Microsporum in woods lamp
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Microsporum Wood's lamp fluorescence color dermatophyte tinea capitis
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Here is a complete, textbook-grounded answer:
Microsporum and Wood's Lamp (UV) Fluorescence
The Key Fact
Microsporum species cause bright blue-green fluorescence under Wood's lamp (long-wave UV light, ~365 nm). This is one of the most tested dermatology/microbiology facts in clinical exams.
Why Does It Fluoresce?
The fluorescence is produced by pteridine compounds (tryptophan metabolites) synthesized by Microsporum species as they infect the hair shaft. The infected hair itself glows - not the fungus directly.
Dermatophyte Fluorescence Table
| Organism | Fluorescence | Color |
|---|---|---|
| Microsporum canis | Yes | Bright blue-green |
| Microsporum audouinii | Yes | Bright blue-green |
| Microsporum gypseum | Yes (weak) | Dull yellow |
| Trichophyton schoenleinii | Yes (weak) | Dull blue (pale) |
| Trichophyton tonsurans | No | - |
| Trichophyton rubrum | No | - |
| Epidermophyton floccosum | No | - |
- Quick Compendium of Clinical Pathology, 5th ed., p. 148
- Red Book 2021 (AAP), Tinea Capitis section
- Fitzpatrick's Dermatology, Vol. 1-2
Clinical Context
- Microsporum causes ectothrix infection - hyphae and spores coat the outside of the hair shaft. This is why the fluorescence is visible on the hair.
- Trichophyton tonsurans (the most common cause of tinea capitis in the USA) causes endothrix infection (spores inside the shaft) and does NOT fluoresce - making Wood's lamp less useful in North America today.
- Wood's lamp was more diagnostically useful historically when M. canis and M. audouinii predominated as the cause of tinea capitis.
Exam Pearl Summary
Microsporum = Blue-Green fluorescence under Wood's lamp (due to ectothrix infection and pteridine metabolite production)
A negative Wood's lamp does not rule out tinea capitis, since T. tonsurans (endothrix, no fluorescence) now dominates in many regions. - Red Book 2021, AAP
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