DESCRIPTIONS OF MEDICAL FUNGI

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Descriptions of Medical Fungi
Synonymy: T. mentagrophytes var. interdigitale (Priestley) Moraes, Anais Bras.
Trichophyton interdigitale is an anthropophilic fungus which is a common cause of
tinea pedis, particularly the vesicular type, tinea corporis, and sometimes superficial
nail plate invasion in humans. It is not known to invade hair in vivo but produces hair
perforations in vitro. Distribution is worldwide. This species may be regarded as a
clonal offshoot of the zoophilic T. mentagrophytes (de Hoog et al. 2016).
RG-2 organism.
Morphological Description: Colonies are usually flat, white to cream in colour with
a powdery to suede-like surface and yellowish to pinkish brown reverse pigment,
often becoming a darker red-brown with age. Numerous subspherical to pyriform
microconidia, occasional spiral hyphae and spherical chlamydospores are present,
the latter being more abundant in older cultures. Occasional slender, clavate, smoothwalled,
multiseptate macroconidia are also present in some cultures.
Confirmatory Tests:
Littman Oxgall Agar: Raised white downy colony with no reverse pigment.
Lactritmel Agar: Macroscopic and microscopic features as described above.
Sabouraud’s Dextrose Agar with 5% Salt: Heaped and folded, buff-coloured, suedelike
surface with a dark reddish-brown submerged fringe and brown reverse.
1% Peptone Agar: Flat, white to cream, suede-like surface with raised white downy
centre. No reverse pigment.
Vitamin Free Agar (Trichophyton Agar No.1): Good growth indicating no special
nutritional requirements, flat cream powdery surface with central downy tuft. Reverse
pale pinkish-brown.
Hydrolysis of Urea: Positive within 7 days (usually 3 to 5 days).
Hair Perforation Test: Positive.
Trichophyton interdigitale Priestley
Key Features: Culture characteristics, microscopic morphology and in vitro perforation
of human hair.
Trichophyton interdigitale can be distinguished from T. rubrum and T. mentagrophytes
by: (a) its culture characteristics and microscopic morphology on SDA and/or lactritmel
agar; (b) its growth and colony morphology on Sabouraud’s salt agar (colonies of T.
interdigitale and T. mentagrophytes unlike T. rubrum, grow very well on this medium
and usually produce a distinctive dark reddish-brown reverse pigment); (c) a positive
urease test (within 7 days), a positive hair perforation test and the production of a
yellow-brown to pinkish-brown reverse pigment on pigment stimulation media like
lactritmel and Trichophyton No.1 agars; (d) T. interdigitale demonstrates profuse growth
and alkalinity on BCP milk solids agar; (e) on 1% peptone agar T. interdigitale has a
suede-like to downy surface whereas T. mentagrophytes has a characteristic granular
appearance.