Utilization of L- and DL-cystine by the fungus Microsporum gypseum.

Growth of the fungus Microsporum gypseum and utilization of cystine during this growth was studied in a glucose-arginine medium containing either sodium sulphate, and L-cystine or DL-cystine. Replacement of sulphate with L-cystine brought about no significant changes in the growth of the microorganism. Utilization of L-cystine as a source of carbon and nitrogen was rapid and complete and excess sulphur was excreted into the medium in the form of sulphate. Similarly excreted were also minute amounts of sulphite which immediately reacted with the remaining cystine to form S. sulphocysteine. Growth of M. gypseum in a medium with DL-cystine was slow. Although this substance was not utilized as readily as L-cystine, its utilization was still complete and excess sulphur was similarly excreted in the form of sulphate and sulphite. The initial step in the utilization of the D-isomer is probably its extracellular deamination.