Yeasts are found worldwide in a variety of natural habitats or organic substrates such as plant leaves, flowers, soil, and salt water and are often able to grow at reduced oxygen levels. They can also be found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they might be considered parasites, but can live symbiotically. Some yeast is reported to be allergenic, and may cause problems in individuals with previous exposure and developed hypersensitivities.

Yeasts are unicellular fungi, spherical to oval, 2.5 to 6 um in diameter that reproduce asexually through a process called budding. This daughter cell is at first much smaller and tends to cling to the parent and often clumps, or chains of cells, are formed. Yeasts are separated into three groups: 1) Blastomycetes, with no known sexual stage (Fungi Imperfecti); 2) Ascomycota (Hemiascomycetes), which produce ascospores as a result of sexual reproduction; and 3) Blastomycota (Heterobasidiomycetes), which form basidiospores as a result of sexual reproduction. Yeast colonies grow rapidly and appear smooth & glabrous, or pasty, moist or dry, white to cream in color, but some may be tan, pinkish or orange.

Yeasts most commonly isolated from human sources include Candida (candidiasis), Cryptococcus (cryptococcosis), Torulopsis (torulopsosis), Trichosporon (trichosporonosis), Geotrichum and many others causing miscellaneous infections. Yeast infections are among the most common fungal infections in humans. Their form ranges from localized cutaneous or mucocutaneous lesions, to fungemia or disseminated systemic mycoses. Candida albicans is the most frequent yeast pathogen isolated because of their part in the normal flora of the gastrointestinal tract and female urogenital area.

The most well-known commercially significant yeast are the species of Saccharomyces cerevisiae, which is used in the production of several types of beers, or known as Brewer’s yeast. It is also known as Baker’s yeast and is used for other types of fermentation. Yeast is often taken as a vitamin supplement because it is 50% protein and is also a good source of B vitamins, niacin, and folic acid.

(A_w-0.61-0.95) Culture - Potato dextrose agar, 20° – 25°C, 7 – 10 days.