Dermatophytes I — Complete Clinical Mycology Guide

Dr Harish M Nair

Sep 27

12 min read

Dermatophytes are the classic keratinophilic fungi responsible for infections of the skin, hair and nails. For a microbiologist, don't study them as three isolated genera. Build the chain:

Genus → conidia → colony → hair invasion → clinical syndrome → specimen → microscopy → culture

The three classical genera are:

Trichophyton
Microsporum
Epidermophyton

A major modern taxonomic point: molecular taxonomy has changed the classification of several dermatophytes, and some organisms historically placed in Trichophyton or Microsporum have been reassigned to other genera. However, the three-genus framework remains essential for traditional medical microbiology teaching and practical identification.

1. What are dermatophytes?

Dermatophytes are filamentous fungi that have an affinity for:

keratinized tissues

They primarily infect:

stratum corneum
hair
nails

They generally do not invade living tissue deeply in ordinary dermatophytosis.

Major clinical diseases are collectively called:

Dermatophytoses / tinea

The word tinea refers to a dermatophyte infection of a particular anatomical site.

2. The three classical genera

Feature	Trichophyton	Microsporum	Epidermophyton
Skin	Yes	Yes	Yes
Hair	Yes	Yes	No
Nail	Yes	Generally not important	Yes
Macroconidia	Present, variable	Numerous, characteristic	Numerous, characteristic
Microconidia	Usually present	Usually absent/rare	Absent
Important clue	Microconidia	Large rough spindle-shaped macroconidia	Smooth club-shaped macroconidia

The single most important table:

Trichophyton = hair + skin + nail

Microsporum = hair + skin, not nails

Epidermophyton = skin + nail, not hair

Memorize this before learning species.

3. TRICHOPHYTON

The name itself can help:

Tricho = hair

Trichophyton species can infect:

skin
hair
nails

They characteristically produce:

microconidia

and may produce macroconidia, although macroconidia are generally less prominent than in Microsporum/Epidermophyton.

4. Trichophyton microconidia

Microconidia are often:

numerous
unicellular
spherical
pyriform
tear-shaped
club-shaped depending on species.

They are often borne singly along the hyphae.

Classic appearance:

       o       o
        \     /
---------\---/----------
          |
     hypha
          |
      o   |   o

Think:

Trichophyton → lots of microconidia

5. Trichophyton macroconidia

Macroconidia are usually:

multicellular
septate
relatively elongated
cigar-shaped or pencil-shaped depending on species
less abundant than microconidia in many species.

A simplified drawing:

      _____________
     /             \
    | | | | | | |   |
     \_____________/
        septa

6. Important Trichophyton species

For clinical microbiology, know at least:

Trichophyton rubrum

Extremely important dermatophyte.

Associated with:

tinea pedis
tinea corporis
tinea cruris
tinea manuum
onychomycosis

Trichophyton interdigitale

Important cause of:

tinea pedis
tinea corporis
tinea cruris
other superficial dermatophytoses.

Trichophyton tonsurans

Important cause of:

tinea capitis

especially in some populations/geographic settings.

Trichophyton violaceum

Important cause of:

tinea capitis

and may produce chronic infection.

Trichophyton schoenleinii

Classically associated with:

favus

Trichophyton mentagrophytes

Traditional medical mycology terminology encompasses organisms that have undergone substantial taxonomic revision; modern identification may distinguish T. interdigitale and T. mentagrophytes complex organisms.

7. MICROSPORUM

Microsporum classically infects:

skin
hair

but:

does not typically infect nails

The classic laboratory clue is:

large, abundant, rough, spindle-shaped macroconidia

8. Microsporum macroconidia

Think:

M = Macro = Microsporum

They are typically:

large
multicellular
spindle-shaped/fusiform
thick-walled
rough-walled
several septa.

Simplified:

          /\
         /  \
        /----\
       /------\
      /--------\
      \--------/
       \------/
        \----/
         \  /
          \/

       spindle-shaped
       macroconidium

9. Microsporum microconidia

Microconidia are generally:

absent or not diagnostically prominent

This is why the distinction from Trichophyton is so useful.

10. Important Microsporum species

Microsporum canis

Very important.

