Microsporum: Species, Identification, and the Rice Grain Test

A child comes in with a scaly, itchy scalp patch, and there is a kitten at home. That single detail, the kitten, points the laboratory toward one genus before any plate is read, because Microsporum canis is caught from cats and dogs and is a leading cause of scalp ringworm in children. Under a Wood's lamp the infected hairs glow bright green, and on culture the plate grows a mold whose microscopy settles it: large, thick-walled, spindle-shaped macroconidia, the signature of Microsporum. This article is about recognizing that signature and telling the Microsporum species apart.

Microsporum infects skin and hair but not nails. That single restriction is diagnostically useful: a nail infection is never Microsporum. For how the genus sits alongside Trichophyton and Epidermophyton, and the full laboratory workflow, see the dermatophytes lab diagnosis article.

What defines the genus Microsporum

Microsporum is identified by a set of features that contrast neatly with Trichophyton:

• Tissues infected: skin and hair only, never nails. A nail infection rules Microsporum out.
• Macroconidia: the genus signature. They are large, thick-walled, rough (echinulate) or spiny, and spindle-shaped (tapered at both ends), with multiple internal cross-walls (septa). These carry the identification.
• Microconidia: present but few, small, and club-shaped along the hyphae. They are not the identifying feature.
• Hair invasion: characteristically ectothrix (spores coat the outside of the hair shaft).
• Wood's lamp: several Microsporum species fluoresce bright green in infected hair, a feature not shared by most Trichophyton.

The contrast to fix in your mind: Microsporum is the macroconidia genus (big, thick-walled, spindle-shaped), while Trichophyton leans on abundant microconidia with few thin-walled macroconidia. If the slide is dominated by large rough spindle-shaped macroconidia, think Microsporum.

The Microsporum species that matter

Microsporum canis

The most clinically important Microsporum species. It is zoophilic, caught from cats and dogs, and is a leading cause of tinea capitis and tinea corporis in children.

• Colony: cottony or woolly, white to pale surface, with a characteristic bright yellow to orange pigment on the reverse.
• Microscopy: abundant, large, thick-walled, spindle-shaped macroconidia with a curved, knob-like tapered end and numerous septa (commonly 6 or more, up to about 15). This is the textbook Microsporum macroconidium.
• Wood's lamp: infected hairs fluoresce bright green.
• Hair perforation test: positive. This separates it from Microsporum equinum, which does not perforate. See the [hair perforation test](PLACEHOLDER — hair perforation spoke URL) article.

Microsporum gypseum

A geophilic species, found in soil, causing sporadic tinea corporis and tinea capitis, often in people with soil contact (gardeners, farmers).

• Colony: flat, powdery or granular, buff to cinnamon (yellowish-brown), fast growing.
• Microscopy: abundant, thin-walled, ellipsoidal macroconidia with fewer septa (about 4 to 6) and rounded (less tapered) ends compared with M. canis. The thinner wall and fewer septa distinguish it.
• Wood's lamp: does not fluoresce.

Microsporum audouinii

An anthropophilic species (spread person to person), historically a major cause of epidemic tinea capitis in children.

• Colony: velvety, flat, greyish to light brown, slow growing, with a salmon to brown reverse.
• Microscopy: macroconidia and microconidia are rare and irregular; the colony sporulates poorly. Identification leans on colony features and the rice grain test rather than on conidia.
• Wood's lamp: infected hairs fluoresce bright green.
• Rice grain test: does not grow on sterile rice, the confirmatory feature (see below).

Other Microsporum species

• Microsporum ferrugineum and Microsporum distortum: anthropophilic causes of tinea capitis; infected hair fluoresces blue-green under Wood's lamp.
• Microsporum nanum: zoophilic, from pigs.
• Microsporum fulvum: geophilic, from soil.

Microsporum species at a glance

The rice grain test

The rice grain test is the specific confirmatory test for Microsporum audouinii. It separates the poorly sporulating M. audouinii from M. canis and most other Microsporum species, which look similar on primary culture but grow and sporulate readily on rice.

Principle. M. audouinii cannot use the nutrients in sterile, non-fortified (polished) rice grains and fails to grow, while M. canis and most other dermatophytes grow well on rice and produce abundant conidia. The differential growth is the readout.

Method. Autoclave a small amount of sterile, non-fortified white rice grains with a little water in a flask. Inoculate lightly with hyphae from the test isolate. Incubate at room temperature (about 25°C) for up to 10 days.

Reading.

• M. audouinii: poor or no growth; the rice grains stay largely unchanged.
• M. canis and most other Microsporum species: good growth with abundant aerial mycelium and sporulation, often turning the grains a yellow-brown.

A memory anchor: audouinii abstains from rice. The species that will not grow on rice grains is M. audouinii.

How to Remember

Microsporum = the macroconidia genus. Microsporum carries big, thick-walled, spindle-shaped macroconidia, and that is its identity down the microscope. Trichophyton is the opposite (abundant microconidia, few thin-walled macroconidia). Learn the two genera as a mirrored pair.

Microsporum misses nails. From the hub's neighbor trick: "M" sits near "N," and Microsporum is the genus that does not infect Nails. Skin and hair only.

Canis is thick and curved, gypseum is thin and blunt. The two most-tested macroconidia: M. canis has a thick, rough wall with many septa and a curved, knobbed tip; M. gypseum is thinner-walled, has fewer septa (4 to 6), and blunter ends.

Audouinii abstains from rice. The rice grain test confirms M. audouinii, the one Microsporum that will not grow on rice grains.

Green glow, think Microsporum. Bright-green Wood's lamp fluorescence in scalp hair points to M. canis or M. audouinii. Most Trichophyton do not fluoresce, so a glowing scalp shifts suspicion toward Microsporum.

Key exam facts

Where Students Get Confused

"Trichophyton gypseum" does not exist. Gypseum is a Microsporum species (Microsporum gypseum), a soil mould. The mix-up is common because the species names run together, but the genus is Microsporum. Soil source plus buff powdery colony plus thin-walled macroconidia all say Microsporum gypseum.

Microsporum is the macroconidia genus, not the microconidia one. The name contains "micro," which misleads students into expecting microconidia to identify it. The reverse is true: its large macroconidia carry the identification, and microconidia are few and unhelpful.

A negative Wood's lamp does not exclude Microsporum. M. canis and M. audouinii fluoresce, but M. gypseum does not, and the lamp only helps for scalp hair. A non-fluorescent scalp infection can still be Microsporum, so culture is needed.

Microsporum never infects nails. A fungal nail infection is caused by Trichophyton or Epidermophyton, never Microsporum. If a report says Microsporum from a nail, suspect a labeling or identification error.

M. canis comes from the animal, not the soil. Students confuse the habitat categories. M. canis is zoophilic (cats and dogs), M. gypseum is geophilic (soil), and M. audouinii is anthropophilic (person to person). The source often guides empiric suspicion, so the categories are worth keeping straight.

References

• Chander J. Textbook of Medical Mycology. 4th ed. Jaypee Brothers Medical Publishers; 2018.
• Tille PM. Bailey and Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2022.
• Procop GW, Church DL, Hall GS, et al. Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 7th ed. Wolters Kluwer; 2017.
• Larone DH. Larone's Medically Important Fungi: A Guide to Identification. 6th ed. ASM Press; 2018.