Saccharomyces is a genus in the kingdom of fungi that includes many species of yeast. Saccharomyces is from Greek σάκχαρ (sugar) and μύκης (mushroom) and means sugar fungus. Many members of this genus are considered very important in food production. It is known as the brewer's yeast or baker's yeast. They are unicellular and saprophytic fungi. One example is Saccharomyces cerevisiae, which is used in making wine, bread, and beer. Other members of this genus include Saccharomyces bayanus, used in making wine, and Saccharomyces boulardii, used in medicine.
Colonies of Saccharomyces grow rapidly and mature in three days. They are flat, smooth, moist, glistening or dull, and cream to tannish cream in color. The inability to use nitrate and ability to ferment various carbohydrates are typical characteristics of Saccharomyces.
Cellular morphology 
Generally, they have a diameter of 2-8μm and length of 3-25μm. Blastoconidia (cell buds) are observed. They are unicellular, globose, and ellipsoid to elongate in shape. Multilateral (multipolar) budding is typical. Pseudohyphae, if present, are rudimentary. Hyphae are absent.
Saccharomyces produces ascospores, especially when grown on V-8 medium, acetate ascospor agar, or Gorodkowa medium. These ascospores are globose and located in asci. Each ascus contains 1-4 ascospores. Asci do not rupture at maturity. Ascospores are stained with Kinyoun stain and ascospore stain. When stained with Gram stain, ascospores appear Gram-negative, while vegetative cells appear Gram-positive.
The presence of yeast in beer was first suggested in 1680, although the genus was not named Saccharomyces until 1837. It was not until 1876 that Louis Pasteur demonstrated the involvement of living organisms in fermentation and in 1883, Emil C. Hansen isolated brewing yeast and propagated leading to the importance of yeast in brewing. The use of microscopes for the study of yeast morphology and purity was crucial to understanding their functionality.
Use in brewing 
Brewing yeasts are polyploid and belong to the Saccharomyces genera. The brewing strains can be classified into two groups; the ale strains (Saccharomyces cerevisiae) and the lager strains (Saccharomyces pastorianus or Saccharomyces carlsbergensis in the old taxonomy). Lager strains are a hybrid strain of S. cerevisiae and S. eubayanus and are often referred to as bottom fermenting. In contrast, ale strains are referred to as top fermenting strains, reflecting their separation characteristics in open square fermenters. Although the two species differ in a number of ways, including their response to temperature, sugar transport and use, the S. pastorianus and S. cerevisiae species are closely related within the Saccharomyces genus.
Saccharomyces cause food spoilage of sugar-rich foods, such as maple sap, syrup, concentrated juices and condiments. Case report suggest extended exposure to S. cerevisiae can result in hypersensitivity.
See also 
- Boulton, Chris and Quain, David. Brewing yeast and fermentation. John Wiley & Sons, 2001, p. 18
- MICROBES INVOLVED IN FOOD SPOILAGE Authors: Gabriel Chavarria, Julia Neal, Parul Shah, Katrina Pierzchala, Bryant Conger
- Yamamoto Y, Osanai S, Fujiuchi S, et al. (2002). "[Saccharomyces-induced hypersensitivity pneumonitis in a dairy farmer: a case report]". Nihon Kokyuki Gakkai Zasshi (in Japanese) 40 (6): 484–8. PMID 12325333.