DESCRIPTIONS OF MEDICAL FUNGI

Descriptions of Medical Fungi 207
Trichophyton rubrum (Castellani) Semon
Synonymy: Trichophyton fischeri Kane.
Trichophyton raubitschekii Kane, Salkin, Weitzman & Smitka.
Trichophyton kanei Summerbell.
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.
a b c d
Trichophyton rubrum culture morphology; (a) “downy strain” with typical wine-red
reverse; (b) “Y variety” with both yellow and red pigmentation; (c) “var. flava” with
yellow pigmentation; (d) “granular strain” with red surface and reverse pigmentation.