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