Biological features and ecology
The causative agent of pityriasis versicolor belongs to the group of yeast-like fungi. In the older taxonomic classification, this microorganism was known as Pityrosporum orbiculare. Today, the genus Malassezia includes a significant species diversity: besides M. furfur itself, eleven other species are distinguished, among which M. globosa is also of major clinical importance.
A key physiological feature of the pathogen is its lipophilicity. This means that complex fatty acids are vital for the fungus's normal life activity and reproduction. It is this physiological requirement that determines its localization under natural conditions: M. furfur is a full member of the normal human skin microbiota and predominantly colonizes areas with the highest sebaceous gland activity.
Pathogenesis and clinical presentation of the disease
Upon transition from a commensal to a pathogenic state, M. furfur causes a specific disease — pityriasis versicolor. In clinical practice, this pathology is also frequently called tinea versicolor.
The pathological process has a strictly defined localization: the fungus affects exclusively the most superficial parts of the stratum corneum. Anatomically, the rashes are most often located on the skin of the trunk, neck, and upper extremities.
Characteristic features of the rash:
- Visually, they appear as pinkish-yellow spots.
- The spots are strictly non-inflammatory in nature.
- The so-called "variegation" is observed: hyperpigmented (darker) and hypopigmented (lighter) areas of lesions are simultaneously present on the patient's skin.
- A crucial diagnostic sign: upon gentle scraping of the spot surface, characteristic desquamation is noted, wherein small scales separate, visually very similar to bran (hence one of the disease's names).
Microscopic diagnostics
Laboratory diagnosis of pityriasis versicolor begins with direct microscopy of pathological material. Skin scales taken from the lesion foci serve as the test sample.
For better visualization of fungal elements, the scales are pre-treated with a 20% alkali solution, which dissolves keratin, making the preparation transparent.
What can be seen under the microscope (in pathological material):
- Yeast-like budding cells that often accumulate in clusters.
- Pseudomycelium, representing short and slightly curved hyphae (filaments).
Such a combined presence of elements in the scales is a typical microscopic pattern for this disease.
Culture method
Isolation of a pure culture of M. furfur requires special conditions due to the specific nutritional requirements of the pathogen. The basic medium is Sabouraud agar supplemented with an antibiotic (tetracycline) to suppress the growth of accompanying bacterial flora.
Culture specifics:
- A mandatory condition is the presence of lipid components. To ensure the lipophilicity of the fungus, Tween-80 is added to the nutrient medium or sterile olive oil is applied to the surface.
- The waiting time for culture growth is about one week.
- The grown colonies are white with a characteristic creamy consistency.
Microscopy of the culture itself (unlike the material from scales) reveals only oval or bottle-shaped budding cells measuring $2 \times 6$ µm. Important to remember: true mycelium is completely absent in this fungus.
Treatment approaches
Since the disease is caused by a fungus, the basis of therapy is antimycotic (antifungal) drugs. According to clinical data, the following medications are used to suppress the activity of M. furfur:
- Amphotericin B;
- Itraconazole;
- Ketoconazole;
- Fluconazole.
Briefly: answers to topical questions
Which specific clinical tests and instrumental methods are used to confirm the diagnosis of pityriasis versicolor upon examination?
During physical examination, the diagnosis is confirmed by scraping, and for further verification, microscopic and culture methods are used.
- Scraping — mechanical action on the spots causes scaling with bran-like flakes.
- Microscopy — the collected material is treated with 20% alkali to visualize yeast-like cells and pseudomycelium.
- Culture method — inoculation is performed on lipid-containing nutrient media (Sabouraud agar with tetracycline and the addition of olive oil or Tween-80).
To remember the physiology of M. furfur, use the association: "The gourmet fungus loves olive oil." This will help you not to forget its lipophilicity, habitation in the sebaceous areas of the skin, and the need to add oil to Sabouraud agar during inoculation.
Class and exam questions
In pathological material (skin scales), yeast-like cells in clusters and pseudomycelium (short curved filaments) are detected. In pure culture in vitro, pseudomycelium is not formed; only oval or bottle-shaped budding cells are visible.
Yeast-like cells and pseudomycelium are visible in the scales. Only oval/bottle-shaped budding cells are present in culture; there is no true mycelium.
Why are the spots of different colors in pityriasis versicolor?
The disease is characterized by the presence of both hyperpigmented (dark) and hypopigmented (light) lesion areas, creating a characteristic "mottled" picture.
What additives are necessary for the growth of this fungus on nutrient media?
Since M. furfur is a lipophilic microorganism, lipid components are essential for its cultivation on Sabouraud agar (with tetracycline): sterile olive oil or Tween-80.
What else to study on this topic
- Taxonomy and species diversity of the genus Malassezia (the role of M. globosa)
- Mechanisms of hypopigmentation and hyperpigmentation formation in pityriasis versicolor
- Differential diagnosis with other superficial mycoses
- Features of the composition of nutrient media for lipophilic fungi
- Mechanism of action of antimycotics (ketoconazole, fluconazole) on the fungal cell
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