Mycobacterium avium complex


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. These bacteria are common and are found in fresh and salt water, in household dust and in soil. MAC bacteria usually cause infection in those who are immunocompromised or those with severe lung disease.

Description

In the Runyon classification, both bacteria are nonchromogens. They can be differentiated from M. tuberculosis and each other by commercially available DNA probes.
They are characterized as Gram-positive, nonmotile, acid-fast, short to long rods.
Colony characteristics
  • Usually, colonies are smooth, rarely rough, and not pigmented colonies. Older colonies may become yellow.
Physiology
Differential characteristics
  • M. intracellulare and M. avium form the M. avium complex.
  • Remarkable ITS heterogeneity is seen within different M. intracellulare isolates.

    Species

  • Mycobacterium avium
  • * ''Mycobacterium avium subsp. paratuberculosis''

    Type strains

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

    Human health

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. MAC infection can cause COPD and lymphadenitis, and can cause disseminated disease, especially in people with immunodeficiency.
During the last decade Mycobacterium chimaera infections following cardiothoracic surgery, especially open-heart surgery, have been increasingly reported worldwide. 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.

History

In 2004, Tortoli et al. proposed the name M. chimaera for strains that a reverse hybridization–based line probe assay suggested belonged to MAIS, 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.