Dermatophytes II — Identification

Dr Harish M Nair

Sep 27

10 min read

Absolutely. Dermatophytes II should be approached as a laboratory identification exercise, not just a morphology chapter. The key is to move from KOH → colony morphology → LPCB → physiological tests → species-level identification.

1. The master identification table

Fungus / genus	Macroconidia	Microconidia	Hair invasion	Important clue
Trichophyton	Usually few, variable; pencil/cigar-shaped, thin-walled, septate when present	Usually abundant; spherical, pyriform, tear/club-shaped depending species	Yes	Microconidia prominent; skin + hair + nail
Microsporum	Numerous, large, spindle/fusiform, thick and often rough-walled	Usually absent or sparse	Yes	Large rough spindle-shaped macroconidia; usually no nail infection
Epidermophyton	Numerous, smooth, broad club-shaped, septate	Absent	No hair invasion	Club-shaped macroconidia + no microconidia

The fastest exam recognition

Trichophyton = MicroconidiaMicrosporum = MacroconidiaEpidermophyton = only macroconidia

And:

T = Three tissues → skin + hair + nailM = Missing nails → skin + hairE = Excludes hair → skin + nail

2. Identification workflow

When you receive an unknown dermatophyte culture:

Clinical specimen
       ↓
Direct KOH
       ↓
Hyaline septate fungal hyphae?
       ↓
Culture on appropriate fungal media
       ↓
Colony morphology
 ┌─────┴─────┐
Surface     Reverse
colour      pigment
Texture     Growth rate
       ↓
LPCB / slide culture
       ↓
Macroconidia + microconidia
       ↓
Urease / hair perforation / rice grain
       ↓
Species identification

The important principle:

Do not identify a dermatophyte from colony colour alone.

You combine:

Colony morphology
Microscopic morphology
Growth characteristics
Urease
Hair perforation
Rice grain growth
DTM reaction
Clinical/ecological information

3. Urease test

Principle

Some dermatophytes produce urease, which hydrolyses urea:

Urea → ammonia → increased pH

The alkaline pH causes a colour change in the indicator.

Practical use

Urease is particularly useful in distinguishing members of the T. mentagrophytes/interdigitale complex from T. rubrum.

Dermatophyte	Urease
T. mentagrophytes/interdigitale complex	Usually positive
T. rubrum	Usually negative
E. floccosum	Usually negative
M. canis	Variable/not routinely relied upon

Classic exam question

T. rubrum vs T. mentagrophytes

Feature	T. rubrum	T. mentagrophytes/interdigitale
Urease	Negative	Positive
Microconidia	Numerous, often along hyphae	Numerous
Microconidia arrangement	Often teardrop/club-shaped	Often grape-like clusters
Macroconidia	Few	More frequent
Reverse pigment	Often red/wine-red	Usually yellowish/brownish or no striking red
Clinical association	Chronic tinea pedis/onychomycosis	Tinea pedis, inflammatory infections

Important caveat

Urease is supportive, not an absolute species-identification test.

4. Hair perforation test

This is one of the classic physiological tests.

Principle

Certain dermatophytes produce an enzyme system capable of penetrating/perforating the hair shaft.

In the test, sterile hair is incubated with the fungus.

After incubation, the hair is examined microscopically.

Positive test

Characteristic wedge-shaped perforations develop along the hair shaft.

Important classic distinction

Dermatophyte	Hair perforation
T. mentagrophytes	Positive
T. rubrum	Negative

This is particularly useful when trying to separate these two Trichophyton groups.

Memory

Mentagrophytes = Makes holes

Not every isolate behaves perfectly, so again, interpret with morphology and other tests.

5. Rice grain test

Another classical identification test.

Principle

The dermatophyte is inoculated onto sterile rice grains and observed for growth and pigmentation.

It can help distinguish dermatophytes based on their ability to grow and pigment on rice.

Why is it useful?

Some dermatophytes show characteristic growth patterns or pigment production on rice, making it a supplementary identification test.

