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.
Predisposing factors: broad-spectrum antibiotic use, immunosuppression, diabetes mellitus, indwelling devices, and 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), distinguishing it from most other Candida species; chlamydospore formation on cornmeal agar — a confirmatory identification feature.
Clinical spectrum:
- Mucocutaneous candidiasis — oral thrush (white, curd-like plaques, classically in infants, immunocompromised/HIV patients, and after broad-spectrum antibiotics), vulvovaginal candidiasis, cutaneous candidiasis (intertrigo).
- 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.
- Chronic mucocutaneous candidiasis — seen in specific T-cell immunodeficiency states.
Laboratory diagnosis: direct microscopy (KOH mount/Gram stain) showing budding yeast, with/without pseudohyphae; culture on Sabouraud dextrose agar; germ tube test and chlamydospore formation; CHROMagar Candida for rapid speciation; blood culture for invasive disease.
Treatment: topical or oral azoles for mucocutaneous disease; echinocandins or amphotericin B for invasive/systemic candidiasis, given rising azole resistance in some non-albicans species.