Summary: The morphology of cells was determined from their maximum length, maximum diameter and septal diameter in a mathematical ratio, the morphology index (Mi), which usually ranged from approximately 1 for spherical yeast cells to approximately 4 for true hyphae, with elongated yeast cells and pseudohyphae giving intermediate values. Mi could be determined with high reproducibility for grown in a variety of environments. The highest mean Mi was seen with cells grown in serum and Eagle's medium at 37°C, the lowest with cells grown in Sabouraud glucose broth at 26°C. Variant strains of gave Mi values that remained constant in a variety of growth environments. The Mi facilitated detection of two variants that grew exclusively in the yeast form, one that grew as elongated yeasts but could be induced to form pseudohyphae in serum, and one consistently pseudohyphal variant. Cells with a mean Mi up to 2·5 could be easily separated at septal junctions by mild ultrasonication, whereas cells with a mean Mi greater than 3·5 tended not to separate under these conditions. The chitin content of cells was almost twice as great in cells with a Mi approaching 4 as in cells with a Mi close to 1. The wide range of Mi distributions for a single isolate in different environments demonstrates that the fungus does not undergo abrupt changes of morphological phase: rather there are continual changes in morphology between spherical yeasts and true hyphae at the extremes. The study shows that Mi can be used reliably in place of subjective descriptions of morphology to indicate the shape of a cell. It should facilitate the detection of molecular and cellular markers specific for morphogenesis in the fungus.


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