It is particularly associated with classical differentiation of Trichophyton species.

Important concept

Rice grain testing is not a standalone identification method.

Think:

Morphology first → physiological tests for confirmation.

Modern laboratories may use molecular identification or MALDI-TOF where validated, so these classical tests are increasingly confirmatory rather than definitive.

6. Dermatophyte Test Medium — DTM

DTM is a selective/differential medium designed to facilitate recovery and recognition of dermatophytes.

Principle

The medium contains:

Nutrients
Selective agents that suppress many contaminating organisms
A pH indicator

Dermatophyte metabolism can produce alkaline products, causing a colour change.

Typical interpretation

A dermatophyte may cause:

yellow/orange → red

colour change due to alkalinization.

Major caution

DTM colour change ≠ automatically dermatophyte.

Some non-dermatophyte fungi can produce similar reactions.

Therefore:

DTM tells you “suspect dermatophyte”; microscopy tells you what organism you have.

Practical approach

If DTM becomes red:

Red DTM
  ↓
Examine colony
  ↓
LPCB / microscopy
  ↓
Look for dermatophyte conidia
  ↓
Confirm identification

7. Colony pigmentation

Colony morphology is incredibly important.

Always record:

A. Rate of growth

Rapid
Moderate
Slow

B. Surface

Powdery
Velvety
Cottony
Granular
Suede-like
Smooth

C. Colour

Observe:

Top + reverse

D. Reverse pigmentation

Often particularly useful.

8. Major species — colony + microscopy

Trichophyton rubrum

One of the most important dermatophytes clinically.

Colony

Typically:

Slow-growing
White to cream surface
Velvety/powdery
Characteristic red to wine-red reverse pigment in many strains

But pigmentation can be variable.

Microscopy

Numerous microconidia
Often pyriform/tear-shaped
Microconidia commonly arise singly from hyphae
Macroconidia uncommon

Important tests

Test	Result
Urease	Negative
Hair perforation	Negative
Reverse pigment	Often red
Microconidia	Present

Classic identification pattern

Red reverse + urease negative + hair perforation negative → think T. rubrum

9. Trichophyton mentagrophytes/interdigitale group

This group deserves special attention because modern taxonomy has separated historically overlapping organisms, and older textbooks may use terminology differently.

Colony

Often:

White/cream
Powdery or granular
May develop radial grooves

Microscopy

Characteristic:

Numerous microconidia
Often arranged in clusters
“Grape-like” appearance
Spiral hyphae may be seen
Macroconidia can occur

Physiological tests

Test	Typical result
Urease	Positive
Hair perforation	Positive
T. rubrum-like red reverse	Usually absent

Classic comparison

T. mentagrophytes = urease +, hair perforation +

versus

T. rubrum = urease –, hair perforation –

10. Trichophyton tonsurans

Important cause of tinea capitis.

Microscopy

Microconidia are highly variable:

Teardrop
Club-shaped
Irregular
Along the hyphae

Macroconidia are uncommon.

Clinical clue

Tinea capitis + endothrix hair invasion

Think:

T. tonsurans

Wood's lamp is generally negative.

11. Trichophyton violaceum

Colony

Characteristic:

Slow-growing
Violet/purple pigmentation
Folded or heaped colony

Microscopy

Often poorly sporulating.

This is important.

You may have to rely heavily on:

Colony morphology
Clinical context
Hair invasion
Molecular methods where necessary

Clinical association

Tinea capitis, particularly endothrix infection.

12. Trichophyton schoenleinii

The classic cause of:

Favus

Microscopy

May show:

Poor sporulation
Favic type of hair invasion
Hyphae within hair
Characteristic air spaces

Clinical

Scutula → crusted lesions → scarring alopecia

Think:

Favus = T. schoenleinii

13. Microsporum canis

Very important dermatophyte.

