Further characterization of pigment-producing Malassezia strains

Reference as well as field strains of Malassezia furfur (30), M. sympodialis (49), M. globosa (52), M. obtusa (one), M. restricta (one), M. slooffiae (seven), and M. pachydermatis (373) were investigated for their ability to produce pigment and fluorochromes when tryptophan (Trp) is offered as the main nitrogen source. Only the M. furfur strains produced pigment on a pigment-inducing medium (p-medium). Remarkably, the optical activity of Trp was not significant for pigment synthesis. Other nitrogen sources that are structurally similar to Trp (gramine, tryptamine, serotonin) did not induce pigment formation. All lipophilic non-furfur species failed to grow and to form pigment on this agar. However, growth of all lipid-dependent species was achieved on a modified Dixon agar in which peptone had been substituted by an equal amount of l-Trp. Here, too, the M. furfur colonies were characterized by rapidly developing dark brown halos. Furthermore, about 11% of the M. pachydermatis strains tested produced pigment formation on the p-medium, which was enhanced by addition of d-glucose. In contrast to M. furfur, pigment formation occurred after a markedly longer incubation time (4 weeks unlike 3-5 days) with a lower yield and limited color spectrum (thin layer chromatography, TLC). The UV filter pityriacitrine recently described for M. furfur was also demonstrated for M. pachydermatis by extraction, high-performance liquid chromatography (HPLC) analysis with co-elution and mass spectroscopy. The phenotypic feature of pigment formation in some strains of M. pachydermatis may confirm recent molecular-genetic findings suggesting a relationship between some strains of M. pachydermatis and M. furfur.