Associated with:

tinea capitis
tinea corporis

Often associated with:

cats and dogs

Therefore, when a child has recurrent tinea capitis and there is a symptomatic pet at home, zoonotic transmission should enter your differential.

Microsporum audouinii

Classically associated with:

tinea capitis

and historically important in anthropophilic transmission.

Microsporum gypseum

Now generally classified as Nannizzia gypsea in modern taxonomy.

Historically important as a geophilic dermatophyte.

11. EPIDERMOPHYTON

The classic medically important species is:

Epidermophyton floccosum

It causes:

tinea cruris
tinea corporis
tinea pedis
tinea manuum
onychomycosis

But:

It does NOT infect hair.

This is one of the most frequently tested facts in dermatophytes.

12. Epidermophyton macroconidia

Macroconidia are:

smooth-walled
broad
club-shaped
multicellular
often arranged singly or in clusters.

Classic description:

club-shaped macroconidia

Simplified:

       ______
      /      \
     | | | | |
      \      /
       \____/
         |
         |

13. Epidermophyton microconidia

The key fact:

Microconidia are absent.

So:

Trichophyton
    ↓
Microconidia present

Microsporum
    ↓
Large spindle-shaped macroconidia

Epidermophyton
    ↓
Club-shaped macroconidia
    ↓
No microconidia

14. MASTER CONIDIA TABLE

This is worth memorizing almost verbatim.

Feature	Trichophyton	Microsporum	Epidermophyton
Microconidia	Present	Usually absent/rare	Absent
Macroconidia	Few/variable	Numerous, spindle-shaped	Numerous, club-shaped
Macroconidia wall	Thin/smooth or variable	Rough	Smooth
Hair infection	Yes	Yes	No
Nail infection	Yes	Generally no	Yes
Classic clue	Microconidia	Spindle	Club

15. THE CLASSIC EXAM MNEMONIC

T-M-E

Trichophyton:

Three sites — skin, hair, nail

Microsporum:

Missing nails

Epidermophyton:

Excludes hair

Or simply:

T = 3 tissues

M = hair + skin

E = skin + nail

16. TINEA

"Tinea" means dermatophyte infection involving a particular body site.

The suffix tells you the location.

17. TINEA CORPORIS

Ringworm of the body

Involves:

glabrous skin of the trunk and limbs.

Typical lesion:

annular
erythematous
scaly
advancing active border
central clearing.

Classic appearance:

        _____________
      /               \
     /   active        \
    |     border        |
    |      ______       |
    |     |      |      |
    |     |clear  |     |
     \    |center |    /
      \___|______|___/

The active border contains more fungal activity.

Diagnostic specimen

Skin scraping from the active edge

Direct examination

KOH

Look for:

hyaline septate hyphae.

18. TINEA CORPORIS — CLINICAL PEARL

Do not scrape the middle of the lesion if you can sample the active border.

Think:

"Edge contains the enemy."

19. TINEA CAPITIS

Infection of:

scalp hair and scalp

Usually occurs predominantly in children.

Clinical presentations can include:

scaling
alopecia
black-dot pattern
inflammatory kerion
favus

Important organisms include:

Trichophyton tonsurans
Trichophyton violaceum
Microsporum canis
other regionally important dermatophytes.

20. TINEA CAPITIS — LABORATORY DIAGNOSIS

Specimens:

affected hairs
scalp scales
broken hairs

Direct microscopy

KOH.

Look for:

fungal hyphae
arthroconidia
hair invasion pattern.

Culture

Important for identification.

21. HAIR INVASION

This is one of the most important concepts in dermatophyte mycology.

There are three major patterns to know:

Ectothrix
Endothrix
Favic

22. ECTOTHRIX

Break the word down:

Ecto = outside

In ectothrix infection:

Arthroconidia develop outside the hair shaft, surrounding it.

       ● ● ● ● ●
     ●             ●
    ●    HAIR       ●
    ●      │        ●
     ●     │       ●
       ● ● ● ● ●

     spores outside
     hair shaft

The fungus invades the hair and produces arthroconidia around the exterior.

