Trichophyton: Species, Identification, and Microscopic Appearance

Two culture plates sit on the bench. Both grew a white-to-buff mold from skin scrapings. Both are Trichophyton, the most common dermatophyte genus. But one is Trichophyton rubrum and the other is Trichophyton mentagrophytes, and the difference is not academic: it changes which follow-up tests you run and confirms which species is driving infection in your patient.

Turn one plate over and the answer starts to appear as a deep red pigment bleeding into the agar. This is the everyday Trichophyton problem, and this article is about recognizing each species down the microscope and on the plate.

Trichophyton is the genus that infects all three keratinized tissues: skin, hair, and nails. That range is why it turns up in nearly every kind of tinea infection. For how Trichophyton fits alongside Microsporum and Epidermophyton, and the full laboratory workflow, see the dermatophytes lab diagnosis article.

What defines the genus Trichophyton

Trichophyton is identified in the laboratory by a consistent set of features:

• Tissues infected: skin, hair, and nails (all three). This distinguishes it from Microsporum (no nails) and Epidermophyton (no hair).
• Microconidia: abundant, and the more useful identifying feature for the genus. They are borne along the sides of the hyphae or in clusters.
• Macroconidia: few, thin-walled, smooth, and pencil- or cigar-shaped when present. In many isolates they are scarce or absent, so identification leans on the microconidia.
• Hair invasion: Trichophyton species can invade hair by either the endothrix pattern (inside the shaft) or the ectothrix pattern (outside the shaft), depending on the species.

The contrast to keep in mind: Microsporum is defined by its large, thick-walled, spindle-shaped macroconidia, while Trichophyton leans on abundant microconidia and produces macroconidia that are thin-walled and sparse. If a slide is dominated by microconidia with few thin-walled macroconidia, think Trichophyton.

Endothrix versus ectothrix: reading hair invasion

When Trichophyton infects hair, the pattern of invasion is visible on KOH microscopy and helps point to the species.

• Endothrix (inside the shaft). Spores form inside the hair shaft, which weakens it so the hair breaks at the scalp surface. This produces the black-dot pattern of tinea capitis. T. tonsurans and T. violaceum are the classic endothrix species.
• Ectothrix (outside the shaft). Spores coat the outside of the hair shaft. T. mentagrophytes shows ectothrix invasion.
• Favic (hyphae and air spaces). Hairs infected with T. schoenleinii show hyphae and characteristic air spaces within the shaft, the pattern seen in favus.

Endothrix ends at the scalp. Growth inside the shaft weakens it, the hair snaps flush with the skin, and each stub shows as a black dot.

The Trichophyton species that matter

Trichophyton rubrum

The single most common dermatophyte worldwide, and the one to know best. It infects skin and nails most often, and is the leading cause of tinea corporis, tinea pedis (moccasin type), and tinea unguium.

• Colony: velvety or downy, white to pinkish on the surface, with a characteristic deep red pigment on the reverse (the underside of the plate). The red reverse is the classic clue.
• Microscopy: abundant tear-drop or peg-shaped microconidia arranged along the sides of the hyphae. Macroconidia are few, long, thin-walled, and pencil-shaped when present.
• Key tests: does not perforate hair (hair perforation test negative) and does not hydrolyze urea (urease test negative). These separate it from T. mentagrophytes.

Trichophyton mentagrophytes

The second most common dermatophyte, and the species most often confused with T. rubrum. It is a frequent cause of inflammatory tinea, including tinea pedis and tinea barbae, and some forms are zoophilic (acquired from animals).

• Colony: white to tan (cream), cottony or granular/powdery. Reverse pigment is variable, usually absent or tan, not the deep red of T. rubrum.
• Microscopy: abundant round microconidia in grape-like clusters, and cigar-shaped macroconidia, sometimes with coiled or spiral hyphae. Some isolates show a terminal thread on the macroconidia.
• Key tests: perforates hair (hair perforation test positive, wedge-shaped) and hydrolyzes urea (urease positive within 2 to 4 days). Both tests are positive, which is the quickest way to separate it from T. rubrum.

Trichophyton tonsurans

A major cause of black-dot tinea capitis, especially in children, and a common cause of scalp outbreaks.

• Colony: flat, powdery to velvety, cream to yellow or brown, often with central folds or furrows.
• Microscopy: abundant microconidia of varying size and shape ("balloon" forms), borne on stalks at right angles to the hyphae. Macroconidia are rare and irregular.
• Hair invasion: endothrix, producing the black-dot pattern. Does not fluoresce under Wood's lamp.
• Note: growth is enhanced by thiamine, a feature used in some identification schemes.

Trichophyton schoenleinii

The cause of favus, a chronic scarring form of tinea capitis with honeycomb-like yellow crusts.

• Colony: slow growing, waxy, heaped, cream to brownish, often cracking the agar.
• Microscopy: microconidia and macroconidia are usually absent. The identifying features are swollen hyphal tips that branch like antlers, called favic chandeliers, along with chlamydospores.
• Hair invasion: favic pattern with hyphae and air spaces in the shaft.

Trichophyton violaceum

A cause of endothrix black-dot tinea capitis, more common in parts of Africa, the Middle East, and Asia.

• Colony: very slow growing, waxy or heaped, with a distinctive violet to purple pigment.
• Microscopy: distorted, irregular hyphae; conidia are rare or absent, so the colony color and slow growth carry the identification.
• Hair invasion: endothrix.

Trichophyton verrucosum

A zoophilic species acquired from cattle, causing inflammatory tinea (including kerion and tinea barbae) in farm and rural settings.

• Colony: very slow growing, small, heaped, white to cream, waxy.
• Growth: grows best at 37°C (warmer than the usual 25 to 30°C), and growth is enhanced by thiamine and inositol.
• Microscopy: conidia usually absent; chlamydospores in chains ("chains of pearls") are characteristic.

Trichophyton species at a glance

Differentiating T. rubrum from T. mentagrophytes

These two are the most frequently isolated dermatophytes, and separating them is the most common Trichophyton identification problem in the laboratory. Four features agree with each other, so you rarely need all four.

The memory anchor from the hub applies here: T. mentagrophytes is the "active" one. It perforates hair and splits urea. T. rubrum does neither and shows off its red reverse pigment instead. "Mentagrophytes moves, rubrum rests and reddens." The full procedures are in the hair perforation test and urease test articles.

How to Remember

Trichophyton = tricho = hair, and it takes all three tissues. The genus name comes from the Greek for hair (thrix), and it is the one genus that infects skin, hair, and nails. If a dermatophyte is in a nail, it is not Microsporum; if it is Trichophyton, it can be anywhere.

Micro over macro. Trichophyton is the microconidia genus: abundant microconidia, few thin-walled macroconidia. Microsporum is the opposite: big thick-walled macroconidia carry its identity. Same first syllable, opposite conidia emphasis, so learn them as a pair.

Mentagrophytes moves, rubrum rests and reddens. The two-species problem in one line: mentagrophytes is positive on hair perforation and urease; rubrum is negative on both and shows the red reverse.

Pencil versus cigar. T. rubrum macroconidia are long and pencil-shaped; T. mentagrophytes macroconidia are cigar-shaped. Both are thin-walled, unlike the thick spindle macroconidia of Microsporum.

Key exam facts

Where Students Get Confused

There is no "Trichophyton gypseum." Gypseum is a Microsporum species (Microsporum gypseum), a geophilic mould from soil. It is a common mix-up because the species names blur together, but the genus is Microsporum. If you see "gypseum," think soil and Microsporum, not Trichophyton.

Microconidia are the genus clue, not macroconidia. Students expect macroconidia to identify Trichophyton because macroconidia identify Microsporum. For Trichophyton it is the reverse: macroconidia are often scarce or absent, and the abundant microconidia carry the identification.

Endothrix and ectothrix are patterns, not species. T. mentagrophytes is ectothrix and T. tonsurans is endothrix, but the terms describe where the spores sit on KOH microscopy, not the organism's name. You report the pattern from the hair; the species comes from culture.

The red reverse of T. rubrum is on the underside. The surface of a T. rubrum colony is usually white to pinkish. The diagnostic deep red is the pigment diffusing into the agar, seen by turning the plate over. Students looking only at the surface miss it.

Colony color is a clue, not a confirmation. T. violaceum is genuinely violet and T. rubrum has a red reverse, but colony color alone never names a dermatophyte. It narrows the field; microscopy and differential tests confirm.

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.
• Su H, Packeu A, Ahmed SA, et al. Species distinction in the Trichophyton rubrum complex. J Clin Microbiol. 2019;57(9):e00352-19. https://doi.org/10.1128/JCM.00352-19