Exophiala hongkongensis

When grown on Sabouraud dextrose agar culture media, the fungus grows slowly as black, slimy, yeast-like colonies that reach a diameter of 2 mm after 14 days at 25 °C (77 °F). Then the colony centers develop a velvety texture and become greenish-gray and dome-shaped, while retaining a slimy and yeast-like margin. After 14 days, the ellipsoidal yeast-like cells are brown when stained with lactophenol cotton blue, and measure 6 by 3.5 μm. They have ringed zones that pinch off to create budding cells. The mycelia of the fungus are swollen and form torulose hyphae (swollen and constricted at intervals) that are pinched at their ends. When the colonies become velvety, the mycelia are not swollen, and conidia are formed in clusters. The average length of the conidiophores is about 10 μm. The conidia are oval to roughly spherical, and typically measure 3 by 2 μm.^[1]

Exophiala hongkongensis is known only from Hong Kong. Known cases of onychomycosis caused by Exophiala are rare, although this genus commonly causes related skin infections such as phaeohyphomycosis, chromoblastomycosis, and eumycetoma. Previous instances of onychomycosis had been previously traced to E. dermatiditis and E. jeanselmei. In addition to Exophiala hongkongensis, Woo and colleagues identified additional cases caused by E. oligosperma and E. bergeri. All known cases are associated with an underlying disease that causes an immunosuppressive state; in the case of E. hongkonensis, the patient had hypertension.^[1]