Ultrastructural Evidence of Chlamydospore Formation in Exophiala dermatitidis Under Environmental Stress

Exophiala dermatitidis is an extremotolerant black yeast with clinical significance and environmental resilience, which exhibits dimorphic growth between yeast-like and hyphal forms. Chlamydospore formation, however, has not been reported in this species. This study investigated the structural modification in E. dermatitidis under hydrocarbon-rich conditions and prolonged incubation at 37°C. Exophiala dermatitidis was cultured on Sabouraud dextrose agar (SDA), rose bengal potato dextrose agar chloramphenicol (R-PDA) and in soil extract solution with engine oil (SE). To explore survival mechanisms and promote hyphal development, cultures were maintained for extended periods on R-PDA and potato flakes agar (PFA). Morphological and ultrastructural changes were examined using transmission electron microscopy. Cell wall thickness was measured and analyzed. Thin-walled yeast cells transformed into thick-walled yeast cells on R-PDA and SE. After prolonged incubation, melanized, lipid-rich chlamydospores were observed at hyphal tips and between hyphae. Growth on PFA confirmed chlamydospores development through the conversion into hyphal structures. This study presents the first ultrastructural evidence of chlamydospore formation in E. dermatitidis.