An acid-fast, slow-growing scotochromogenic mycobacterium was isolated from a medium specific for hydrogen-utilizing chemolithotrophs. The organism grew well in pure culture in simple mineral salts media under an atmosphere of hydrogen, oxygen, and carbon dioxide. No growth occurred in the absence of the gas mixture unless organic substrates were added. Four tested strains of the tap-water scotochromogen, , were also able to grow autotrophically, whereas none of eight tested strains of grew using hydrogen. Twenty-one other mycobacterial strains were negative for autotrophic growth; a strain of grew very slowly. The isolated scotochromogen conformed to the properties of M. scrofulaceum except for its ability to grow at 19 C and its autotrophic ability. This organism exhibited two major colony types. During autotrophic cultivation, a flat, rough colony form was dominant; heterotrophic cultivation caused a population shift to a smooth, domed variety. The two colony forms exhibited qualitatively similar biochemical properties, and the unusual rough-to-smooth transition seemed to correlate with the quantitatively enhanced heterotrophic growth capacities of the smooth strain. Rough-to-smooth variation was reversible, and predominantly smooth inocula gave rise to predominantly rough populations under conditions of chemoautotrophy. The ability to grow autotrophically may be a useful characteristic for distinguishing the saprophytic scotochromogens from the more pathogenic strains.


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