All varieties of T. rubrum.
Trichophyton rubrum is an anthropophilic fungus that has become the most widely distributed dermatophyte of humans. 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.
Morphologically T. rubrum exhibits a spectrum of overlapping characters; for example culture surface texture may vary from downy to suede-like; culture surface pigmentation may vary from white to cream to deep red; culture reverse pigmentation may vary from colourless to yellowish to yellow-brown to wine red; numbers of microconidia range from none to scanty to many; shape of microconidia vary from slender clavate to pyriform; numbers of macroconidia range from none to scanty to many and may or may not have terminal projections. This is why so many varieties or synonyms have been described in the past. However, molecular evidence by Gräser et al. (1999b) reveals little variation between strains of T. rubrum and determined that the species is largely clonal.
The majority of isolates, especially those causing tinea pedis and onychomycosis, are characterised by the production of scanty to moderate numbers of slender clavate microconidia and no macroconidia (formerly the “downy strain”). Some isolates, usually from cases of tinea corporis, are characterised by the production of moderate to abundant numbers of clavate to pyriform microconidia and moderate to abundant numbers of thin-walled, cigar-shaped macroconidia (formerly the “granular strain”).
It should be stressed that intermediate strains do occur as many culture and morphological characteristics overlap.
Trichophyton rubrum culture morphology; “downy strain” with typical wine-red reverse; “Y variety” with both yellow and red pigmentation; “var. flava” with yellow pigmentation; “granular strain” with red surface and reverse pigmentation.
Note: Trichophyton rubrum produces both red and yellow pigments. Culture colours may range from none to dark red to dark yellow, with all combinations in between. The images above show the same strain grown on lactritmel agar that promotes red pigmentation and on mycobiotic agar that shows an underlying yellow pigmentation.
Morphological Description: Colonies are mostly flat to slightly raised, white to cream, suede-like to downy, with either no reverse pigment or a yellow-brown to wine-red reverse. Most cultures show scanty to moderate numbers of slender clavate to pyriform microconidia. Macroconidia are usually absent, but when present are smooth, thin-walled multiseptate, slender and cylindrical to cigar-shaped. Older cultures may show numerous chlamydospores with a few clavate to pyriform microconidia.
Note: On primary isolation some cultures may lack reverse pigmentation and fail to produce microconidia. These 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 Trichophyton agar No.1.
Littman Oxgall Agar: Raised, greyish-white, suede-like to downy colony with no reverse pigment. Some cultures may show a greenish-yellow diffusible pigment.
Lactritmel Agar: Flat, white to rose pink, downy to granular colonies with a deep wine-red reverse pigment.
Sabouraud’s Dextrose Agar with 5% salt: Very stunted, white to cream, downy to glabrous colony with a pale yellow-brown reverse pigment.
1% Peptone Agar: Flat, white to cream, downy to glabrous colony with no reverse pigment.
BCP Agar: restricted colony growth and neutral (unchanged) pH. Colonies typically demonstrate red pigment on reverse
Hydrolysis of Urea: Typically, negative at 7 days (some may be positive).
Vitamin Free Agar (Trichophyton Agar No.1): Good growth indicating no special vitamin requirements. Colonies are flat, white to cream, suede-like to downy with a deep wine-red reverse pigment.
Hair Perforation Test: Negative at 28 days.
Key Features: Include clinical history, culture characteristics, microscopic morphology and failure to perforate hair in vitro.
Trichophyton rubrum showing typical slender clavate microconidia.
Microscopically, the granular type is characterized by the production of moderate to abundant numbers of clavate to pyriform microconidia and moderate to abundant numbers of thin-walled, cigar-shaped macroconidia. The macroconidia may or may not have terminal appendages.
Trichophyton rubrum showing slender clavate microconidia and cigar-shaped macroconidia, some with terminal appendages.
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.