Trichophyton rubrum downy strain

On Sabouraud's dextrose agar, colonies are flat to slightly raised, white to cream, suede-like to downy, with a yellow-brown to wine-red reverse. Most cultures show scanty to moderate numbers of slender clavate to pyriform microconidia. Macroconidia are usually absent, however closterospore-like projections may be present in some mounts. Note: on primary isolation some cultures may lack reverse pigmentation and fail to produce microconidia. These will need to be subcultured onto media like Lactritmel agar or potato dextrose agar which stimulate pigmentation and sporulation. If sporulation still fails subculture the fungus onto Difco Trichophyton Agar No.1.  RG-2 organism.

Culture of Trichophtyon rubrum showing wine-red reverse pigment.

Typical slender clavate microconidia of T. rubrum downy type

Kaminski's Dermatophyte Identification Scheme:

Littman Oxgall Agar (Difco): Raised, greyish-white, suede-like to downy colony with no reverse pigment. Some cultures may have showed a faint greenish-yellow diffusible pigment.

Lactritmel Agar (Mycopathologia 91:57-59, 1985): Flat, white, downy colonies with a deep wine-red reverse pigment. Microscopically, cultures show the typical downy type morphology of pyriform to slender clavate microconidia described above.

Sabouraud's Dextrose Agar with 5% NaCl: Very stunted, white downy colony with a pale yellow-brown reverse pigment.

1% Peptone Agar: Flat, white to cream, downy colony often with a raised centre. No reverse pigment produced.

Hydrolysis of Urea: Negative at 7 days.

Vitamin Free Agar (Difco Trichophyton Agar No.1): Good growth indicating no special vitamin requirements. Cultures are flat, white to cream, suede-like to downy with a deep wine-red reverse pigment.

Hair Perforation Test ("in vitro"): Negative at 28 days.

The above morphological and cultural characteristics are typical of Trichophyton rubrum downy type. Seven other strains or varieties of the T. rubrum downy type have been recognized and are listed below.

1. T. rubrum "Y" variant

Microscopically similar to downy type. Yellow reverse pigment produced on 1% peptone agar, however characteristic red reverse pigment is produced on Sabouraud's dextrose agar and on pigment stimulating media like Lactritmel and Trichophyton No.1 agars. May or may not show pigmentation on primary isolation media. Basically, these strains produce both yellow and red pigments.

2. T. rubrum var. flava

Microscopically similar to downy type. Yellow reverse pigment is produced on 1% peptone agar and on Sabouraud's dextrose agar. Some red reverse pigment may be produced on pigment enhancing media like Lactritmel agar, but never on Sabouraud's dextrose agar. These strains normally produce only yellow pigment.

3. T. rubrum "P" variant

Microscopically similar to downy type. Purple reverse pigment produced on 1% Peptone agar, however characteristic red reverse pigment is also produced on Lactritmel agar and on other pigment stimulating.

4. T. rubrum melanoid type

Microscopically similar to downy type. A diffusible brown, melanoid pigment is present on all media, especially 1% peptone agar. Characteristic red reverse pigment, although often discoloured by the presence of the diffusable melanoid pigment is also produced on Lactritmel agar and other pigment stimulating media.

* Note 1% peptone agar is the key medium for distinguishing the above types of T. rubrum.

5. T. rubrum hyperpigmented type

Microscopically similar to downy type. All cultures show a violet to red-violet glabrous surface with radial furrows and a deep violet to red-violet reverse.

6. T. rubrum colourless variant

Microscopically similar to downy type. No reverse pigment is produced on any media.

7. T. rubrum dysgonic type

Microscopically restricted to distorted hyphae. All media show tiny slow growing colonies which are usually hyperpigmented. Primary isolates of this type are often confused with Trichophyton violaceum. These strains usually revert to the typical downy type following several sub-cultures.

Clinical significance:

Trichophyton rubrum is an anthropophilic fungus which has become the most widely distributed dermatophyte of man. It frequently causes chronic infections of skin, nails and rarely scalp. Granulomatous lesions may sometimes occur. Infected hairs do not fluoresce under Wood's ultraviolet light, and microscopically may show endothrix or ectothrix type of invasion. Key features include culture characteristics, microscopic morphology and failure to perforate hair "in vitro".

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.