Fun With Microbiology (What's Buggin' You?): Tinea capitis

Microsporum audouinii (fungus - dermatophyte)

Ecology: Microsporum audouinii is a cosmopolitan anthropophilic dermatophyte which is spread person to person. Now rarely seen in North America or Europe however it remains the most common cause of human ringworm, primarily tinea capitis (scalp infection) on the African continent. Travelers and migrant workers may bring this infection from endemic areas. M.audouinii infection is more prevalent in prepubescent children. Hairs will fluoresce under a ultra-violet Wood’s Lamp.

Macroscopic Morphology: On Sabouraud-Dextrose agar the colonies grew with a moderately rapid pace. Colonies were rather flat and spreading with a radiating margin. They appeared to have a greyish-white to tan to beige colouration with the reverse a salmon colour to rose-brown.

M.audouinii on SAB at 3 weeks* (reverse shown below right)
*Correction: This appeared to be very luxuriant growth for a Microsporum to have developed in only 10 days as I had originally stated. On reviewing my original files I see that this culture was incubated on SAB/SDA at 30ᵒC. (mea culpa)

May 2nd, 2016: I've added another photograph from my files of M.audounii on Sabouraud Dextrose Agar (SAB or SDA, if you prefer) incubated at 30ᵒC for 14 days. As you can see, the growth is much more sparse that that above. I'm not sure what my thought process was some four years ago when I originally posted the culture photo.

M.audounii on SAB (SDA) incubated at 30ᵒC for 14 days (Nikon)

* * *

Microscopic Morphology: Hyphae are septate and often show pectinate (comb-like) and racquet cells (See photo). Both intercalary (in the middle of a hyphae) and terminal (at the end of a hyphae) chlamydospores may be present. Terminal chlamydospores may have a small point or peak at their apical end. Sources report that microconidia and macroconidia are rare. Microconidia are ovoidal to clavate (club shaped). Macroconidia can be elongated or spindle shaped and may be irregular or distorted in their shape. They may also show a slight constriction along their length often closer to the center. The macroconidia are thick-walled, variable in size (30-80 µm X 8-14 µm) and number of internal cells. They are usually echinulate (small spiny like projections on surface) to some degree.

Photos below were taken with the Nikon Coolpix camera or the Leica DMD-108 Digital Microscope as indicated. Read more about the photography in the post entitled 'Toys'.

M.audouinii hyphae bearing microconidia (LPCB X 400 -Nikon)

(authoratative sources state that microconidia are rare however the isolate I'm presenting here seems to be capable of producing ample numbers.)

M.audouinii - photo just to show the septate hyphae (S) and microconidia (M)

(LPCB X1000 DMD-108)

M.audouinii - starting the formation of racquet hyphae which appear somewhat like a tennis racquet, narrower on one end and wider-round on the opposite end usually stacked in greater numbers than in my photo. (LPCB X1000 DMD-108)

M.audouinii - what appears to be racquet hyphae though not "text-book" in shape.

(LPCB X1000)

As above

M.audouinii -Pectinate hyphae (comb-like structure of hyphae)

(LPCB X1000)

M.audouinii -Pectinate hyphae (comb-like structure of hyphae)

(LPCB X1000)

M.audouinii - another example of pectinate hyphae (LPCB X1000 DMD-108)

M.audouinii - jumble of structures showing pectinate hyphae and one intercalary chlamydospore

(LPCB X1000 DMD-108)

M.audouinii - Terminal chlamydospore (T) shown, often exhibits a little apical point or peak. Also an intercalary chlamydospore (IC) (in the middle of a hyphae or perhaps a hyphae running through it) (LPCB X1000)

M.audouinii - A better view of an intercalary chlamydospore showing it in between the run of a hyphal element. (LPCB X1000+10X digital zoom DMD-108)

M.audouinii - 2 young macroconidia (and lots of microconidia!!)

(LPCB X1000 Nikon)

M.audouinii - Macroconidia forming with constriction near middle. Internal cells not visible.

(LPCB X1000 Nikon)

M.audouinii - 2 macroconida, long thin on left and more typical spindle shaped on right

(LPCB X1000 DMD-108)

M.audouinii - macroconidia with slight narrowing near middle

(LPCB X1000 Nikon)

M.audouinii - macroconidia with more drastic narrowing near middle than the photo above. Internal cellular structure visible. (LPCB X1000 Nikon)

M.audouinii -macroconidia somewhat distorted with echinulate surface texture. (rough, small projections) Internal cells clearly visible. (LPCB X1000 Nikon)

M.audouinii - macroconidia with four internal cells (LPCB X1000 Nikon)

M.audouinii - Distorted 2 celled Macroconidia (LPCB X1000 Nikon)

M.audouinii - fairly young macroconidia forming (LPCB X 1000+10X digital zoom DMD-108)

M.audouinii - Short 2 celled macroconidia (LPCB X1000+10 digital zoome DMD-108)

M.audouinii - okay, last one - Macroconidia with rounder apical end than M.canis

(LPCB X1000 DMD-108)

Physiological Tests: Hair perforation test is negative. M.audouinii has no specific growth factor requirements. Alkalinization occurs in BCP-milk solids glucose (BCPCG) media. It shows weak to little growth on autoclaved rice grains with possible production of a brownish pigment.

Compare: Microsporum canis

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Fungus

Tinea capitis refers to dermatophytosis, a fungal infection of the scalp. Three types of in vivo hair invasion are recognized:

Ectothrix invasion is characterised by the development of arthroconidia on the outside of the hair shaft. The cuticle of the hair is destroyed and infected hairs usually fluoresce a bright greenish yellow colour under Wood's ultraviolet light. Common agents include M. canis, M. gypseum, T. equinum and T. verrucosum.

Single hair invaded by Tinea capitis (Calcofluor White Stain under Ultra Violet Microscope)

Magnification was not noted
(click on photo to enlarge for better viewing)

Endothrix hair invasion is characterized by the development of arthroconidia within the hair shaft only. The cuticle of the hair remains intact and infected hairs do not fluoresce under Wood's ultraviolet light which the physician might use in a presumptive diagnosis.. All endothrix producing agents are anthropophilic eg T. tonsurans and T. violaceum.

Favus usually caused by T. schoenleinii, produces favus-like crusts or scutula and corresponding hair loss.

Ditto

Tinea capitis infection of a single hair (Calcofluor White Stain under U.V. Scope)

The fungi that instigates tinea capitis thrives in moist and wet environments, and your risk increases if you have poor hygiene, prolonged skin wetness (such as that caused by sweating), and minor scalp injuries. Oral antifungal medication and medicated shampoos, such as those that contain selenium sulfide may be necessary to eliminate the infection.

The microphotographs above are of a single human hair with the arthroconidia visible within the hair shaft. They fluoresce because they were stained using Calcofluor White prior to being placed under a microscope cover slip and slide. Viewed under a fluorescent microscope at a magnification of x800.

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