23. ENDOTHRIX

Endo = inside

Arthroconidia are produced:

within the hair shaft

      _____________
     | ● ● ● ● ● ● |
     | ● ● ● ● ● ● |
     | ● ● ● ● ● ● |
      -------------
       spores inside
       hair shaft

24. ECTOTHRIX VS ENDOTHRIX

Feature	Ectothrix	Endothrix
Arthroconidia	Outside hair	Inside hair
Hair shaft	Coated externally	Filled internally
Fluorescence	Variable by species	Generally not fluorescent
Classic organisms	Microsporum spp., some Trichophyton	T. tonsurans, T. violaceum

The exact pattern varies by species.

25. WHY HAIR INVASION PATTERN MATTERS

It can help you predict:

organism group
epidemiology
Wood's lamp behavior
clinical presentation
laboratory identification.

This is a beautiful example of:

Microscopic morphology → organism identification

26. MICROSPORUM AND ECTOTHRIX

Many Microsporum species produce:

ectothrix invasion

Classic example:

Microsporum canis

often produces a large-spore ectothrix pattern.

Some Microsporum species can fluoresce under Wood's lamp.

27. TRICHOPHYTON AND ENDOTHRIX

Important anthropophilic Trichophyton species such as:

T. tonsurans

and

T. violaceum

classically produce:

endothrix invasion

The arthroconidia are within the hair shaft.

28. FAVIC INVASION

This is associated classically with:

Trichophyton schoenleinii

and the disease:

Favus

Instead of producing the typical mass of arthroconidia seen in ectothrix/endothrix patterns, the fungus produces characteristic:

air spaces/hyphae within the hair

with degeneration of the hair shaft.

29. FAVUS

Favus is a chronic form of tinea capitis.

Classical organism:

Trichophyton schoenleinii

Classic clinical lesion:

scutula

These are yellowish cup-shaped crusts.

They may be associated with:

mousy odor
alopecia
chronic inflammation
permanent scarring
hair destruction.

30. FAVIC HAIR

Classic microscopy:

       Hair shaft
   __________________
  |   \  \  \  \     |
  |    \  \  \  \    |
  |     air spaces   |
  |__________________|

       ↓
    favic hair

The term:

favic

therefore refers to the characteristic hair invasion pattern associated with favus.

31. WOOD'S LAMP

Although not specifically listed in your syllabus, you should connect it with hair invasion.

A Wood's lamp uses:

long-wave ultraviolet light

Some dermatophytes produce fluorescent metabolites.

Classic example

Microsporum canis

→ often produces:

greenish fluorescence

Some other Microsporum species also fluoresce.

However:

T. tonsurans and T. violaceum generally do not fluoresce.

Therefore:

A negative Wood's lamp does not exclude tinea capitis.

32. TINEA CRURIS

Groin infection

Commonly called:

"jock itch"

Typically affects:

groin
inner thighs
adjacent skin.

Classically:

itchy
erythematous
scaly
advancing border.

A useful clinical distinction:

Tinea cruris usually spares the scrotum, whereas candidal intertrigo commonly involves the scrotal skin.

But real-world presentations can overlap.

33. TINEA PEDIS

Athlete's foot

One of the most common dermatophyte infections.

Patterns include:

Interdigital

Most common form.

Moccasin type

Diffuse scaling of the sole and lateral foot.

Vesiculobullous

Vesicles/bullae may occur.

Ulcerative

More severe inflammatory disease, especially in susceptible hosts.

Important organisms:

T. rubrum
T. interdigitale

34. TINEA MANUUM

Dermatophyte infection of the hand.

Classically:

dry, scaly palm

and may occur with tinea pedis.

A classic clinical association:

"two feet, one hand" syndrome

i.e., bilateral tinea pedis with unilateral tinea manuum.

35. TINEA BARBAE

Dermatophyte infection involving:

beard and moustache region

Usually occurs in adult men.

Can involve:

beard hair
moustache hair
facial skin.

May be:

superficial
inflammatory
follicular.

Inflammatory cases can resemble bacterial folliculitis.

36. TINEA FACIEI

Dermatophyte infection of:

glabrous facial skin

Not including the beard area in the classic terminology.

May present as:

annular lesion
erythema
scaling
pruritus.

