Decreased susceptibility of Malassezia furfur to UV light by synthesis of tryptophane derivatives.

Recently, tryptophane (Trp)-dependent synthesis of pigments and fluorochromes in Malassezia furfur was described. The possible significance of this metabolic pathway for the microorganism remains to be explored. Since the upper parts of the human epidermis are a natural habitat of M. furfur, increased exposure to UV light may be hazardous. Five reference strains and one wild type strain of M. furfur were grown on m-Dixon agar, in which the nitrogen source peptone had been substituted either by pigment-inducing tryptophane or arginine. The yeast cells thus obtained were harvested after 6 days, washed with physiological saline and inoculated on to the modified Dixon medium. Immediately after inoculation, the yeast cells were irradiated with UVA (100, 150 and 200 Jcm-2, single dose) or UVB (100, 500, 1000, 1500, 2000 mJcm-2, single dose; 500, 1500, 2500 mJcm-2 cumulative dose). Irrespective of the primary nitrogen source (Trp or Arg), unexposed controls showed nearly identical cell yield after 5 days. In the case of irradiation, however, growth reduction of cells cultured on Trp was lesser than that of cells fed with arginine. High significance (p < 0.0001) was found especially with the upper UVA and UVB doses. Differences were also found among the individual test strains, the wild strain being most sensitive. One strain (CBS 6094) failed to produce pigment on Trp medium, and there were no differences in the growth behavior of subcultures of this strain fed with either arginine or tryptophane under irradiation. In conclusion, synthesis of pigments and fluorochromes by M. furfur implies the generation of potent UV filters in the UVA and UVB spectrum.