Utilization of cystine by dermatophytes on a gelatin medium.

All 16 strains of dermatophytes investigated utilized cystine (added to the gelatin medium) as a source of sulfur and also of carbon and nitrogen. Excess sulfur oxidized and excreted to the medium, primarily as inorganic sulfate. Six strains used up all cystine and excreted more than 90% stoichiometric amount of sulfur. Cystine utilization proceeded in parallel with the development of the culture and was terminated during the stationary phase or as late as in the autolytic phase. Other strains did not use up cystine completely and excreted 17-70% sulfur in the oxidized form. In addition to sulfate, sulfite was always produced during the initial growth phases and in poorly growing strains. Free sulfite was only rarely detected; it usually reacted with the residual cystine yielding S-sulfocysteine that was also used up later. Specific features of cystine metabolism (known from Microsporum gypseum) are generally valid in dermatophytes.