Saccharomyces

Overview
Saccharomyces is a genus in the kingdom of fungi that includes many species of yeast. Saccharomyces is from Latin meaning sugar fungi. Many members of this genus are considered very important in food production. 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.

Morphology
Colonies of Saccharomyces grow rapidly and mature in 3 days. They are flat, smooth, moist, glistening or dull, and cream to tannish cream in color. The inability to utilize nitrate and ability to ferment various carbohydrates are typical characteristics of Saccharomyces.

Cellular morphology
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 are gram-negative while vegetative cells are gram-positive.

History
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 1888, 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 yeast are polyploid and belong to the Saccharomyces genera. The brewing strains can be classified into two groups; the ale strains (Saccharomyces cerevisiae, generally used for ale and stout) and the lager strains (Saccharomyces pastorianus or Saccharomyces uvarum). Lager strains are a hybrid strain of S. cerevisiae (ale strains) and S. bayanus (wine strains) 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 utilisation, the S. pastorianus and S. cerevisiae species are closely related within the Saccharomyces genus.

Saccharomyces yeasts can form symbiotic matrices with bacteria, and are used to produce kombucha, kefir and ginger beer.