Introduction The fungal genus Malassezia comprises lipid-dependent and lipophilic yeast species that are part of the normal skin microbiota [1]. The 14 species are classified in class Malasseziomycetes in the Ustilaginomycotina of Basidiomycota [2]. Malassezia species can be involved in skin disorders, such as pityriasis versicolor, seborrheic dermatitis, atopic eczema, and folliculitis, and occur at higher population densities on scalps with dandruff than on scalps without dandruff [3], [4]. Occasionally, invasive infections by Malassezia pachydermatis and lipid-dependent Malassezia spp. occur in neonates, most often in those who are receiving intravenous lipid supplementation, or in immunocompromised patients receiving parenteral nutrition via a catheter. Malassezia spp. have not yet been cultured from the environment, but metagenomics identified Malassezia phylotypes from terrestrial and marine habitats [5]. For instance, Malassezia ribosomal DNA (rDNA) has been reported from soil nematodes [6], sponges [7], and rocks [8]. Undeniably, much remains to be discovered about the spectrum of habitats exploited by Malassezia that would advance our knowledge on the ecological relationships between the Malassezia skin biotic community, their hosts, and the environment. The aim of this article is to review and discuss the literature available on the pathogenesis, detection, typing, and treatment of Malassezia infections in humans and animals. Pathophysiology on Human Skin The pathophysiology of Malassezia-caused or Malassezia-exacerbated skin conditions is largely unknown, owing to the complex interactions of this commensal with the skin, an organ that has been on the edge of extreme selection pressure during evolution. In healthy skin, Malassezia yeasts exploit essential nutrients for their growth without inflicting disease (Fig. 1). When this process is perturbed, Malassezia yeasts adapt by modifying the expression of enzymes involved in the acquisition of energy, such as lipases and phospholipases [9], [10], and at the same time synthesize an array of bioactive indoles that act through the aryl-hydrocarbon receptor (AhR), which is expressed on almost all cell types found in the epidermis [11]. 10.1371/journal.ppat.1004523.g001 Figure 1 Model showing the putative interactions of Malassezia yeasts with the skin. Malassezia yeasts take up nutrients as well as sebum lipids that are used to form the outer layer of the yeast or amino acids that are needed for the formation of melanin or the synthesis of AhR indolic ligands. In parallel they modify the expression of lipases and phospholipases under the action of β-endorphin. Cellular components (enzymes, proteins, glyceroglycolipids, and mannosyl fatty acids) are recognized by the innate and adaptive immune system and alter its function. AhR ligands potentially down-regulate immune stimulation, modify the function of epidermal cells, interfere with AhR-induced ultraviolet (UV) damage and melanogenesis, and probably inhibit antagonist microbes. A major challenge would be to comprehend the multifaceted interactions of Malassezia yeasts with the human skin during health and disease. These include (a) commensalism (healthy skin), as there is no strong evidence for a mutualistic or beneficial relationship of the Malassezia microbiome and the skin; (b) subtle alterations in the function of skin melanocytes, resulting in hypo- or hyperpigmented plaques with characteristic clinical absence of inflammation and mild alterations in the epidermal barrier function (pityriasis versicolor) (Fig. 1); (c) inflammation without generation of antibody-mediated immunity (seborrheic dermatitis and dandruff); (d) induction of specific immunity (atopic dermatitis); and (e) invasion and inflammation of the hair follicle (Malassezia folliculitis). Interestingly, the high lipase activity of M. globosa from folliculitis specimens during the summer months may be promoted by sweat components [12], such as sodium chloride and lactic acid, thus laying a framework for examining potential metabolome, structure and function relationships between M. globosa lipases and the human skin. In seborrheic dermatitis and dandruff, there is a difference in the quality of sebum lipids between healthy and diseased skin [13], while the expression and function of Malassezia lipases in addition to barrier function defects and individual susceptibility take part in the exacerbation of these conditions [14], [15]. Recently, culture and biopsy evidence supported an association of M. restricta and M. globosa with rare nipple hyperkeratotic lesions [16] in young women, who responded to a combination therapy of oral itraconazole and topical ketoconazole. This denotes that the metabolome of strains involved in rare presentations of skin diseases should be thoroughly investigated, clearly in conjunction with key host and environmental factors. In that respect, at least two Malassezia yeast metabolic pathways, i.e., phospholipase production [17], [18] and indole pigment synthesis, have been associated with strains isolated from human and animal diseased skin. Malassezia produces potent indolic AhR ligands, such as indirubin and indolo [3,2-b] carbazole (ICZ) [19], which potentially