Trichosporon species are urease-positive, non-encapsulated basidiomycetous yeasts characterised by the development of hyaline, septate hyphae that fragment into oval or rectangular arthroconidia. Some blastoconidia are also seen. The colonies are usually raised and have a waxy appearance, which develop radial furrows and irregular folds. They are widely distributed in the environment and many have different habitats, usually occupying narrow ecological niches. Some are soil borne and others are associated with humans and animals (Colombo et al. 2011, Sugita 2011, Arendrup et al. 2014).
The genus has undergone major taxonomic revision (Gueho et al. 1992, de Hoog et al. 2000, Rodriguez-Tudela et al. 2005). In particular, the name Trichosporon beigelii is now obsolete, and previously described infections reported in the literature under this name could in fact be due to any one of the species listed below.
Six species are of clinical significance: T. asahii, T. asteroides, T. cutaneum, T. inkin, T. mucoides and T. ovoides. Other species reported from human and animal infections include T. dermatis, T. domesticum, T. faecale, T. jirovecii, T. loubieri and T. mycotoxinovorans (Rodriguez-Tudela et al. 2005, Chagas-Neto et al. 2008, Colombo et al. 2011).
Trichosporon species are a minor component of normal skin flora, and are widely distributed in nature. They are regularly associated with the soft nodules of white piedra, and have been involved in a variety of opportunistic infections in the immunosuppressed patient. Disseminated infections are most frequently (75%) caused by T. asahii (Arendrup et al. 2014) and have been associated with leukaemia, organ transplantation, multiple myeloma, aplastic anaemia, lymphoma, solid tumours and AIDS. Disseminated infections are often fulminate and widespread, with lesions occurring in the liver, spleen, lungs and gastrointestinal tract. Infections in non-immunosuppressed patients include endophthalmitis after surgical extraction of cataracts, endocarditis usually following insertion of prosthetic cardiac valves, peritonitis in patients on continuous ambulatory peritoneal dialysis (CAPD), and intravenous drug abuse.
Note: Genus identification is mandatory for clinical management and should be performed and provided in a timely manner. Species identification remains difficult and requires molecular analysis or MALDI-TOF MS (with an extensive database) (Arendrup et al. 2014).
Molecular Identification: ITS and D1/D2 sequencing is required for accurate species identification (Arendrup et al. 2014).
MALDI-TOF MS: A promising identification tool to accurately identify species (with an extensive database) (Kolecka et al. 2013).
Comment: The API 20C yeast identification system is recommended for sugar assimilation tests.
References: Kurtzman and Fell (1988), Gueho et al. (1992), de Hoog et al. (2000, 2015), Rodriguez-Tudela et al. (2005), Chagas-Neto et al. (2008), Guo et al. (2011), Xiao et al. (2013).
Key to medically important species (de Hoog et al. 2000).
1. Growth with melibiose 2
No growth with melibiose 3
2. Tolerant to cycloheximide T. mucoides
Not tolerant to cycloheximide T. cutaneum
3. Growth with myo-inositol, no growth with L-arabinose T. inkin
No growth with myo-inositol, growth with L-arabinose 4
4. Colony with very slow growth; thallus consisting of clumps
of meristematic cells T. asteroides
Colonies and microscopy otherwise 5
5. Appressoria present in slide cultures T. ovoides
Appressoria absent in slide cultures 6