A complex group of fungi resembling one another in existing as single cells that "bud" directly to form new cells. The colonies are pasty in appearance. Some yeasts may form ascospores within their cells. Common in moist habitats and often able to grow at reduced oxygen levels. See the discussion of yeasts under the broader topic of fungal growth.
The pictures above left illustrates Saccharomyces cerevisiae, a yeast used for the rising of bread and the brewing of beer. The one at right is probably Pichia membranaefaciens, a species found in a variety of habitats. It often colonizes stored foods where it may form a thin film over the surface. The picture at left shows a jar of olives colonized by P. membranaefaciens
Holomorphs: various and not necessarily related to one another. Ref: Arx, Rodrigues de Miranda, Smith, and Yarrow 1977; Kurtzman et al., 2011