modify the function of almost all cells found in the epidermis and express this receptor (Fig. 1). In view of the AhR participation in (a) carcinogenesis, (b) immune regulation, and (c) the mediation of ultraviolet radiation damage, a hypothesis on the potential contribution of Malassezia yeasts in skin carcinogenesis has been formulated [20]. Risk Factors for Malassezia Fungemia and Disseminated Disease Patients under total parenteral nutrition (TPN) and immunocompromised patients with increased length of stay (LOS) in intensive care units are at risk for Malassezia infections. Risk for Malassezia infections is also high in very-low-birth-weight infants ( 2 months) azole treatment is required for suppression of symptoms in the Malassezia-triggered head and neck variant of atopic dermatitis [55]. Although the in vitro susceptibility testing is not yet standardized for Malassezia spp., the Clinical and Laboratory Standards Institute (CLSI) broth microdilution protocol was adapted by modifying media, time of incubation, and inocula, showing that itraconazole, ketoconazole, and posaconazole are the most effective drugs [50], [56]. Malassezia infections in animals are frequently treated with topical and/or systemic azole antifungal drugs [36]–[38], [57]–[59] usually combined with antibiotics and glucocorticoids in dogs with otitis externa [37], [38]. The emergence of azole-resistant M. pachydermatis [57], [58], as well as the increasing number of Malassezia infections in both humans and animals, emphasizes the importance of susceptibility tests as a guide for proper antifungal treatment [56]. Alternative therapeutic protocols, i.e., desensitization to Malassezia by immunotherapy or administration of inhibitors of yeast adherence factors, have been proposed to avoid repeated administration of antifungals and the occurrence of drug resistance phenomena [60]. Recently, the daily administration (150 µl, 2 mg/ml for 8 days) of a killer decapeptide, engineered from the variable region of a single-chain recombinant anti-idiotypic antibody, was shown to be a safe and effective treatment for Malassezia otitis externa in dogs [60]. Conclusions Over the last few decades, advances in research and technologies have greatly contributed to elucidating the role of Malassezia species in human and animal skin diseases and in human bloodstream infections. Molecular and alternative approaches have provided insights into the identification, taxonomy, and epidemiology of Malassezia species. In particular, PCR-RFLP, random amplified polymorphic DNA (RAPD), AFLP, PCR-single strand conformation polymorphism (SSCP) analysis, multilocus sequence typing (MLST, e.g., of ITS, IGS, chs2, and RNA polymerase 1 and 2), and MALDI-TOF MS resulted in the accurate identification and genotyping of Malassezia strains from humans or animals, thus resolving questions related to the geographical distribution of the infection agents and the characterization of strains causing outbreaks [61], [62]. Nevertheless, these studies showed that the diversity within a single Malassezia species can more likely be attributed to a high degree of evolution driven by ecology, host adaptation, and pathogenicity. In particular, the pathogenic role of Malassezia yeasts seems to be related to changes in the normal physical, chemical, or immunological processes in the skin, which may enhance or down-regulate the molecular production of yeast virulence factors or antigens [23], [39]. The chemical composition of host epidermis seems to play a pivotal role in influencing the pathogenic or commensal phenotype of Malassezia yeasts by selecting different genetic populations with specific physiological requirements, different cell wall compositions, and different antifungal susceptibility profiles. In addition, molecular and physiological studies suggest the possibility of sexual or parasexual reproduction that might have a role in the process of adaptation of different Malassezia genotypes on different hosts or skin sites. As a consequence, antifungal therapy in Malassezia infections requires careful appraisal of drugs chosen, especially in cases of unresponsiveness to the treatment or recurrent infections. So far, restoring the epidermal-barrier function and avoiding immunoglobulin E (IgE) sensitization seems to be useful for the prevention and treatment of skin diseases complicated by Malassezia [63], even if antifungal therapy remains the main effective treatment in the near future. Alternative future treatments seem to be the use of selected cell-penetrating peptides that are harmless for mammalian cells but have antifungal activity, as shown for Malassezia otitis in dogs [60]. Undoubtedly, proteomic and genomic studies are needed in order to better understand the relationship between particular species/genotypes of Malassezia and the host at molecular and biochemical levels. Detailed biochemical analysis of the cell wall of the various species, as recently performed for M. restricta [31], and studies on the genotypic variants and their interaction with the immune system seem important here. Such studies might be the base for designing methods for the prevention, treatment, and control of infections caused by these fungi.
