Candidiasis (Candida)

Pathogen characteristics

Fungi of the genus Candida mainly belong to Deuteromycetes (imperfect fungi) because a sexual reproduction stage has not been discovered in them. However, teleomorphic genera with a sexual stage also exist (e.g., Pichia, Kluyveromyces).

C. albicans and C. tropicalis are of the greatest clinical significance. The fungi are aerobes and grow well on simple nutrient media (at 25–27 °C), forming convex, shiny, creamy colonies.

Morphologically, they are polymorphic:

• Yeast cells: oval, budding, 3–4 µm in size.
• Pseudohyphae: elongated cells (5–10 µm) connected in chains with constrictions.
• True hyphae: filaments with partitions (septa).

Both yeast forms and pseudohyphae are found in the host organism (in vivo).

Epidemiology and pathogenesis

Normally, Candida inhabit the environment (plants, fruits) and are part of the normal microflora of humans and mammals.

Routes of transmission:

• Endogenous (primary): invasion of resident fungi into tissues when host defenses are weakened.
• Exogenous: vertical (from mother to child) or sexual route.

Risk factors contributing to the development of candidiasis include: irrational use of antibiotics, immunodeficiencies, endocrine disorders, as well as local factors (mucosal trauma, skin moisture). The virulence of fungi (especially C. albicans) is ensured by the production of proteases and the ability to adhere to matrix proteins via integrin-like molecules.

Clinical forms and immunity

The interaction between the fungus and the macroorganism can take 5 forms:

1. Superficial candidiasis: involvement of mucous membranes (oral thrush, vulvovaginitis), skin (nodules and pustules, more common in newborns), and nails.
2. Chronic granulomatous candidiasis.
3. Visceral candidiasis: involvement of individual organs (gastritis, respiratory tract, and urinary tract infections).
4. Disseminated (systemic) candidiasis: progressing to sepsis. Its characteristic marker is fungal endophthalmitis.
5. Allergy: allergic reactions affecting the gastrointestinal tract and eyes.

Protection against infection is mediated by phagocytosis (neutrophils, macrophages, eosinophils), the complement system, and antibodies (for opsonization). Specific immunity is cell-mediated (delayed-type hypersensitivity) and manifests as granuloma formation.

Microbiological diagnostics

The diagnosis is established based on a set of methods:

• Smear microscopy: looking for blastoconidia, pseudomycelia, and true mycelia.
• Culture method: inoculation on Sabouraud agar or wort agar. Blood culture is necessary to confirm candidemia, and in urinary tract infections, the detection of more than $10^5$ CFU per 1 ml of urine.
• Species identification: filamentation test, rapid germ tube test in serum (specific for C. albicans), chlamydospore formation on rice agar (for C. albicans). It is important to distinguish them from Saccharomyces, which do not form pseudomycelia and possess intracellular ascospores.
• Immunodiagnostics: serological reactions (ELISA, complement fixation test, precipitation test) and skin allergy tests.

Principles of treatment and prevention

When prescribing antimycotics, susceptibility testing of the fungus is mandatory, as some species exhibit intrinsic resistance (e.g., C. lusitaniae is resistant to amphotericin B, and C. krusei and C. glabrata to fluconazole). Flucytosine, caspofungin, and itraconazole are also used.

For prevention, strict adherence to aseptic rules during invasive procedures (catheterization, bronchoscopy) and drug prophylaxis in patients with severe neutropenia are essential.

Class and exam questions

What else to study on this topic

• Mechanisms of antifungal resistance in Candida species
• Features of the immune response in chronic granulomatous candidiasis
• Teleomorphic genera of yeast-like fungi: significance and properties
• Differential diagnosis of candidiasis and other superficial mycoses
• Adhesion factors and enzymatic activity as the basis of Candida spp. virulence

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

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