Mycology of dermatophyte infections

What is mycology?

Mycology is the study of fungi.

What are dermatophyte fungi?

Dermatophyte fungi are the ringworm fungi (tinea). They depend on their host, which may be an animal ("zoophilic") or a human ("anthropophilic") and need to spread from one host to another to survive. Dermatophytes may also prefer to live in the soil ("geophilic").

Anthropophilic dermatophytes are so well adapted to living on human skin that they provoke a minimal inflammatory reaction. Zoophilic or geophilic dermatophytes will often provoke a more vigorous inflammatory reaction when they attempt to invade human skin.

There are nine genera of dermatophytes, recognised by the nature of their macroconidae (asexual spores). The common dermatophytes infecting humans are:

• Trichophyton (abbreviated as "T")
• Microsporum ("M")
• Epidermophyton ("E"). 

Other genera are Arthroderma, Ctenomyces, Lophophyton, Nannizzia, Guarromyces and Paraphyton.

There are about 50 species. Their spores can live for more than a year in human skin scales in the environment.

Anthropophilic organisms include:

• T. rubrum (most common in New Zealand)
• T. interdigitale
• T. tonsurans (very common in the USA)
• M. audouinii
• T. violaceum
• M. ferrugineum
• T. schoenleinii
• T. megninii
• T. soudanense
• T. yaoundei

Zoophilic organisms include:

• M. canis (originating from cats and dogs)
• T. equinum (originating from horses)
• T. erinacei (originating from hedgehogs and other animals)
• T. verrucosum (originating from cattle)
• M. nanum (originating from pigs)
• M. distortum (a variant of M. canis).

Geophilic organisms include:

• Nannizzia gypsea
• M. fulvum.

How are dermatophyte fungi diagnosed?

Fungal infection may be suspected clinically or with the help of dermatoscopy. The presence of a dermatophyte infection is confirmed by:

• Microscopy and culture of skin scrapings
• Histopathological examination of skin or nail biopsy using periodic acid-Schiff (PAS) stains to reveal fungal elements
• Specific antigen tests using molecular biology techniques such as polymerase chain reaction.

Dermatophyte hyphae seen using potassium hydroxide treated skin scrapings using light microscopy

Lactophenol Cotton Blue bright field microscopy of trichophyton species showing fungal elements

Fluorescent KOH stain of dermatophyte hyphae amongst epithelial cells.

Non-inoculated skin scraping slopes. Blue top is Sabouraud dextrose agar with antibiotics and white top is Dermatophyte test medium.

Trichophyton rubrum colony morphology on potato dextrose agar culture plates showing reddish pigmentation at the back of the slope.

Trichophyton mentagrophytes cultured in Sabouraud dextrose agar slope with antibiotics.