Morphology and Cultural Characteristics
Fungi of the genus Penicillium belong to the group of mold fungi. They possess a characteristic branching septate mycelium (with cross-walls). The primary morphological marker of the genus is the specific structure of the fruiting body (conidiophore), which visually resembles a small brush.
The branching process of the conidiophore is organized as follows:
- Branching structures called primary and secondary metulae form at the ends of the growing hypha, creating multi-whorled structures.
- Dense bundles of phialides—specialized flask-shaped cells—arise from the metulae.
- The phialides, in turn, bear long chains of rounded conidia (spores).
When cultured on artificial nutrient media, penicilla form colonies of diverse colors: from classical green to yellow-brown, pink, and even purple.
Different species of fungi have unique structural features of their "brush-like" structures:
- P. crustaceum: form bi-, tri-, and multi-whorled brushes.
- P. notatum: characterized by asymmetric bi- and tri-whorled structures.
- P. glaucum: can be represented by both single- and multi-whorled variants.
- P. mycetomagenum: feature single-, bi-, or tri-whorled brushes and produce the smallest conidia (up to 2.2 µm in size).
Epidemiology and Pathogenesis
In nature, fungi of the genus Penicillium are ubiquitous. Their natural reservoirs include soil, decaying plant parts, as well as the air and environments of vegetable storage facilities. The infection is transmitted via an aerogenic mechanism: humans are infected by accidentally inhaling dust containing spores and other fungal elements.
Penicilliosis is a classic opportunistic infection. This means that the development of a clinically significant disease requires a decline in host defenses—risk groups consist exclusively of immunocompromised patients.
The clinical presentation of classical penicilliosis lacks strict specific features and largely resembles the manifestations of aspergillosis. Cellular immunity factors play the primary role in defending the body against the pathogen, and a delayed-type hypersensitivity (DTH) reaction develops during the infection process.
Characteristics of Penicillium marneffei
Among all representatives of the genus, Penicillium marneffei occupies a very special place. Unlike its cosmopolitan relatives, it is endemic to countries in Southeast Asia. Bamboo rats are considered the presumed natural reservoir of this pathogen.
Clinically, infection caused by P. marneffei more closely resembles histoplasmosis. The key biological distinction of this species from other penicilla is true thermal dimorphism:
- At 25 °C in the external environment, the fungus grows in a mycelial form, capable of producing a characteristic red pigment.
- At 37 °C, as well as directly within the tissues of the infected host, it transforms into a yeast form.
Laboratory Diagnostics and Treatment
A wide range of clinical specimens is used for laboratory confirmation of the diagnosis. The choice depends on the localization of the pathological process. Scrapings (from skin, nail plates, cornea), nasal and ear sinus discharge, sputum, purulent masses, and biopsies of affected tissues may be sent to the laboratory.
Various staining methods are used during microscopic examination: hematoxylin and eosin, Giemsa stain, or Wright's stain.
On microscopy, the classical picture includes the detection of long, branching septate hyphae and large rounded conidia. If the causative agent is P. marneffei, the microscopic picture changes fundamentally: characteristic oval cells corresponding to the yeast-like form of the fungus are identified in the tissues.
Specific treatment of penicilliosis requires the use of systemic antifungal agents. The main prescribed drugs are amphotericin B and itraconazole.