Ultrastructure of proteinase-secreting cells of Candida albicans studied by alkaline bismuth staining and immunocytochemistry

Abstract

The ultrastructure of Candida albicans cells induced to secrete extracellular proteinase (EPR) has been studied. Electron microscopy employing alkaline bismuth staining, a method which stains polysaccharides, clearly revealed Golgi-like bodies and secretory vesicles in C. albicans cells. After EPR induction, there was no apparent increase in the number of these structures. Instead, many flocculent granules appeared at the periphery of induced cells. The granules were similar to secretory vesicles in size, but were more irregular in shape. Similar granules were observed in non-induced cells, though less frequently than in induced cells. Brefeldin A, a specific inhibitor of membrane transport in the secretory pathway, caused the accumulation of EPR and Golgi-like bodies in EPR-induced cells, but did not affect the accumulation of the granules. These results suggest that the granules are unrelated to EPR secretion. Electron microscope immunocytochemistry with affinity-purified anti-EPR antibodies showed that the granules in EPR-induced cells were recognized by the antibodies. This recognition was completely inhibited by the presence of glycogen, suggesting that antibodies cross-react with glycogen-like polysaccharides in the granules. Although the location of EPR within the cells remains unclear, the results suggest that EPR might be secreted via the constitutive secretory pathway, and that EPR is glycosylated to give a structure with some similarity to glycogen.