SUMMARY: Morphological and biochemical features of 42 strains of ‘atypical’ mycobacteria and one strain of were studied. Of the 42 atypical mycobacteria, 16 were originally classified in Runyon's group I, 4 in group II, 19 in group III, and 3 in group IV. The characteristics studied were bacillary morphology and staining properties on Kirschner and Löwenstein-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 chemotherapeutic drugs was tested. The niacin test proved to be the most useful method for distinguishing the atypical mycobacteria from 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.


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