[Yeast capsule ultrastructure]

The ultrastructure of capsules was studied in 12 yeast species producing extracellular polysaccharides and belonging to ascomycetous, basidiomycetous and asporogenous organisms using the method of ultrathin sections. On the whole, the structure of capsules was similar in these organisms: the capsules are formed by fibrils radially coming out of the outer surface of the cell wall. Unlike those of basidiomycetous yeasts, the capsules of ascomycetous organisms are characterized by a more ordered orientation of fibrils which run strictly parallel to one another. In the capsules of basidiomycetous organisms, fibrils usually interweave, often producing a fibrillar network; in certain species, fibrils stick together forming thick threads on the periphery of the capsule. Moreover, as a rule, these yeasts possess a distinct bright zone (halo) above the outer surface of the cell wall. These results as well as data available about the chemical composition and functions of the capsule indicate that there are radical differences the latter and the cell wall which make it possible to regard the capsule as an individual organelle of the yeast cell rather than as part of the cell wall.