What are yeast?

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Introduction to Fungi Credit: Kandis Elliot, Mo Fayyaz, University of Wisconsin, Madison

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General Information

Yeast are single-celled eukaryotic microorganisms that are classified, along with molds and mushrooms, as members of the kingdom Fungi. Yeasts are phylogenetically diverse, and as such are classified in two separate phyla, the Ascomycota and the Basidiomycota. Budding yeast (also called “true yeasts”), such as the well-known species Saccharomyces cerevisiae, are classified in the order Saccharomycetales under the phylum Ascomycota. Such classifications are based on characteristics of the cell, ascospore, and colony, as well as on physiology. One of the most well known characteristics of yeast is its ability to ferment sugars for the production of ethanol and carbon dioxide.

C. albicans, Credit: E. Gueho/Science Photo Library


Yeast are characterized by a wide dispersion of natural habitats. They are common on plant leaves, flowers, and fruits, as well as in soil. Yeast are also found on the skin surfaces and in the intestinal tracts of warm-blooded animals, where they may live symbiotically or as parasites. The common "yeast infection" