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Microbiology · Molds and dermatophytes

Dermatophytes (Trichophyton, Microsporum, Epidermophyton) !

  • Fungus
  • Exam priority: medium

Fungi that feed on keratin and cause tinea of the skin, scalp, groin, feet and nails. Spread by contact with people, animals or objects.

Updated 2026-11-01

Also called: ringworm, tinea, athlete's foot, nail fungus

At a glance

Shape
Septate hyphae and arthroconidia in scale, hair or nail
Family
Arthrodermataceae (Onygenales)
Type
Keratinophilic molds (do not invade living tissue)
Stain
KOH: branching septate hyphae; Wood's lamp: some Microsporum species fluoresce
Size
Hyphae about 2–4 µm wide

Morphology

  • In scale on KOH: septate hyphae that branch and break into arthroconidia
  • Trichophyton: many microconidia; Microsporum: large rough-walled macroconidia; Epidermophyton: club-shaped macroconidia, no microconidia
  • In hair they can invade inside the shaft (endothrix) or outside it (ectothrix)

Transmission

  • Direct skin contact with infected people (anthropophilic, such as T. rubrum)
  • Contact with animals, mainly cats and dogs (zoophilic, such as M. canis)
  • Contact with soil (geophilic)
  • Shared objects and wet floors: towels, combs, showers, pools

Reservoir: Humans; Cats, dogs and livestock; Soil

Isolation precautions

What the care team adds to standard precautions.

  • Standard

Virulence factors

  • Keratinases that digest keratin
  • Grow in the stratum corneum, hair and nail; tolerate deep body temperature poorly
  • Inflammation is stronger with animal or soil species

Diseases it causes

Clinical picture

  • Tinea corporis: annular plaque with a scaly active border and clearer centre
  • Tinea capitis in children: patches with broken hairs and scaling; kerion is an inflammatory form
  • Tinea pedis: scaling and cracks between the toes
  • Onychomycosis: thick, yellow, brittle nail, mostly toenails
  • Tinea incognito: atypical look after steroid creams

Diagnosis

  • KOH direct exam of scale, hair or nail
  • Culture on Sabouraud (or dermatophyte medium) for the species
  • Wood's lamp: green fluorescence in some Microsporum tinea
  • Biopsy with PAS in doubtful onychomycosis

Treatment

Skin tinea: topical antifungals (azoles such as clotrimazole or allylamines such as terbinafine). Scalp tinea and onychomycosis: oral treatment (terbinafine, griseofulvin, itraconazole). Terbinafine-resistant strains (such as T. indotineae) have emerged; steroid creams make tinea worse.

Drug class only; doses and regimens are set by the clinician from the current guideline.

In Pharmacology:

Prevention

  • Dry feet and skin folds well; footwear in showers and pools
  • Do not share towels, combs or caps
  • Treat pets with lesions
  • Confirm the diagnosis before treating, to avoid needless steroids and antifungals

Test yourself

Think of the answer, then tap the question to see it.

Which quick office test confirms tinea?

A KOH direct exam of scale, hair or nail, showing septate hyphae.

Which dermatophyte does not affect hair?

Epidermophyton (it affects skin and nails).

Why is tinea capitis not treated with creams alone?

The fungus sits inside the follicle and hair shaft, where cream does not reach; it needs an oral antifungal.

What happens if a steroid cream is put on tinea?

The infection worsens and changes its look (tinea incognito), which makes diagnosis harder.

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Educational content on microorganisms; not a treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Ringworm Basics (CDC, 2024); Clinical Overview of Ringworm (CDC, 2024); Ringworm: MedlinePlus Medical Encyclopedia (MedlinePlus (NLM), 2024).

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Representative model, for educational purposes only.