Taxonomy and differential diagnostics
Dermatophytes have septate mycelium and are capable of forming various spores: arthroconidia, chlamydoconidia, macro- and microconidia.
In microbiology, three main genera are distinguished. The main criterion for distinguishing them in pure culture is macroconidia morphology (large asexual spores):
- Genus Microsporum: macroconidia are spindle-shaped (fusiform) with a thick multicellular wall. Their surface is rough and covered with spines.
- Genus Trichophyton: macroconidia are large, septate (have cross-walls), with a smooth wall. Cylindrical or cigar-shaped.
- Genus Epidermophyton: macroconidia are smooth, club-shaped. A distinctive feature is that they are arranged in multiple groups, often forming characteristic clusters.
Dimorphism: in vivo and in vitro morphology
Dermatophytes exhibit dimorphism—their structure in infected tissues (parasitic phase, in vivo) differs from their structure in pure culture on a nutrient medium (saprophytic phase, in vitro).
- Microsporum canis
In the parasitic phase on hair, it forms an Ectothrix type of infection—arthroconidia are located on the outside. When subcultured into a pure culture, spindle-shaped macroconidia, microconidia, chlamydoconidia, and arthroconidia are formed.
- Trichophyton tonsurans
In the hair shaft, it causes an Endothrix type of infection—arthroconidia form chains that densely fill the hair shaft from the inside. In vitro, the fungus produces abundant small microconidia, rare macroconidia, and resting forms (chlamydoconidia).
- Trichophyton mentagrophytes
On the hair, it forms an "ectothrix" (arthroconidia lie in a mosaic pattern or in chains on the surface). In pure culture, it is characterized by the presence of spiral (corkscrew-like) hyphae, microconidia gathered in clusters, and typical cigar-shaped macroconidia.
- Epidermophyton floccosum
Septate mycelial filaments (hyphae) and chains of arthroconidia penetrating the stratum corneum of the epidermis are found in skin scales. In pure culture, club-shaped macroconidia and chlamydoconidia grow. Microconidia are usually absent in this genus.
Physiology and cultivation
The life cycle of dermatophytes includes two types of reproduction. The main pathway is asexual reproduction (anamorphic stage), during which conidia (micro- and macroconidia) are formed. Sexual reproduction (teleomorphic stage) is less common, accompanied by the formation of asci with spores.
To isolate the pathogen in the laboratory, Sabouraud agar and its various modifications are used. On these media, fungi show good growth. Colony characteristics depend on the dermatophyte species:
- Texture can be fluffy, granular, or powdery.
- Pigmentation is diverse—fungi can synthesize pigment that either stains the mycelium itself or is secreted into the surrounding nutrient medium.
Stability in the external environment
Dermatophytes have high tolerance to drying and freezing, which contributes to their long-term survival in the environment. In biological material (e.g., shed infected hair or scales), they remain viable for years. For instance, fungi of the genus Trichophyton can survive in hair for 4 to 7 years.
Inactivation factors (susceptibility):
- Temperature: boiling (100 °C) kills fungi in 10–20 minutes.
- Physical factors: pathogens are susceptible to ultraviolet (UV) radiation.
- Chemical agents: alkaline solutions, formaldehyde, and iodine-based preparations are effective for disinfection and topical therapy.
Briefly: answers to topical questions
Which dermatophyte species belong to the anthropophilic group?
- Microsporum audouinii — cause microsporia.
- Microsporum ferrugineum — cause microsporia.
- Trichophyton tonsurans — cause trichophytosis.
- Trichophyton violaceum — cause trichophytosis.
- Trichophyton schoenleinii — causes favus.
- Epidermophyton floccosum — tinea cruris, nail infection.
How to remember macroconidia shape in culture: Microsporum — spindle (pricking, has spines), Trichophyton — smooth cigar (T-tube), Epidermophyton — smooth clubs (in clusters).
Class and exam questions
What microscopic feature is primary for differentiating dermatophyte genera?
The main feature when studying pure culture is the morphology of macroconidia—large asexual spores (their shape, wall thickness, presence of spines).
What is the difference between endothrix and ectothrix hair infection types?
In endothrix infection, arthroconidia densely fill the hair shaft from the inside, while in ectothrix infection, spores are located on the outside of the hair (in mosaic pattern or chains).
What nutrient medium are dermatophytes typically cultured on?
Sabouraud agar and its modifications are well suited for cultivating these molds.
Does Epidermophyton form microconidia?
No, a characteristic feature of the genus Epidermophyton is the absence of microconidia in pure culture.
What else to study on this topic
- Clinical forms of dermatophytosis: trichophytosis, microsporia, favus
- Mechanisms of endothrix and ectothrix infection development
- Features of teleomorphic (sexual) stages of dermatophytes
- Pigment production by fungi on Sabouraud agar and its diagnostic significance
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Other topics in the section "Epidermophytoses (microsporia, trichophytosis, favus, rubroophytosis, etc.)"
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