Pathogen Characteristics
Fungi of the genus Candida are primarily classified as Deuteromycetes (imperfect fungi) because a sexual reproductive 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. These fungi are aerobes that grow well on simple nutrient media (at 25–27 °C), forming convex, shiny, cream-like 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 cross-walls (septa).
Inside the host organism (in vivo), both yeast forms and pseudohyphae are found.
Epidemiology and Pathogenesis
Normally, Candida species inhabit the environment (plants, fruits) and are part of the normal microbiota of humans and mammals.
Modes of transmission:
- Endogenous (primary): Invasion of the host's own fungi into tissues during host defense impairment.
- Exogenous: Vertical (mother-to-child) or sexual transmission.
Risk factors predisposing to candidiasis include irrational antibiotic use, immunodeficiencies, endocrine disorders, as well as local factors (mucosal trauma, skin moisture). Fungal virulence (especially in C. albicans) is mediated by protease production and the ability to adhere to matrix proteins via integrin-like molecules.
Clinical Forms and Immunity
The interaction between the fungus and the host can manifest in 5 forms:
- Superficial candidiasis: Involvement of mucous membranes (oral thrush, vulvovaginitis), skin (nodules and pustules, frequently in newborns), and nails.
- Chronic mucocutaneous (granulomatous) candidiasis.
- Visceral candidiasis: Involvement of individual organs (gastritis, respiratory tract, and urinary tract infections).
- Disseminated (systemic) candidiasis: Progressing to sepsis. A hallmark feature is fungal endophthalmitis.
- Allergy: Allergic reactions involving the gastrointestinal tract and eyes.
Defense against the infection is mediated by phagocytosis (neutrophils, macrophages, eosinophils), the complement system, and antibodies (for opsonization). Specific immunity is cell-mediated (delayed-type hypersensitivity) and is characterized by granuloma formation.
Microbiological Diagnosis
Diagnosis relies on a combination of methods:
- Smear microscopy: Searching for blastoconores, pseudomycelia, and true mycelia.
- Culture method: Inoculation on Sabouraud agar or wort agar. Blood cultures are required to confirm candiduria or candidemia (with $>10^5$ CFU/mL in urine indicating significant bacteriuria/candiduria).
- Species identification: Filamentation test, rapid germ tube test in serum (specific for C. albicans), and chlamydospore formation on rice agar (for C. albicans). It is crucial to differentiate them from Saccharomyces, which do not form pseudomycelia and possess intracellular ascospores.
- Immunodiagnostics: Serological assays (ELISA, complement fixation, precipitation tests) and skin allergy tests.
Principles of Treatment and Prevention
Antifungal susceptibility testing is mandatory when prescribing antimycotics, as certain species exhibit intrinsic resistance (e.g., C. lusitaniae is resistant to amphotericin B, while C. krusei and C. glabrata are resistant to fluconazole). Flucytosine, caspofungin, and itraconazole are also utilized.
Prevention relies on strict adherence to aseptic techniques during invasive procedures (catheterization, bronchoscopy) and prophylactic pharmacotherapy in patients with severe neutropenia.