Mycobacterium avium complex is a group of mycobacteria comprising Mycobacterium intracellulare and Mycobacterium avium that are commonly grouped because they infect humans together; this group, in turn, is part of the group of nontuberculous mycobacteria. These bacteria cause Mycobacterium avium-intracellulare infections or Mycobacterium avium complex infections in humans.^[2] These bacteria are common and are found in fresh and salt water, in household dust and in soil.^[3] MAC bacteria usually cause infection in those who are immunocompromised or those with severe lung disease.

In the Runyon classification, both bacteria are nonchromogens. They can be differentiated from M. tuberculosis and each other by commercially available DNA probes.^[4]: 245

They are characterized as Gram-positive, nonmotile, acid-fast, short to long rods.^{[citation needed]}

Colony characteristics

• Usually, colonies are smooth, rarely rough, and not pigmented colonies. Older colonies may become yellow.

Physiology

• Growth on Löwenstein-Jensen medium and Middlebrook 7H10 agar occurs at 37°C after seven or more days.
• The complex can be (but is not often) resistant to isoniazid, ethambutol, rifampin, and streptomycin.^[5]

Differential characteristics

• M. intracellulare and M. avium form the M. avium complex (MAC).
• Remarkable ITS heterogeneity is seen within different M. intracellulare isolates.

• Mycobacterium avium
  • Mycobacterium avium subsp. paratuberculosis

• M. intracellulare type strains include ATCC 13950, CCUG 28005, CIP 104243, DSM 43223, JCM 6384, and NCTC 13025.^[6]
• M. avium type strains include ATCC 25291, DSM 44156, and TMC 724.^[7]

MAC bacteria enter most people's body when inhaled into the lungs or swallowed, but only cause infection in those who are immunocompromised or who have severe lung disease such as those with cystic fibrosis or chronic obstructive lung disease (COPD).^[3] MAC infection can cause COPD and lymphadenitis, and can cause disseminated disease, especially in people with immunodeficiency.^[4]: 245 During the last decade Mycobacterium chimaera (see below) infections following cardiothoracic surgery, especially open-heart surgery, have been increasingly reported worldwide.^[8] Infections usually involve the respiratory system. Mycobacterium chimaera is acquired during cardiopulmonary bypass via bioaerosols emitted from contaminated heater-cooler units water systems. Due to nonspecific symptoms and long latency, postoperative Mycobacterium chimaera infections may not be promptly diagnosed and treated, and may become life-threatening.

In 2004, Tortoli et al. proposed the name M. chimaera for strains that a reverse hybridization–based line probe assay suggested belonged to MAIS (M. avium–M. intracellulare–M. scrofulaceum group), but were different from M. avium, M. intracellulare, or M. scrofulaceum. The new species name comes from the Chimera, a mythological being made up of parts of three different animals.^[9][10]

• Mycobacterium avium Complex at the U.S. National Library of Medicine Medical Subject Headings (MeSH)