Trichophyton interdigitale var. nodulare
Supplementary description for Trichophyton interdigitale var. nodulare, a dysgonic variant of T. interdigitale with distinctive bright yellow to apricot-coloured colonies with a suede-like to powdery surface and a bright yellow-brown to orange reverse. On primary isolation, colonies are often glabrous with minimal surface mycelium. Microscopically characteristic “nodular organs” are observed in the vegetative hyphae. Usually, no conidia are seen but some isolates, especially with subculture, may produce subspherical to pyriform microconidia similar to those of T. interdigitale. RG-2 organism.
Culture of T. interdigitale var. nodulare showing typical bright apricot-yellow pigmentation.
Nodular organs of T. interdigitale var. nodulare.
Kaminski's Dermatophyte Identification Scheme:
Littman Oxgall Agar (Difco): Raised, heaped and sometimes folded white suede-like colony with yellow-green diffusable pigment.
Lactritmel Agar (Mycopathologia 91:57-59, 1985): Cultures are flat, white, powdery to suede-like with yellow-brown reverse. Microscopic morphology as described above, with nodular organs, and subspherical to pyriform microconidia and spiral hyphae present in some cultures.
Sabouraud's Dextrose Agar with 5% NaCl: Stunted, heaped and folded dark-brown glabrous colony with dark reddish-brown submerged fringe and reverse.
1% Peptone Agar: Heaped and folded glabrous colony with yellow to orange peripheral fringe. No reverse pigment.
Hydrolysis of Urea: Positive within 7 days (usually 3-5).
Vitamin Free agar (Difco Trichophyton Agar No.1): Flat, white suede-like colony with pale yellow-brown reverse (ie. no special nutritional requirements).
Hair Perforation Test ("in vitro"): Positive.
It should be noted that primary isolates of T. interdigitale var. nodulare are often glabrous and apricot orange in colour. Generally, it can be differentiated from T. mentagrophytes by (a) its microscopic morphology showing moderate numbers of nodular organs (often no conidia are seen but some isolates have subspherical to pyriform microconidia); and (b) its culture characteristics on the dermatophyte identification scheme, especially the 1% peptone and plain Sabouraud's agar.
T. interdigitale var. nodulare is an unusual cause of tinea pedis. It has a world-wide distribution. It is not known to invade hair in vivo, but produces hair perforations in vitro.
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.