On Sabouraud's dextrose agar, colonies are flat, spreading, greyish-white to light tan-white 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 only rarely produced, most cultures are sterile or produce only occasional thick-walled terminal or intercalary chlamydoconidia. 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. So called pectinate (comb-like) hyphae and racquet hyphae (a series of hyphal segments swollen at one end) may also be present. RG-2 organism.
Culture of Microsporum audouinii.
Chlamydoconidium of M. audouinii.
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).
Lactritmel Agar: Colonies are usually flat, spreading, with a fine, whitish suede-like surface and a very pale yellow-brown reverse. Microscopic morphology as described above.
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-brown 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.
Microsporum audouinii is an anthropophilic fungus causing non-inflammatory infections of scalp and skin especially in children. Once the cause of epidemics of tinea capitis in Europe and North America, it is now becoming less frequent. Invaded hairs show an ectothrix infection and usually fluoresce a bright greenish-yellow under Wood's ultra-violet light. Only rarely found in Australia, most reports are in fact non-sporulating strains of M. canis.
Rippon, J.W. 1988. Medical Mycology. 3rd Edition. W.B. Saunders Co., Philadelphia, USA.