Microbiology of Dermatophytes

Dermatophytosis:

Tinea or ringworm (dermatophytoses) are defined as cutaneous fungal infections of keratinized tissues (hair, nail, skin,etc.) by a group of related mould fungi called dermatophytes. Skin acts as a barrier to fungal infection. The fatty acid content and acid pH of skin, ciliary mucosal epithelia of respiratory tract and humoral factors restrict the growth of dermatophytes to the outer layers of skin.

Common dermatophytes and their features:

Features	Trichophytom	Microsporum	Epidermophyton
Site of infection	Skin, hair, nail	Skin, hair	Skin, nail
Macroconidia	Sparse, thin-walled, smooth, Pencil shaped	Numerous, thick-walled, rough, Spindle shaped	Numerous, smooth-walled, Club shaped
Microconidia	Abundant	Rare	Absent
Examples	T.rubrum, T.schoenleinii, T.violaceum, T.concentricum, T.mentagrophytes, T.tonsurans, T.equinum	M.canis, M.fulvum, M.nanum, M.andouinii, M.gypseum, M.racemosum, M.equinum, M.gallinae	E.floccosum

Clinical Forms:

Dermatophytes grow only on the keratinized layers of epidermis and their appendages. Usually they do not invade living tissues. These fungi secrete an enzyme called keratinase, which digests keratin. Since keratin is the primary structural protein of skin, nails, and hair, the digestion of keratin manifests as scaling of the skin, loss of hair, and crumbling of the nails.

Dermatomycosis	Site Involved	Lesions	Causative Agent
Tinea capitis	Scalp	Scaly red lesions with loss of hair	Trichophyton, Microsporum (most species) T.schoenleinii causes favus
Tinea corporis	Non-hairy skin of the body	Ring shape with red raised border	T.rubrum
Tinea cruris (Jock itch)	Groin and perineum	Itchy red patches	E.floccosum
Tinea barbae	Bearded areas of face and neck	Kerion like plaques	Trichophyton (most common)
Tinea pedis	Sole and toe clefts (Athlete’s foot)	Cracking and peeling of the skin	T.rubrum
Tinea unguim (Onychomycosis)	Nails	Thickened, discolored and brittle nails	Trichophytom (many species) and Epidermotophyton

Laboratory Diagnosis:

Specimen: Nail samples, scrapings from the edges of skin lesions, plucked infected hairs

Direct Microscopy:

1. Keratinized specimen is placed on a slide
2. A drop of 10 to 20% KOH is added to dissolve keratin and cellular material, leaving the alkali resistant fungi intact
3. Covered by a cover slip
4. Left for 20 minutes in incubator at 37 c to digest keratin
5. Microscopy reveals filamentous branching hyphae and arthrospore is oftenly seen in hair

Culture:

The specimen is inoculated into a slide or in a plate of Sabourad’s dextrose Agar (SDA) containing chloramphenicol or cycloheximide. Slightly acidic nature of SDA allows pathogenic fungi to grow but doesn’t allow bacterial growth. And antibiotics in the culture medium further prevent bacterial or saprophytic fungal contamination.

The slide/plate is incubated aerobically at 25-30c upto 3 weeks

Identification:

Trichophyton:

• Colonies show colors
• Examination of macro and microconidia

Microsporum:

• Microsporum on scalp can be identified because of bright green appearance of infected hair with an ultraviolet Wood’s light
• Examination of macro and microconidia

Epidermophyton:

• Colonies may appear white, yellow or olive in color
• Examination of macro and microconidia

Treatment:

• Topical imidazoles as 1^st line drugs
• Oral griseofulvin for tinea capitis and tinea unguium