Colony

Typically:

Rapid/moderately growing
White/cottony/fluffy surface
Yellowish pigmentation may be present

Microscopy

Classic:

Large spindle-shaped macroconidia

Features:

Numerous
Multicellular
Thick-walled
Rough/echinulate surface
Fusiform/spindle-shaped
Several septa

Microconidia

Usually sparse or absent.

Clinical

Especially:

Tinea capitis
Tinea corporis
Exposure to cats/dogs

Wood's lamp

Classically:

Greenish fluorescence

But remember:

A negative Wood's lamp does not exclude dermatophytosis.

14. Microsporum audouinii

Important anthropophilic dermatophyte.

Clinical

Tinea capitis.

Microscopy

Macroconidia can be sparse.

Therefore, identification may rely on:

Colony morphology
Microscopy
Clinical context
Molecular methods when needed

15. Nannizzia gypsea

Older literature:

Microsporum gypseum

Modern taxonomy places this organism in Nannizzia.

Ecology

Geophilic

Microscopy

Large spindle-shaped macroconidia.

Compared with M. canis, the macroconidia are generally:

More symmetrical
Thinner-walled
Less rough

Clinical

Can cause inflammatory:

Tinea corporis
Tinea capitis

16. Epidermophyton floccosum

The species you should immediately associate with the genus.

Colony

Slow/moderate growth
Khaki/yellowish/olive tones
Velvety to suede-like
Radial grooves/folds may develop

Microscopy

The diagnostic clue:

Broad, smooth, club-shaped macroconidia

They are:

Multicellular
Septate
Smooth-walled
Club-shaped

Microconidia

Absent

Hair

Does not infect hair

Classic identification

Club-shaped macroconidia + no microconidia + no hair invasion = E. floccosum

17. Master morphology table

Species	Macroconidia	Microconidia	Special morphology
T. rubrum	Few	Numerous	Tear/pyriform microconidia; red reverse
T. mentagrophytes/interdigitale	More common	Numerous	Clustered “grape-like” microconidia; spiral hyphae
T. tonsurans	Few	Numerous, variable	Variable microconidia
T. violaceum	Usually sparse	Sparse/variable	Poor sporulation, violet colony
T. schoenleinii	Poor	Poor	Favic morphology
M. canis	Numerous, spindle-shaped, rough	Few/absent	Large fusiform macroconidia
M. audouinii	Sparse	Sparse	Often poor sporulation
Nannizzia gypsea	Numerous spindle-shaped	Few	Geophilic
E. floccosum	Numerous, smooth club-shaped	Absent	Club macroconidia

18. Microscopy — what should your eye immediately recognize?

Trichophyton

Imagine:

Hypha ─────●────●────●────●
            ↑    ↑
        microconidia

Microconidia arise along the hyphae.

Some species produce characteristic clusters.

Microsporum

Think:

          /\
        /    \
       /      \
      |  | |  |
       \      /
        \____/
      spindle-shaped
       macroconidium

The large spindle-shaped macroconidium is the giveaway.

Epidermophyton

Think:

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

Broad club-shaped macroconidium.

No microconidia.

19. Identification of the three genera — practical unknown

Suppose you receive an unknown dermatophyte.

Step 1 — KOH

You see:

Hyaline, septate, branching hyphae.

This establishes fungal infection but doesn't identify the genus.

Step 2 — Culture

Suppose you see a white colony.

Still insufficient.

Step 3 — LPCB

Now you see:

A. Numerous microconidia

Think:

Trichophyton

B. Large rough spindle macroconidia

Think:

Microsporum

C. Broad smooth club-shaped macroconidia and no microconidia

Think:

Epidermophyton

That's the core diagnostic skill.

20. The important T. rubrum vs T. mentagrophytes problem

This is one of the most useful laboratory exercises.

Feature	T. rubrum	T. mentagrophytes/interdigitale
Microconidia	Numerous	Numerous
Arrangement	Usually solitary along hyphae	Often clustered
Spiral hyphae	Usually absent	May be present
Urease	Negative	Positive
Hair perforation	Negative	Positive
Reverse pigment	Often red	Usually not classic red
Clinical	Chronic tinea/onychomycosis	Tinea pedis and inflammatory lesions

Exam trigger

White colony + grape-like microconidia + spiral hyphae + urease positive

→ T. mentagrophytes/interdigitale group

21. Clinical Cases — 10 Unknown Dermatophytes

Now let's make this into a microbiology resident-level spotter exercise.