Diagnosis:

KOH of skin scraping from the active margin.

37. TINEA UNGUIUM

This term traditionally refers to:

dermatophyte infection of the nail

It is essentially dermatophytic onychomycosis.

Most commonly:

Trichophyton rubrum

Other Trichophyton species are also important.

38. IMPORTANT TERMINOLOGY

Onychomycosis

Any fungal infection of the nail.

Tinea unguium

Specifically:

dermatophyte infection of the nail

Therefore:

All tinea unguium is onychomycosis, but not all onychomycosis is tinea unguium.

Because nails may also be infected by:

Candida
non-dermatophyte moulds.

39. DERMATOPHYTE ECOLOGY

Another concept worth learning early:

Dermatophytes can be classified according to their usual ecological reservoir.

Anthropophilic

Primarily associated with humans.

Transmission:

human → human

Usually produce:

more chronic
less inflammatory
persistent infections.

Examples include:

T. rubrum
T. tonsurans

Zoophilic

Associated with animals.

Transmission:

animal → human

Often produces:

more inflammatory disease.

Example:

Microsporum canis

Geophilic

Associated with soil.

Transmission to humans is less common.

Often produces:

inflammatory infection.

Example historically/classically:

M. gypseum / Nannizzia gypsea.

40. ECOLOGY → CLINICAL PHENOTYPE

This is a useful conceptual relationship:

Anthropophilic
      ↓
Human adapted
      ↓
Chronic
      ↓
Less inflammation


Zoophilic
      ↓
Animal adapted
      ↓
Human is abnormal host
      ↓
More inflammation


Geophilic
      ↓
Soil adapted
      ↓
Human infection
      ↓
Often inflammatory

Not an absolute rule, but highly useful.

41. COLONY MORPHOLOGY

Don't learn colony morphology as a list of colors.

Always assess:

1. Growth rate

rapid
moderate
slow

2. Texture

powdery
granular
velvety
cottony
waxy

3. Surface

smooth
folded
heaped
radial grooves.

4. Pigmentation

Especially:

surface + reverse pigmentation

The reverse side of the colony can be extremely useful.

42. TRICHOPHYTON RUBRUM

Classic laboratory characteristics:

relatively slow-growing
white/cream surface
fluffy or velvety
often reddish/wine-red pigmentation on reverse

The classic clue:

red pigment on reverse

But remember:

Colony morphology can vary with strain, medium and incubation conditions.

Do not identify a dermatophyte solely from colony color.

43. TRICHOPHYTON INTERDIGITALE

Colonies can be:

white
cream
powdery
granular depending on strain.

Microscopy may show:

numerous microconidia.

44. MICROSPORUM CANIS

Classically:

white to cream
fluffy/cottony
yellowish pigmentation may be present
characteristic reverse pigmentation can occur.

Microscopy:

large rough spindle-shaped macroconidia

This is much more useful diagnostically than colony color alone.

45. EPIDERMOPHYTON FLOCCOSUM

Classic colony:

khaki/yellowish
suede-like/velvety to powdery
may develop radial grooves or folds with age.

Microscopy:

characteristic smooth, club-shaped macroconidia.

And:

no microconidia.

46. WHY CULTURE MORPHOLOGY MUST BE COMBINED WITH MICROSCOPY

Imagine:

"White colony."

That tells you very little.

But:

White colony + red reverse + numerous pyriform microconidia

is much more informative.

And:

Cottony colony + large rough spindle-shaped macroconidia

strongly points toward:

Microsporum-type morphology.

Therefore:

Colony + LPCB microscopy > colony alone.

47. MACROCONIDIA VS MICROCONIDIA

This is a fundamental practical skill.

Macroconidia

Large.

Usually:

multicellular
septate
more conspicuous.

Microconidia

Small.

Usually:

unicellular
much smaller
often produced abundantly by Trichophyton.

