Physiology and Biochemistry

Due to the ecologic niche of Malassezia yeasts, host-specific adaptations are an important issue in their biology. The commensal status of Malassezia yeasts is not clearly distinguished from the pathogenic stage, as transition from the one to the other is probably a continuum and not an on/off condition. Part of this pathogenic potential is determined by the activities of the enzymatic systems of Malassezia yeasts. The efficiency with which Malassezia yeasts utilize nutrients on the skin surface and in the sebaceous gland determines the size of their population, and also the quality and quantity of the produced metabolic by-products. This organic material ranges from free fatty acids that apply their action through an irritating or toxic effect to highly bioactive indole derivatives that bind to specific cellular receptors and regulate the expression of downstream metabolic pathways. The recently reported genome and secretory proteome of M. globosa and, in part, of M. restricta provide a molecular basis to understand the adaptions of Malassezia yeasts to their environment and to identify factors of pathogenicity. As many experiments on Malassezia biochemistry and physiology had been performed before the current taxonomy of Malassezia species was developed, and the corresponding strains were not deposited in official fungal collections, the genus name Malassezia will be used to include strains that were categorized as Malassezia furfur (sensu lato), Pityrosporum ovale and Pityrosporum orbiculare (see Chap. 2).