(Megnin) Grigorakis, 1929
Microsporum ferrugineum colonies grow slowly and reach a diameter of 0.5-1 cm on Sabouraud dextrose agar incubated at 25°C for 7 days. Two colony types may be observed. The first type is glabrous, heaped, wrinkled and often has furrows and folds. The color is yellow to rust from the front and a dull orange pigmentation is observed from the reverse. The second type is flat, spreading and leathery to downy. The color is white.
Microsporum ferrugineum produces sterile, septate hyphae which are usually deformed and irregularly branched, having a bamboo-like appearance. Long, straight, thick-walled, hypersegmented hyphae are often present. This Microsporum spp. does not produce macro- and microconidia. Fusiform macroconidia may be produced on rice grains or diluted Sabouraud dextrose agar.
In vitro hair perforation test is negative. Urease activity is positive. No special growth factor is required to grow Microsporum ferrugineum. It produces pale yellow colonies on Lowenstein-Jensen medium. This property helps to differentiate the Microsporum ferrugineum strains that produce rusty yellow colonies from those of Trichophyton soudanense. Trichophyton soudanense colonies are brownish black on Lowenstein Jensen medium. In addition, Microsporum ferrugineum is urease positive while Trichophyton soudanense is urease negative.