Morphological and Structural Changes During the Yeast-to-Mold Conversion of Phialophora dermatitidis

The details of the morphological and structural events occurring during yeast-to-mold conversion of the human pathogenic fungus Phialophora dermatitidis as seen by phase-contrast microscopy and electron microscopy are described and illustrated. Budding yeasts growing exponentially were observed to have thin walls and a cytoplasm exhibiting the characteristics of rapidly growing cells including numerous mitochondria, abundant ribosomes, few vacuoles, and little accumulation of storage material. In contrast, thick-walled yeasts were characterized by less apparent or significantly fewer mitochondria and ribosomes and the presence of considerable amounts of storage materials. Microscope observations of yeast-to-mold conversion revealed that only thick-walled yeasts having prominent lipid bodies in their cytoplasm converted to hyphal forms. Typically, the thick-walled yeast formed two to a number of moniliform hyphal cells which in turn often produced true hyphae. The results indicated that yeasts of P. dermatitidis must acquire spore-like characteristics by becoming thick-walled and by accumulating considerable endogenous substrate reserves before they convert and produce hyphae.