Morphologic Features of Yeast Colonies

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The page below is a sample from the LabCE course Mycology: Yeasts and Dimorphic Pathogens. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Morphologic Features of Yeast Colonies

Yeast will grow on bacteriological media (sheep blood agar and chocolate agar). They may appear as small, creamy or white colonies that are somewhat more raised than staphylococcal colonies. A presumptive identification of Candida albicans can be made by observing pasty, yellow-white colonies from which "feet" extend out from the margins into the surrounding agar. C. albicans can be seen in Image A.
On BHI agar or fungal media, such as Saboraud's dextrose agar or Mycosel, yeast typically have a smooth (glabrous) texture and are larger than bacterial colonies on the same medium. Candida species are generally creamy white, although Candida krusei exhibits a flat, dry colony morphology. The distinctly mucoid appearance of the colonies in image B provides for a presumptive identification of Cryptococcus species with its production of abundant polysaccharide capsular material. Rhodotorula rubra can virtually be recognized by the distinctive orange-red pigmentation of the colonies, as seen in Image C.
Colonies that demonstrate characteristic morphology should be examined by wet mount to confirm the presence of a yeast in culture.