Etiology and ecology
The primary causative agent of the infection is Cryptococcus neoformans. There are two main variants: var. neoformans, which has a worldwide distribution, and var. gattii, which predominates in warm-climate regions. Sporadic cases may be caused by C. albidus and C. laurentii. The sexual (teleomorph) form of the fungus is called Filobasidiella neoformans and is capable of forming hyphae with basidiospores.
In nature, these fungi inhabit soil, air, grass, and food products. An important reservoir is bird droppings, especially pigeon guano. The variant var. gattii has a specific niche: it is associated with soil under flowering eucalyptus trees.
The main route of transmission is aerogenous (inhalation of dust containing the pathogen). Contact transmission (through damaged skin) and alimentary transmission are less common.
Morphology and virulence factors
In the human body (in tissues and macrophages), cryptococci appear as large rounded cells ranging from 3 to 25 µm in diameter with a single small bud. Their main distinguishing feature is a massive polysaccharide capsule composed of glucuronoxylomannan. It appears as a light halo around the cell and acts as the primary virulence factor, effectively inhibiting phagocytosis. Occasionally, reduced mycelium can be found in tissues.
When cultured in vitro (on artificial media), the cells are smaller — 4–8 µm. They retain their capsule, reproduce by narrow-based budding, and can form both pseudomycelium and true mycelium.
In addition to the capsule, an important virulence factor is the enzyme phenoloxidase, which is a unique biochemical marker found exclusively in cryptococci.
Clinical presentation and immunity
The body's defense against cryptococci relies predominantly on cell-mediated immunity. If a patient shows a negative delayed-type hypersensitivity (DTH) skin reaction to fungal antigens, it indicates profound cell-mediated immunodeficiency and is an extremely poor prognostic sign. The disease rarely occurs in immunocompetent individuals.
The infection is often asymptomatic or has mild clinical manifestations, but patients in risk groups develop severe involvement:
- Cryptococcal meningitis: develops slowly (though the course is more acute in AIDS). It manifests with fever, severe headache, nuchal rigidity, dizziness, visual disturbances, and agitation.
- Pulmonary involvement: accompanied by low-grade fever, sweating, and a cough producing mucoid sputum.
- Skin involvement: characterized by the appearance of papules and ulcers with raised borders.
In the absence of an adequate immune response, generalized dissemination of the fungus throughout the body occurs.
Microbiological diagnostics
For laboratory testing, cerebrospinal fluid (CSF), sputum, pus, urine, or scrapings from skin ulcers are collected.
- Microscopy. A classic rapid method for examining cerebrospinal fluid sediment is staining with India ink. Against the dark ink background, budding yeasts surrounded by an unstained light halo are clearly visible. Gram staining, Gram-Weigert staining, Giemsa staining, Schiff reaction (PAS), and acridine orange fluorescence microscopy are also used.
- Culture method. Inoculation of the specimen is performed on Sabouraud agar. Incubation is carried out at 37 °C for two weeks. Shiny, slightly convex colonies with a characteristic mucous (creamy) consistency grow out.
- Serological method. Used to detect specific fungal antigens in the patient's biological fluids.
Briefly: answers to topical questions
What virulence factors are characteristic of Cryptococcus neoformans?
The pathogen Cryptococcus neoformans is characterized by the presence of two main virulence factors that ensure its survival in the host organism. These include:
- Polysaccharide capsule — a pronounced structure whose main function is to inhibit phagocytosis.
- Phenoloxidase enzyme — a unique marker identified only in fungi of the genus Cryptococcus.
The presence of a thick capsule is a key feature and the primary virulence factor of this opportunistic fungus.
What antigens are detected during serological diagnosis of cryptococcosis, and by what method?
During serological diagnosis of cryptococcosis, a specific fungal antigen — the capsular polysaccharide glucuronoxylomannan — is detected in the test samples. The following methods are used for its detection:
- Latex agglutination test (latex test) — a qualitative assay with high sensitivity and specificity.
- Enzyme-linked immunosorbent assay (ELISA) — used for precise quantification of the capsular polysaccharide.
Blood, cerebrospinal fluid, urine, and bronchoalveolar lavage fluid may be subjected to antigen testing.
What antifungal drugs are used to treat cryptococcal meningitis?
Sources indicate amphotericin B, fluconazole, itraconazole, and flucytosine as targeted therapy for cryptococcosis. Ketoconazole and itraconazole are also mentioned as alternative azoles. In severe cases, intravenous administration of amphotericin B at a dose of 0.3–0.5 mg/kg for a course of 10–14 days is recommended.
To remember CSF rapid diagnostics: "Cryptococcus in ink like the moon in the night" — India ink creates a dark background against which the unstained polysaccharide capsules of the fungus stand out brightly.
Class and exam questions
What indicates a poor prognosis in cryptococcosis?
Negative delayed-type hypersensitivity (DTH) reaction to fungal antigens. This indicates a severe deficiency in cell-mediated immunity, which is necessary to fight the infection.
What unique enzyme is detected in cryptococci?
Phenoloxidase. This enzyme is a specific marker used to identify the pathogen in the laboratory.
What is the difference in cryptococcal morphology in the host organism versus in culture?
In human tissues, the cells are significantly larger (up to 25 µm) and have a very thick capsule. On in vitro nutrient media, the cells are smaller (4–8 µm), although the ability to form capsules and bud is retained.
What else to study on this topic
- Structure and properties of the glucuronoxylomannan capsule
- Mechanisms of phagocytosis inhibition by fungi of the genus Cryptococcus
- Life cycle and sexual stage of Filobasidiella neoformans
- Differential diagnosis of cryptococcal and bacterial meningitis
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Other topics in the section «Systemic endemic mycoses (histoplasmosis, blastomycosis, coccidioidomycosis, cryptococcosis)»
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