The yield of Aspergillus niger mycelium from a synthetic medium can be increased by the addition of microgram quantities of cholesterol, ergosterol, cholestanol, 7-dehydrocholesterol, stigmasterol, sitosterol, pregnenolone, and the vitamins D. The stimulation is not due to degradation to the acetate level. It is obtained only in highly aerated cultures. The rate of growth of Torula utilis was not increased.

Both organisms were inhibited by desoxycorticosterone, testosterone, androstenedione, cortisone acetate, progesterone, and diethylstilbestrol. T. utilis was also inhibited by estradiol.

A small decrease in progesterone inhibition of T. utilis was obtained by adding ergosterol, cholesterol, or pregnenolone.

Of the compounds which have been adequately tested the order of stimulatory activity for A. niger is: ergosterol > cholesterol > stigmasterol > 7-dehydrocholesterol > cholestanol > pregnenolone. Progesterone was inhibitory at low concentrations but stimulatory at higher ones, while 17-hydroxyprogesterone was neither inhibitory nor stimulatory. Desoxycorticosterone and testosterone were inhibitory at all concentrations. Complete inhibition of the growth of the fungus was not obtained with any of the steroids.

It is concluded that A. niger has a metabolic requirement for a steroid with a hydroxy group on carbon 3, a double bond in the 5–6 position, and a side chain similar to that in ergosterol or cholesterol and that this material is growth-limiting in the early stages of the cultures described.

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