Question
Candida albicans.
Answer
Candida albicans is a dimorphic (yeast and pseudohyphal/hyphal forms), oval, budding yeast, part of normal human commensal flora (mouth, gut, vagina, skin), and the most common cause of candidiasis — becoming pathogenic when local or systemic host defences are compromised (broad-spectrum antibiotic use disrupting competing flora, immunosuppression, diabetes, indwelling devices, pregnancy).
Key laboratory features: germ tube test — when incubated in serum at 37°C for 2–3 hours, C. albicans produces a characteristic germ tube (a thin filamentous outgrowth without constriction at its point of origin from the parent cell) — this rapid presumptive test distinguishes C. albicans (and closely related C. dubliniensis) from most other Candida species, which are germ-tube negative; chlamydospore formation on cornmeal agar — C. albicans forms characteristic thick-walled chlamydospores, a confirmatory identification feature.
Clinical spectrum:
- Mucocutaneous candidiasis — oral thrush (white, curd-like plaques on an erythematous base, classically seen in infants, immunocompromised/HIV patients, and after broad-spectrum antibiotics or inhaled corticosteroid use), vulvovaginal candidiasis, and cutaneous candidiasis (intertrigo, particularly in moist skin folds).
- Oesophageal candidiasis — an AIDS-defining opportunistic infection at low CD4 counts.
- Invasive/systemic candidiasis — candidaemia and disseminated disease in severely immunocompromised, critically ill, or catheterized patients — a significant cause of healthcare-associated bloodstream infection.
- Chronic mucocutaneous candidiasis — seen in specific T-cell immunodeficiency states.
Laboratory diagnosis: direct microscopy (KOH mount/Gram stain) showing budding yeast cells with or without pseudohyphae; culture on Sabouraud dextrose agar (SDA) — cream-coloured, smooth colonies; germ tube test and chlamydospore formation for species identification; blood culture for invasive disease.
Treatment: topical or oral azoles (fluconazole, clotrimazole) for mucocutaneous disease; echinocandins or amphotericin B for invasive/systemic candidiasis, particularly given rising azole resistance in some non-albicans species.