Try to identify each before looking at the answer.

Case 1

A 24-year-old man has chronic scaling between the toes and thickened toenails.

KOH:

Hyaline septate hyphae.

Culture:

Slow-growing white colony
Red/wine-coloured reverse pigment

LPCB:

Numerous small pyriform microconidia
Few macroconidia

Urease: negative.

What is the organism?

Answer: Trichophyton rubrum

Why?

The combination of:

Red reverse + numerous microconidia + urease negative

is highly characteristic.

Case 2

A 19-year-old athlete has inflammatory tinea pedis.

Culture:

White powdery colony

LPCB:

Numerous spherical/pyriform microconidia
Clustered around hyphae
Spiral hyphae present

Urease: positive.

Hair perforation: positive.

Answer

Trichophyton mentagrophytes/interdigitale group

Key clue

Grape-like microconidia + spiral hyphae + urease positivity.

Case 3

A 7-year-old child has tinea capitis with patchy alopecia.

Culture produces a fluffy white colony.

LPCB:

Numerous large spindle-shaped macroconidia
Thick rough walls
Several septa

History:

The child has a kitten at home.

Answer

Microsporum canis

Key clues

Pet exposure + tinea capitis + large rough spindle macroconidia.

Case 4

A 10-year-old child has tinea capitis.

KOH of infected hair shows fungal elements inside the hair shaft.

Culture:

Slowly growing violet/purple colony
Poor sporulation

Answer

Trichophyton violaceum

Key clues

Endothrix + violet colony + poor sporulation.

Case 5

A 9-year-old child develops black-dot tinea capitis.

Culture shows a dermatophyte.

Microscopy:

Numerous variable-shaped microconidia
Few macroconidia

Hair invasion is endothrix.

Answer

Trichophyton tonsurans

Key clue

Endothrix tinea capitis + variable microconidia + generally nonfluorescent Wood's lamp.

Case 6

A 30-year-old man has chronic tinea cruris and onychomycosis.

Culture:

Khaki/yellowish colony
Velvety/suede-like surface

LPCB:

Broad smooth club-shaped macroconidia
No microconidia

Hair invasion absent.

Answer

Epidermophyton floccosum

Key clue

Club macroconidia + no microconidia.

Case 7

A patient has a chronic scalp infection with crusting and scarring alopecia.

Characteristic yellowish crusts/scutula are present.

Microscopy demonstrates favic hair invasion and relatively poor sporulation.

Answer

Trichophyton schoenleinii

Diagnosis

Favus

Key clue

Scutula + scarring alopecia + favic infection.

Case 8

A 35-year-old gardener develops a highly inflammatory annular lesion.

Culture produces a rapidly growing dermatophyte.

Microscopy:

Numerous large spindle-shaped macroconidia
Smooth to relatively thin walls
Geophilic ecological association

Answer

Nannizzia gypsea

Older name: Microsporum gypseum

Key clue

Geophilic dermatophyte + spindle macroconidia + inflammatory infection.

Case 9

A patient has tinea corporis.

Culture shows a dermatophyte with abundant large spindle-shaped macroconidia.

The macroconidia have:

Rough walls
Multiple septa
Characteristic fusiform shape

Microconidia are absent/sparse.

Answer

Microsporum

If the morphology is strongly typical and clinical context fits:

Microsporum canis would be a major consideration.

Key clue

Large rough spindle macroconidia = Microsporum.

Case 10 — The tricky one

A patient has chronic tinea pedis.

Culture:

White colony
No obvious red reverse pigment

LPCB:

Numerous microconidia
Some clustered arrangement
Spiral hyphae seen

Urease: positive.

Hair perforation: positive.

