DESCRIPTIONS OF MEDICAL FUNGI

Descriptions of Medical Fungi 141
Synonymy: Hendersonula toruloidea Nattras.
Scytalidium dimidiatum (Penzig) Sutton & Dyko.
Scytalidium hyalinum Campbell & Mulder.
Neoscytalidium dimidiatum is a coelomycete and recognised agent of onychomycosis
and superficial skin infections, especially in tropical regions. The primary isolation
of this fungus from clinical specimens may be difficult as isolates are sensitive to
cycloheximide (actidione), which is commonly added to primary isolation media used
for culturing skin scrapings.
The taxonomy of this species has been very confusing; the conidial state of S.
dimidiatum was originally described under the name Hendersonula toruloidea.
However it is phylogenetically remote from Scytalidium and has now been placed
into Neoscytalidium (Crous et al. 2006, Machouart et al. 2012). Colourless mutants
(previously known as Scytalidium hyalinum) often occur and have now been listed as
variety N. dimidiatum var. hyalinum (Madrid et al. 2009).
Note: Nattrassia mangiferae, previously considered the teleomorph form of S.
dimidiatum, is now considered a distinct species, and has been placed in the genus
Neofusicoccum, based on the lack of an arthoconidial anamorph (Crous et al. 2006,
Machouart et al. 2012).
RG-2 organism.
Neoscytalidium dimidiatum (Penzig) Crous & Slippers
Morphological Description: Cultures are effuse, hairy, dark grey to blackish-brown, or
white to greyish, with a cream-coloured to deep ochraceous-yellow reverse. Colourless
mutants often occur. Arthroconidia are typically present in chains of one to two-cells,
darkly pigmented, 3.5-5 x 6.5-12 µm, produced by the holothallic fragmentation of
undifferentiated hyphae. Pycnidia, only occasionally formed in older cultures are black,
ostiolate and contain numerous hyaline, flask-shaped phialides. Phialoconidia are at
first one-celled and hyaline, later becoming three-celled, brown, with the centre cell
darker than the end cells and are ovoid to ellipsoidal in shape.
Molecular Identification: ITS and D1/D2 sequencing may be used for accurate
species identification (Halliday et al. 2015).
MALDI-TOF MS: Alshawa et al. (2012) developed a spectral database for 12 different
species of dermatophytes which also included Neoscytalidium dimidiatum and N.
dimidiatum var. hyalinum. Correct identification of the species was obtained for 18/21
Neoscytalidium isolates (85.7%).
References: McGinnis (1980), Moore (1986), Rippon (1988), Frankel and Rippon
(1989), Sutton and Dyko (1989) Madrid et al. (2009), de Hoog et al. (2000, 2015),
Crous et al. (2006), Machouart et al. (2012), Alshawa et al. (2012).