Taxonomy and Differential Diagnosis
Dermatophytes have septate mycelium and can form various spores: arthroconidia, chlamydoconidia (chlamydospores), macroconidia, and microconidia.
In microbiology, three main genera are distinguished. The primary criterion for differentiating them in pure culture is macroconidia morphology (large asexual reproductive spores):
- Microsporum species: Macroconidia are spindle-shaped (fusiform) with a thick, multicellular wall. Their surface is rough and covered with spines or echinulations.
- Trichophyton species: Macroconidia are large, septate (having transverse walls), with a smooth wall. They resemble a cylinder or a cigar in shape.
- Epidermophyton species: Macroconidia are smooth and club-shaped (pyriform). A distinctive feature is that they appear in clusters or groups, frequently forming characteristic bunches.
Dimorphism: In Vivo vs. In Vitro Morphology
Dermatophytes exhibit dimorphism—their structure in infected tissues (parasitic phase, in vivo) differs from their structure in pure culture on nutrient media (saprophytic phase, in vitro).
- Microsporum canis
In the parasitic phase on hair, it forms an Ectothrix type of infection, where arthroconidia are arranged on the outside of the hair shaft. When inoculated into pure culture, spindle-shaped macroconidia, microconidia, chlamydoconidia, and arthroconidia are formed.
- Trichophyton tonsurans
In hair tissue, it causes an Endothrix infection, where chains of arthroconidia densely pack the interior of the hair shaft. In vitro, the fungus produces abundant small microconidia, rare macroconidia, and resting forms (chlamydoconidia).
- Trichophyton mentagrophytes
On hair, it forms an ectothrix pattern (arthroconidia lie in a mosaic pattern or chains on the surface). In pure culture, it is characterized by spiral (corkscrew) hyphae, microconidia clustered in grape-like bunches, and typical cigar-shaped macroconidia.
- Epidermophyton floccosum
In skin scrapings, septate hyphae and chains of arthroconidia penetrating the stratum corneum are observed. Club-shaped macroconidia and chlamydoconidia grow in pure culture. Microconidia are typically absent in this genus.
Physiology and Cultivation
The life cycle of dermatophytes includes two types of reproduction. The primary pathway is asexual reproduction (anamorphic stage), which produces conidia (micro- and macroconidia). Sexual reproduction (teleomorphic stage) occurs less frequently and is accompanied by the formation of asci containing spores.
To isolate the pathogen in the laboratory, Sabouraud dextrose agar (SDA) and its various modifications are used, on which the fungi show robust growth. Colony characteristics depend on the dermatophyte species:
- Texture may be woolly, cottony, granular, or powdery.
- Pigmentation varies—fungi can synthesize pigments that either color the mycelium itself or diffuse into the surrounding agar medium.
Environmental Resistance
Dermatophytes exhibit high tolerance to desiccation and freezing, contributing to their prolonged survival in the environment. In biological material (e.g., shed infected hairs or skin scales), they remain viable for years. For instance, Trichophyton species can survive in hair for 4 to 7 years.
Inactivation Factors (Susceptibility):
- Temperature: Boiling (100 °C) kills the fungi within 10–20 minutes.
- Physical factors: Pathogens are sensitive to ultraviolet (UV) radiation.
- Chemical agents: Alkaline solutions, formaldehyde, and iodine-based preparations are effective for disinfection and topical treatment.