48. VISUAL COMPARISON

TRICHOPHYTON

   o   o   o   o
    \  |  |  /
-----\ | /-----
      \|/
      hypha

Lots of microconidia


MICROSPORUM

       /\
      /--\
     /----\
    /------\
    \------/
     \----/
      \--/

Large spindle macroconidium


EPIDERMOPHYTON

      ______
     /      \
    | | | | |
     \      /
      \____/

Club-shaped macroconidium

49. HAIR INVASION — MASTER TABLE

Pattern	Location of arthroconidia	Classic organisms
Ectothrix	Outside hair shaft	Microsporum spp.; some Trichophyton
Endothrix	Inside hair shaft	T. tonsurans, T. violaceum
Favic	Characteristic favus pattern with hyphae/air spaces	T. schoenleinii

50. TINEA CAPITIS — PRACTICAL ALGORITHM

Patient:

Child + scalp scaling + alopecia.

Think:

Step 1

Collect:

affected hair + scalp scales.

Step 2

Direct microscopy:

KOH ± calcofluor.

Step 3

Determine:

ectothrix vs endothrix vs favic pattern.

Step 4

Culture.

Step 5

LPCB examination.

Look for:

microconidia
macroconidia
other characteristic structures.

Step 6

Consider Wood's lamp.

Step 7

Species identification.

This is much better than simply memorizing "tinea capitis = Trichophyton."

51. TINEA — COMPLETE CLINICAL MAP

              DERMATOPHYTES
                    │
     ┌──────────────┼──────────────┐
     ↓              ↓              ↓
   SKIN            HAIR           NAIL
     │              │              │
     ↓              ↓              ↓
 corporis        capitis       unguium
 cruris          barbae
 pedis
 manuum
 faciei

52. CLINICAL SYNDROME TABLE

Tinea	Site	Important organisms
Corporis	Body/trunk/limbs	T. rubrum, Microsporum spp.
Capitis	Scalp/hair	T. tonsurans, T. violaceum, M. canis
Cruris	Groin	T. rubrum, T. interdigitale
Pedis	Feet	T. rubrum, T. interdigitale
Manuum	Hands	T. rubrum
Barbae	Beard	Trichophyton spp.
Faciei	Face	Trichophyton/Microsporum spp.
Unguium	Nails	T. rubrum, other Trichophyton spp.

The precise species distribution varies geographically and over time.

53. A VERY IMPORTANT CLINICAL CONNECTION

If you see:

Tinea pedis + nail dystrophy

think:

possible dermatophyte onychomycosis

because the foot can act as a reservoir for recurrent nail infection.

Similarly:

tinea pedis + tinea manuum

is a classic pattern.

54. SPECIMEN COLLECTION FOR DERMATOPHYTES

Skin

Scrape active edge.

Nail

Nail clipping/scraping + subungual debris.

Hair

Pluck affected hairs + collect scales.

Then:

KOH → culture → microscopy of culture

55. WHAT DOES KOH SHOW?

For dermatophytes:

hyaline, septate, branching hyphae

You may also see:

arthroconidia.

KOH does not normally give you species identification.

It answers:

"Are fungal elements present?"

56. CULTURE

Culture helps establish:

which dermatophyte is present

Traditional identification uses:

colony morphology
growth characteristics
pigmentation
microscopic morphology.

Modern laboratories may additionally use:

MALDI-TOF
PCR
sequencing
other molecular methods.

57. LPCB PRACTICAL

Once a mould-like colony grows:

prepare an LPCB mount.

Look for:

Trichophyton

microconidia ± macroconidia.

Microsporum

large spindle-shaped macroconidia.

Epidermophyton

club-shaped macroconidia + no microconidia.

This is where your theoretical morphology becomes actual laboratory diagnosis.

58. HIGH-YIELD IDENTIFICATION ALGORITHM

When you see a dermatophyte colony:

          DERMATOPHYTE
               ↓
        Examine LPCB mount
               ↓
       ┌───────┼────────┐
       ↓       ↓        ↓
   Microconidia?     Large macroconidia?
       ↓                    ↓
      YES              ┌────┴────┐
       ↓               ↓         ↓
Trichophyton       spindle?    club?
                       ↓         ↓
                  Microsporum  Epidermophyton

This is a starting algorithm, not a complete species identification system.

59. PRACTICAL DRAWINGS YOU SHOULD MASTER

For dermatophytes, I would practice these six diagrams.

