Histopathological examination of in vitro bone degeneration caused by a black yeast, Exophiala spinifera

Exophiala spinifera, a black yeast, rarely causes systemic infection, and only a very few cases of its infection by the invasion of internal organs or bones have been reported. We examined the ability of E. spinifera to invade bone tissues in vitro. The fungus was inoculated on the surface of murine bones, and then these bones were incubated at 30 C for 2, 4, and 12 weeks on water agar plates and on brain heart infusion agar supplemented with 1%glucose (BHIA) plates. Histopathological examination demonstrated that the fungus was initially found in the non-calcified parts of the bone tissue, such as the growth plate and articular cartilage. Thereafter, the fungus invaded the calcified parts: cancellous and cortical bones. Our experiments showed that the capability of E. spinifera to invade bone tissue is higher than that of Candida albicans or other black fungi. E. spinifera grew in the mycelial form and C. albicans in the yeast form in these experiments. Our results suggest that E. spinifera may have a high potential to invade bone tissues, and that the mycelial form can invade bone more deeply than the yeast form. Therefore, bone degeneration should/ must be carefully monitored in any systemic infection with E. spinifera.