Atypical Mycobacterial Infections

Atypical mycobacterial infections -caused by a species of mycobacterium other than Mtuberculosis complex  the causative bacteria of pulmonary TB and extrapulmonary TB including cutaneous TB; and Mycobacterium leprae, the cause of leprosy. Atypical mycobacteria include species such as M. avium, M. intracellularae, M. kansasii, M. xenopi, and M. fortuitum. In the acquired immunodeficiency syndrome (AIDS),  Mycobacterium avium-intracellulare, Mycobacterium kansasii, and Mycobacterium xenopi have caused widely disseminated infection.

Other names used to designate Nontuberculous Mycobacteria

Main differences of Atypical Mycobacteria with Mycobacterium tuberculosis

  1. MOTTs infections are not spread from person to person. With the exception of organisms causing skin lesions (M. kansasii, and possibly M. simiae), there is very little evidence of person-to-person spread of these organisms.
  2. Isolation of MOTTs is not equated with disease where as if M. tuberculosis is always considered pathogen when isolated.

Some characteristics of Atypical Mycobacteria: 

Runyon Classification of NTM

Runyon proposed the following scheme:

Group I-III are slow growers. 

Note: Chromogen (A substance that can be readily converted into a dye or other colored compound.)

Group I: Photochromogens

Mycobacterium marinum

Swimming pool granuloma caused by Mycobacterium marinum

Group II (Scotochromogens)

  1. M. scrofulaceum

Group III (Non chromogens)

Mycobacterium avium-intracellulare Complex (MAC)
Mycobacterium ulcerans

Buruli ulcer presents in two different forms

Group IV: Rapid growers

Colonies appear on solid media in 7 days or less.

  1. M. fortitum-chelonae complex

Laboratory diagnosis of Atypical Mycobacterial diseases

Specimen: Sputum, pus or exudate

Microscopy: Ziehl-Neelsen staining (AFB staining) of smear shows acid fast bacilli.

Culture: Atypical mycobacteria grow well in LJ, Middlebrook, and Dubo’s media.

Differentiation from M. tuberculosis requires