1. Trichophyton

   o  o   o
    \ | /
-----\|/------
      |
      |____

Label:

hypha
microconidia
macroconidium if present.

2. Microsporum

        /\
       /--\
      /----\
     /------\
     \------/
      \----/
       \--/

Label:

rough, spindle-shaped macroconidium.

3. Epidermophyton

       ______
      /      \
     | | | | |
      \      /
       \____/
          |

Label:

smooth, club-shaped macroconidium.

4. Ectothrix

    ● ● ● ● ●
  ●     │     ●
 ●      │      ●
  ●     │     ●
    ● ● ● ● ●

    Hair shaft
    surrounded
    externally

5. Endothrix

    ___________
   | ● ● ● ● ● |
   | ● ● ● ● ● |
   | ● ● ● ● ● |
   |___________|

 Arthroconidia
 INSIDE hair

6. Favic hair

    ______________
   | \   \   \    |
   |  \   \   \   |
   | air spaces   |
   |______________|

60. EXAM-LEVEL RAPID FIRE

Which dermatophyte genus infects hair, skin and nails?

Trichophyton

Which classical genus does not infect hair?

Epidermophyton

Which classical genus generally does not infect nails?

Microsporum

Which produces abundant microconidia?

Trichophyton

Which produces large spindle-shaped macroconidia?

Microsporum

Which produces club-shaped macroconidia?

Epidermophyton

Which lacks microconidia?

Epidermophyton

Classic organism causing favus?

Trichophyton schoenleinii

Classic favic hair invasion?

Favic

Arthroconidia outside hair?

Ectothrix

Arthroconidia inside hair?

Endothrix

Classic tinea pedis organism?

Trichophyton rubrum

Important tinea capitis organism?

T. tonsurans

Zoophilic dermatophyte associated with cats/dogs?

Microsporum canis

61. THE FIVE THINGS I WOULD MEMORIZE TONIGHT

If you're preparing as a postgraduate microbiologist, make these automatic:

1.

Trichophyton = hair + skin + nail

2.

Microsporum = hair + skin

3.

Epidermophyton = skin + nail, not hair

4.

Ectothrix = outside; Endothrix = inside

5.

Trichophyton → microconidia; Microsporum → spindle macroconidia; Epidermophyton → club macroconidia

62. The deeper clinical-microbiology connection

The really important sequence is:

Patient → lesion → specimen → KOH → fungal morphology → culture → LPCB → conidial morphology → identification

For example:

Child with tinea capitis

↓

affected hair + scale

↓

KOH

↓

fungal hyphae/arthroconidia

↓

hair invasion pattern

↓

culture

↓

LPCB

↓

large rough spindle-shaped macroconidia

↓

Microsporum-type dermatophyte

↓

correlate with clinical/epidemiological findings

↓

species identification.

That is the workflow you want to eventually execute almost automatically at the microscope.

One important modern-taxonomy caveat

For postgraduate practice, don't let the traditional Trichophyton–Microsporum–Epidermophyton framework become a rigid modern taxonomy. Molecular phylogeny has resulted in reassignment of several dermatophytes—for example, organisms historically called Microsporum gypseum are now generally placed in Nannizzia, and the T. mentagrophytes/interdigitale complex has undergone substantial taxonomic refinement. The traditional morphology-based framework remains extremely useful for learning and practical microscopy, while species-level reporting increasingly uses modern identification methods.

Core references for this chapter

Larone — Medically Important Fungi
Koneman — Color Atlas and Textbook of Diagnostic Microbiology
Bailey & Scott — Diagnostic Microbiology
Murray — Medical Microbiology
Ananthanarayan & Paniker — Textbook of Microbiology
Mandell, Douglas & Bennett — Principles and Practice of Infectious Diseases
CDC — Ringworm/Dermatophyte resources
Current dermatophyte taxonomy and identification literature

The next major step after this is Dermatophytes II: individual species in depth—T. rubrum, T. interdigitale, T. tonsurans, T. violaceum, T. schoenleinii, M. canis, M. audouinii, Epidermophyton floccosum—including colony photographs, LPCB morphology, Wood's lamp, ecology, transmission, clinical presentations, KOH findings, culture media, differential diagnosis and species-level identification algorithms.