Saturday 1 September 2012

Trichophyton tonsurans

Trichophyton tonsurans (Fungus, Dermatophyte)

Pathogenicity and Distribution: Trichophyton tonsurans is a cosmopolitan (found worldwide) dermatophyte (a fungus of skin, nails or hair).  It is the etiologic agent most frequently implicated in ‘ringworm’ infections of the scalp (Tinea capitis) in America.  Causes an endothrix[i] infection of the hair (penetrates hair shaft to grow within).  T.tonsurans is anthropophilic (prefers humans to animals) however sources vary on its infectivity.  One prominent source[ii] suggests T.tonsurans is Zoophilic, but is frequently transmitted to man.  Others have speculated that equine strains may have mutated to become anthropophilic.  Infections are more commonly found in heavily populated urban regions

Macroscopic Morphology: T.tonserans may have highly variable colony morphology.  Surface growth may be white, beige, greyish or pale to sulphurous yellow, rose coloured to brownish.  The surface texture also can vary from velvety or powdery to suede-like, often with radial or concentric furrows.

 Trichopyton tonsurans: SAB agar at 30o C after 14 days incubation, (Nikon)

Microscopic Morphology:  T.tonsurans produces septate hyphae.  The most prominent feature is the numerous microconidia formed along the hyphae or on short conidiophores which grow perpendicular to the originated hyphae.  The microconidia are sessile (attached by a ‘base’ rather than a ‘stalk’).  They can also be highly variable is shape ranging from pyriform (tear-drop) to clavate (club-like) to cylindrical and even larger round balloon-like forms.  Macroconidia are usually rare and also show variation in shape & size from cylindrical to cigar shaped (10 – 65 µm by 4 – 12 µm).  They are somewhat thick walled with a smooth surface and usually contain between 2 to 4 cells within each.  Terminal and intercalary chlamydospores may also be present, particularly in older cultures.  Another source[iii] has stated that spiral coils and arthroconidia may be present.

 Trichophyton tonsurans: Slide culture - First look at low power shows a mass of hyphae with microconidia visible on hyphae towards lower right of photograph.  (LPCB, DMD-108, X250)

 Trichophyton tonsurans: A closer look shows the microconidia growing along the hyphae.  This typical "birds on a wire" appearance is typical of Trichophyton species.  (LPCB, DMD-108, X400)

Trichophyton tonsurans:  Another look, as above. microconidia growing along hyphae.  Arrow shows a widening of the terminal end of a hyphal element, perhaps a young macroconidium or development of arthrospores?
  (LPCB, DMD-108, X400)

Trichophyton tonsurans: Microconida growing along the septate hyphae.
(LPCB, DMD-108, X1000)

 Trichophyton tonsurans: Another view showing the highly variable shape of the microconidia as well as an intercalary chlamydospore at the middle left of the photograph.
(LPCB, DMD-108, X1000)

Trichophyton tonsurans: Septate hyphae with microconida growing along its length.
(LPCB, DMD-108, X1000)

Trichophyton tonsurans:  Microconidia alongside septate hyphae with possible development of macroconidia at top left of photograph.  (LPCB, DMD-108, X1000)

Trichophyton tonsurans:  Again, microconidia along the length of a septate hyhal element.
(LPCB, DMD-108, X1000+10)

Trichophyton tonsurans:  Thickening of hyphal element - development of macroconida?  or arthrospores?  (LPCB, DMD-108, X1000)

Trichophyton tonsurans: two intercalary chlamydospores within the hyphae.
(LPCB, DMD-108, X1000+10)

Trichophyton tonsurans: Intercalary chlamydospore in an older culture.
(LPCB, DMD-108, Chlamydospore)

Trichophyton tonsurans:  Macroconidium - three visible divisions within.  T.tonsurans' macroconidia often show an irregular or wavy (undulating/ S-shaped) structure.
(LPCB, DMD-108, X1000+10)

Trichophyton tonsurans: Macroconidia in center of photograph.  Note micron bar at top of photo.
(LPCB, DMD-108, X1000)

Trichophyton tonsurans:  Stepping back a magnification, this photo shows the microconidia (MI) along the length of various hyphae, a possible macroconidium (MA) and what appears to be an intercalary chlamydiospore (CHL)  (LPCB, Nikon, X400)

Trichophyton tonsurans: (LPCB, DMD-108, X1000)

Trichophyton tonsurans:  Another photo just to get a better feel for the organism
(LPCB, DMD-108, X1000)

Trichophyton tonsurans:  Macroconidium -micron bar reads 20.23 µm.
(LPCB, DMD-108, X1000)

Physiological Tests:
·         Urease Positive
·         Hair perforation test –Negative (exceptions have been noted)
·         Usually causes alkalinisation of BCP-Milk solids-glucose media
·         Growth is stimulated by the addition of thiamine.
·         Growth at 37oC
Definitive identification by molecular diagnosis is becoming more popular where available.

Differentiation:  The abundant microconidia of varying sizes and shapes assist in distinguishing this species from T.mentagrophytes and T.soudanense.  T.tonsurans response to thiamine also aids in this purpose.

[i] Source states that hair perforation is variable – Guy St-Germain B.S., Richard Summerbell Ph.D. Star Publishing -further details to follow)
ii Atlas of Cinical Fungi, 2nd edition: G.S. de Hoog, J. Guarro, J. Gené & M.J Figueras; Centraalbureau voor Shimmelcultures ‘ Universitat Rovira I Virgili, 2000: ISBN 90-70351-43-9
[iii] Medically Important Fungi – A Guide To Identification  5th ed. Davise H. Larone, MT (ASCP), Ph.D., F(AAM),
ASM Press, Washington D.C.
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