination of a macrolide such as azithromycin or clarithromycin, plus ethambutol and rifampin for pulmonary and disseminated infections.
In MAC lymphadenitis, the affected lymph nodes are surgically excised. and disseminated infections in mac lymphadenitis the affected