DESCRIPTIONS OF MEDICAL FUNGI

Descriptions of Medical Fungi 203
Trichophyton mentagrophytes (Robin) Blanchard
Synonymy: T. mentagrophytes var. mentagrophytes (Robin) Sabour.
Trichophyton mentagrophytes is a zoophilic fungus with a worldwide distribution and
a wide range of animal hosts including mice, guinea-pigs, kangaroos, cats, horses,
sheep and rabbits. Produces inflammatory skin or scalp lesions in humans, particularly
in rural workers. Kerion of the scalp and beard may occur. Invaded hairs show an
ectothrix infection but do not fluoresce under Wood’s ultra-violet light. Distribution is
worldwide.
RG-2 organism.
Morphological Description: Colonies are generally flat, white to cream in colour,
with a powdery to granular surface. Some cultures show central folding or develop
raised central tufts or pleomorphic suede-like to downy areas. Reverse pigmentation is
usually a yellow-brown to reddish-brown colour. Numerous single-celled microconidia
are formed, often in dense clusters. Microconidia are hyaline, smooth-walled, and
are predominantly spherical to subspherical in shape, however occasional clavate
to pyriform forms may occur. Varying numbers of spherical chlamydospores, spiral
hyphae and smooth, thin-walled, clavate-shaped, multicelled macroconidia may also
be present.
Confirmatory Tests:
Littman Oxgall Agar: Raised greyish-white, suede-like to downy colony. Some
cultures may show a diffusible yellow to brown pigment.
Lactritmel Agar: Cultures are flat, white to cream in colour, with a powdery to granular
surface. Some cultures develop raised central tufts or pleomorphic downy areas.
Reverse pigmentation is yellow-brown to pinkish-brown to red-brown. Microscopic
morphology similar to that described above, with predominantly spherical microconidia,
often formed in dense clusters, and varying numbers of spherical chlamydospores,
spiral hyphae and smooth, thin-walled, clavate, multiseptate macroconidia.
Sabouraud’s Dextrose Agar with 5% Salt: Cultures are heaped and folded, buff to
brown in colour, with a suede-like surface texture and characteristically have a very
dark reddish-brown submerged peripheral fringe and reverse pigmentation.
1% Peptone Agar: Flat, cream-coloured, powdery to granular colony with no reverse
pigment.
Hydrolysis of Urea: Positive within 7 days (usually 3 to 5 days).
Vitamin Free Agar (Trichophyton Agar No.1): Good growth indicating no special
nutritional requirements. Cultures are flat, cream-coloured, with a powdery to suedelike
surface, and have a reddish-brown reverse pigmentation.
Hair Perforation Test: Positive within 14 days.
Key Features: Culture characteristics, microscopic morphology and clinical disease
with known animal contacts. T. mentagrophytes can be distinguished from T. interdigitale
by: (a) its granular appearance on the 1% Peptone agar; (b) its microscopic morphology
of more spherical microconidia and generally greater numbers of macroconidia; and
(c) a yellow to brown diffusible pigment is often seen on the Littman oxgall agar. Both
T. interdigitale and T. mentagrophytes demonstrate profuse growth and alkalinity on
BCP milk solids agar.