DESCRIPTIONS OF MEDICAL FUNGI

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Descriptions of Medical Fungi
Microsporum canis is a zoophilic dermatophyte of worldwide distribution and is a
frequent cause of ringworm in humans, especially children. Invades hair, skin and
rarely nails. Cats and dogs are the main sources of infection. Invaded hairs show
an ectothrix infection and fluoresce a bright greenish-yellow under Wood’s ultra-violet
light.
RG-2 organism.
Morphological Description: Colonies are flat, spreading, white to cream-coloured, with
a dense cottony surface which may show some radial grooves. Colonies usually have
a bright golden yellow to brownish yellow reverse pigment, but non-pigmented strains
may also occur. Macroconidia are typically spindle-shaped with 5-15 cells, verrucose,
thick-walled and often have a terminal knob, 35-110 x 12-25 µm. A few pyriform to
clavate microconidia are also present. Macroconidia and/or microconidia are often not
produced on primary isolation media and it is recommended that subcultures be made
onto lactritmel agar and/or boiled polished rice grains to stimulate sporulation.
Confirmatory Tests:
Microsporum canis Bodin
Growth on Rice Grains: good growth of white aerial mycelium with production of
yellow pigment. Microscopy reveals numerous macroconidia and microconidia similar
to those described above.
Reverse Pigment on Potato Dextrose Agar: Bright yellow (both M. audouinii and M.
canis var. equinum are salmon to pinkish-brown).
Vitamin Free Agar (Trichophyton Agar No.1): Good growth indicating no special
nutritional requirements. Cultures are flat, white, suede-like to downy, with a yellow to
pale yellow-brown reverse.
Hair Perforation Test: Positive at 14 days.
Key Features: Distinctive macroconidia and culture characteristics. Abundant growth
and sporulation on polished rice grains and in vitro perforation of hair.
Microsporum canis culture showing a bright golden yellow reverse pigment