Trichophyton interdigitale

On Sabouraud's dextrose agar, colonies are usually flat, white to cream in colour with a powdery to suede-like surface and yellowish and 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, smooth-walled, multiseptate macroconidia are also present in some cultures.

Culture of Trichophyton interdigitale.

Microconidia, macroconidia, chlamydoconidia and spiral hyphae in T. interdigitale.

Kaminski's Dermatophyte Identification Scheme:

Littman Oxgall Agar (Difco): raised white downy colony with no reverse pigment.

Lactritmel Agar (Mycopathologia 91:57-59, 1985): Macroscopic and microscopic features as described for the primary culture.

Sabouraud's Dextrose Agar with 5% NaCl: Heaped and folded, buff-coloured suede-like 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.

Hydrolysis of Urea: Positive 5 days.

Vitamin Free agar (Difco Trichophyton Agar No.1): Good growth indicating no special nutritional requirements, flat cream powdery surface with central downy tuft. Reverse pale pinkish-brown.

Hair Perforation Test ("in vitro"): Positive.

Positive hair perforation test.

Trichophyton interdigitale can be distinguished from T. rubrum and T. mentagrophytes by (a) its culture characteristics and microscopic morphology on Sabouraud's dextrose agar and/or lactritmel agar; (b) its growth and colony morphology on Sabouraud's salt agar (colonies of T. mentagrophytes unlike T. rubrum, grow very well on this medium and usually produce a distinctive dark reddish-brown reverse pigment). Overall, Sabouraud's' salt agar is an excellent test for differentiating T. mentagrophytes from T. rubrum. (c) Other confirmatory tests useful for distinguishing T. mentagrophytes from T. rubrum include 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. Although in must be remembered that some granular strains of T. rubrum can give positive urease and hair perforation tests; and (d) on 1% peptone agar T. interdigitale has a suede-like to downy surface whereas T. mentagrophytes has a characteristic granular appearance.

Clinical significance:

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. It is not known to invade hair in vivo but produces hair perforations in vitro. Distribution is world-wide.

Mycosis: Dermatophytosis

Further reading:

Rebell, G., and D. Taplin. 1970. The Dermatophytes. 2nd. revised edition. University of Miami Press, Coral Gables, Florida. USA.

Rippon, J.W. 1988. Medical Mycology. 3rd Edition. W.B. Saunders Co., Philadelphia, USA.