Differences in biochemical tests performed on photochromogenic mycobacteria isolated from human sources

The photochromogenic mycobacteria do not all belong to one homogeneous species. With a simple heat-stable esterase (HSE) test it is possible to divide photochromogenic mycobacteria into two groups: HSE-positive--strongly pathogenic strains; and HSE-negative--weakly pathogenic strains. HSE-positive strains are mostly associated with pulmonary disease. HSE-negative strains are seldom associated with pulmonary disease, but are often associated with renal disease. It is rather difficult to draw a clear dividing line between pathogenic and non-pathogenic photochromogenic mycobacteria; and such a distinction probably does not exist in nature.