Morphological and Biochemical Features of 'Atypical' Mycobacteria

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Abstract
Morphological and bio-chemical features of 42 strains of "atypical" mycobacteria and one strain of Mycobacterium tuberculosis were studied. Of the 42 atypical mycobacteria, 16 were originally classified in Runyon''s group I, 4 in group n, 19 in group III, and 3 in group IV. The characteristics studied were bacillary morphology and staining properties on Kirschner and Lowenstein-Jensen media; colonial morphology on 7H-10 agar medium; pigmentation in the dark and after exposure to light; rate of growth and temperature requirements, with different methods of inoculation; growth on blood and nutrient agar plates, and in gelatin stabs; catalase activity on drug-free and on isoniazid-containing media; nicotinic acid (niacin) production. The sensitivity of the majority of the strains to 6 chemo- therapeutic drugs was tested. The niacin test proved to be the most useful method for distinguishing the atypical mycobacteria from _M. tuberculosis. In identifying strains of group I, their ability to produce yellow pigment after exposure to light was of most value, and their colonial morphology and their periodic acid-Schiff staining were also helpful. Strains of group II were identified by their ability to form yellow pigment in the dark, by their periodic acid-Schiff staining and by their colonial morphology. Strains of group III were identified by their rate of growth and buff pigmentation. Exceptionally a yellow pigment was formed, and such strains were identified principally by their colonial morphology and periodic acid-Schiff staining. Among the 19 strains classified originally as group III, 3 were reclassified into group IV. Strains of group IV were identified by their ability to grow on blood and nutrient agar plates within 3 days and in gelatin stab cultures within 2 weeks.