Answer

T. mentagrophytes/interdigitale group

Why is this case important?

Because you should not diagnose T. rubrum merely because you see numerous microconidia.

Both can have numerous microconidia.

The decisive supportive features are:

Clustered microconidia + spiral hyphae + urease positivity + hair perforation positivity

22. Bonus Case — a laboratory trap

A culture from a skin scraping turns the DTM indicator red.

The colony looks mould-like.

The resident immediately reports:

“Dermatophyte isolated.”

Is that enough?

No.

DTM is a screening/differential medium, not definitive species identification.

You should perform:

Colony examination → LPCB/slide culture → conidial morphology → correlation with direct microscopy and clinical picture.

This is a very important laboratory principle:

Never let the medium make the identification for you.

23. Hair invasion — extremely important

Pattern	Meaning	Classic organisms
Ectothrix	Arthroconidia outside hair shaft	Many Microsporum spp.; some Trichophyton spp.
Endothrix	Arthroconidia inside hair shaft	T. tonsurans, T. violaceum
Favic	Chronic favus pattern	T. schoenleinii

Wood's lamp

Classically:

M. canis → green fluorescence

But:

Wood's lamp is supportive, not definitive.

24. The complete identification matrix

Feature	T. rubrum	T. mentagrophytes/interdigitale	M. canis	E. floccosum
Microconidia	+++	+++	±	–
Macroconidia	Few	+/++	+++	+++
Macroconidia shape	Pencil/cigar if present	Variable	Spindle	Club
Macroconidia wall	Thin	Thin/variable	Rough	Smooth
Urease	–	+	Variable/not key	Usually –
Hair perforation	–	+	Not primary test	Not applicable
Hair infection	+	+	+	–
Nail infection	+	+	Usually –	+
Red reverse	Often +	Usually –	–/variable	–
Major clue	Red reverse	Grape-like microconidia	Rough spindle macroconidia	Club macroconidia

25. How I want you to examine every dermatophyte culture

As a microbiologist, don't just look at the colony and say a name.

Train yourself to ask seven questions:

1. How fast did it grow?

Slow / moderate / rapid?

2. What does the surface look like?

Powdery / granular / velvety / cottony / suede?

3. What is the reverse?

This is often extremely informative.

4. What conidia are present?

Macro or micro?

5. What is their shape?

Spindle?

Pencil?

Club?

Pyriform?

6. What do the physiological tests show?

Urease?

Hair perforation?

Rice grain?

7. Does everything fit the clinical syndrome?

Because:

A laboratory identification is strongest when morphology + physiology + clinical context agree.

26. Final rapid-recall sheet

Genus

Trichophyton

→ Microconidia→ Skin + hair + nail

Microsporum

→ Large spindle macroconidia→ Skin + hair→ Usually not nail

Epidermophyton

→ Club-shaped macroconidia→ No microconidia→ No hair

Species

T. rubrum

→ Red reverse→ Urease –→ Hair perforation –

T. mentagrophytes/interdigitale

→ Grape-like microconidia→ Spiral hyphae→ Urease +→ Hair perforation +

T. tonsurans

→ Endothrix→ Tinea capitis

T. violaceum

→ Violet colony→ Endothrix→ Poor sporulation

T. schoenleinii

→ Favus→ Scutula→ Scarring alopecia

M. canis

→ Cats/dogs→ Large rough spindle macroconidia→ Green Wood's fluorescence classically

Nannizzia gypsea

→ Geophilic→ Spindle macroconidia→ Often inflammatory

E. floccosum

→ Club-shaped macroconidia→ No microconidia→ No hair invasion

The single most important mental picture

If you are shown an unknown dermatophyte under the microscope:

Tiny numerous conidia → Trichophyton

Huge rough spindle conidium → Microsporum

Broad smooth club conidium → Epidermophyton

Then use urease + hair perforation + colony/reverse pigmentation + clinical context to narrow the species.

That is the level at which I would want you to master dermatophyte identification as a clinical microbiology resident.