Penicilliosis (Penicillium)

Morphology and cultural characteristics

Fungi of the genus Penicillium belong to mold fungi. They possess a characteristic branching septate mycelium (with transverse septa). The main morphological marker of the genus is the specific structure of the fruiting part (conidiophore), which visually resembles a brush.

The branching process of the conidiophore proceeds as follows:

1. At the ends of the growing hypha, branches are formed which are called primary and secondary metulae, forming multi-whorled structures.
2. Dense bundles of phialides—specialized bottle-shaped cells—extend from the metulae.
3. The phialides, in turn, bear long chains of rounded conidia (spores).

When cultured on artificial nutrient media, penicilla form colonies of a wide variety of colors: from classical green to yellow-brown, pink, and even purple.

Different species of fungi have their own unique structural features of the «brush» formation:

• P. crustaceum: form bi-, tri-, and multi-whorled brushes.
• P. notatum: characterized by asymmetrical bi- and tri-whorled structures.
• P. glaucum: can be represented by both single- and multi-whorled variants.
• P. mycetomagenum: have single-, bi-, or tri-whorled brushes and are distinguished by the formation of the smallest conidia (up to 2.2 µm in size).

Epidemiology and pathogenesis

In nature, fungi of the genus Penicillium are ubiquitous. Their natural reservoir is soil, decaying plant parts, as well as air and vegetable store premises. The infection is transmitted via an aerogenic mechanism: humans become infected by accidentally inhaling dust containing spores and other elements of the fungus.

Penicilliosis is a typical opportunistic infection. This means that for the development of a clinically significant disease, a decline in the host's defense mechanisms is required—only immunocompromised patients fall into the risk group.

The clinical presentation of classical penicilliosis lacks strict specific features and is largely similar to the manifestations of aspergillosis. Cellular immunity factors play the primary role in protecting the body against the pathogen, while delayed-type hypersensitivity (DTH) develops in the patient during the course of the infection.

Features of Penicillium marneffei

Among all representatives of the genus, the species Penicillium marneffei occupies a completely special place. Unlike its cosmopolitan relatives, it is endemic to Southeast Asian countries. Bamboo rats are considered the presumptive natural reservoir of this pathogen.

Clinically, the infection caused by P. marneffei more closely resembles histoplasmosis. The key biological distinction of this species from other penicilla is true thermal dimorphism:

• At a temperature of 25 °C in the external environment, the fungus grows in a mycelial form, while being capable of producing a characteristic red pigment.
• At a temperature of 37 °C, as well as directly in the tissues of the infected host, it transforms into a yeast form.

Laboratory diagnostics and treatment

A wide range of clinical material 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.

During microscopic examination, various staining methods for preparations are used: hematoxylin and eosin, Romanowsky-Giemsa, or Wright's stain.

Under microscopy, the classical picture includes the detection of long branching septate hyphae and large rounded conidia. However, if the pathogen is P. marneffei, the microscopic picture changes fundamentally: characteristic oval cells corresponding to the yeast-like form of the fungus are revealed in the tissues.

Specific treatment of penicilliosis requires the use of systemic antimycotic agents. The main prescribed drugs are amphotericin B and itraconazole.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Detailed structure of conidiophores: differences between primary and secondary metulae
• Species-specific brush structures in P. crustaceum, P. notatum, and P. glaucum
• Differential microscopic diagnosis of penicilliosis and aspergillosis
• Mechanisms of thermal dimorphism in pathogenic fungi using P. marneffei as an example
• Mechanism of the fungicidal action of amphotericin B and itraconazole

Other topics in the section "Opportunistic mycoses (candidiasis, aspergillosis, pneumocystosis, etc.)"

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