On and Under the Skin: Emerging Basidiomycetous Yeast Infections Caused by Trichosporon Species

Trichosporon species are basidiomycetous, yeast-like organisms capable of filamentous growth, i.e., dimorphic (Fig 1), that are distributed throughout nature [1]. They are important from a biotechnological point of view as they are capable of decontaminating polluted environments by accumulating large amounts of oils [2–5]. A limited number of reports also show their presence in the human microbiome, such as Trichosporon asahii, which has been isolated from human fecal samples, the skin of healthy individuals, and patients with atopic dermatitis [1,6– 10]. In addition, Trichosporon spp. can cause white piedra, hypersensitivity pneumonitis, superficial infections, and invasive trichosporonosis [11]. Designated 150 years ago, the genus Trichosporon was for many years a collection of many different yeast-like organisms. Until the end of the 20th century, a wide range of species were included under the name of Trichosporon beigelii or synonyms, which were later shown to be phylogenetically distinct. With the advent of molecular techniques, the genus was rearranged, and many of its species were reassigned to other genera and new ones were described. Currently there are 51 accepted Trichosporon species, 16 of which have clinical relevance [1,12–13] (these species are highlighted in Fig 2). Formerly associated with uncommon hair and skin infections, research on Trichosporon has fallen behind more life-threatening fungi such as Candida or Cryptococcus species, until the rise of opportunistic, deeply seated, disseminated Trichosporon infections (especially T. asahii) over the last decades. A lack of background knowledge impairs the proper diagnosis and treatment of Trichosporon infections. Fortunately in the recent years, studies of Trichosporon epidemiology [14], virulence factors [15,16], antifungal susceptibility [16,17], and animal models of infection [18,19] have been conducted. There are a few reports of genetic transformation systems for T. cutaneum, which used both dominant and auxotrophic markers [20,21]. However, forward and reverse genetics is still not common practice in the study of Trichosporon. Molecular techniques have already proved necessary for the detection and correct identification of Trichosporon species [1,17]. However, the use of whole genome sequencing and gene manipulation techniques in Trichosporon is in its infancy, in comparison to other fungi, such as Candida or Aspergillus species. Genome sequences for a T. asahii environmental strain and a strain isolated from a progressive psoriatic lesion are available [22,23]. Both T. asahii genomes are of approximately 25 Mbp and are predicted to encode approximately 9,000 genes. A comparison of these two Trichosporon genomes revealed>99% chromosomal and mitochondrial sequence identity. When compared to the genomes of other skin-associated or