Exopolysaccharides and capsules in human pathogenic Exophiala species

The black yeasts Exophiala spinifera and E. dermatitidis produce extracellular slimes, which may be either in the form of a well-delimited capsule or of diffusely exuded exopolysaccharides (EPS). The optimal conditions for their production were studied. The presence or absence of polysaccharide material can be used for recognition of the two species. Five-day-old cultures grown on potato glucose agar at 24 degrees C were observed in India ink, and positive identification for E. spinifera was obtained when significant halos were seen around yeast cells. In contrast, E. dermatitidis had irregular EPS with a fibrillar substructure made visible by alcian blue staining. Other Exophiala species produce insignificant amounts of extracellular mucus or none at all. The diagnostic method is particularly useful with yeast-like primary cultures, which often consist entirely of budding cells and lack the characteristic structures of the filamentous Exophiala synanamorph.