DESCRIPTIONS OF MEDICAL FUNGI

Descriptions of Medical Fungi 125
Microsporum audouinii is an anthropophilic fungus causing non-inflammatory infections
of the scalp and skin, especially in children. Once the cause of epidemics of tinea
capitis in Europe and North America, it is now less common. Invaded hairs show an
ectothrix infection and usually fluoresce a bright greenish-yellow under Wood’s ultraviolet
light. Only rarely found in Australasia, most reports are in fact misidentified nonsporulating
strains of M. canis.
RG-2 organism.
Microsporum audouinii Gruby
Morphological Description: Colonies are flat, spreading, greyish-white to light tanwhite
in colour, and have a dense suede-like to downy surface, suggestive of mouse
fur in texture. Reverse can be yellow-brown to reddish-brown in colour. Some strains
may show no reverse pigment. Macroconidia and microconidia are rarely produced,
most cultures are sterile or produce only occasional thick-walled terminal or intercalary
chlamydospores. When present, macroconidia may resemble those of M. canis but
are usually longer, smoother and more irregularly fusiform in shape; microconidia,
when present, are pyriform to clavate in shape and are similar to those seen in other
species of Microsporum, Lophophyton and Nannizzia. Pectinate (comb-like) hyphae
and racquet hyphae (a series of hyphal segments swollen at one end) may also be
present.
a
b
10 µm
Microsporum audouinii (a) Culture and (b) a thick-walled intercalary
chlamydospore. Note: Macroconidia and microconidia are only rarely produced.
Confirmatory Tests:
Growth on Rice Grains: Very poor or absent, usually being visible only as a brown
discolouration. This is one of the features which distinguish M. audouinii from M. canis.
Reverse Pigment on Potato Dextrose Agar: Salmon to pinkish-brown (M. canis is
bright yellow).
BCP Milk Solids Glucose Agar: Both M. canis and M. audounii demonstrate profuse
growth, but only M. audouinii shows a rapid pH change to alkaline (purple colour).
Vitamin Free Agar (Trichophyton Agar No.1): Good growth indicating no special
nutritional requirements. Cultures are flat, white, suede-like to downy, with a yellowbrown
reverse. Note: Growth of some strains of M. audouinii is enhanced by the
presence of thiamine (Trichophyton agar No.4).
Hair Perforation Test: Negative after 28 days.
Key Features: Absence of conidia, poor or no growth on polished rice grains, inability
to perforate hair in vitro, and culture characteristics.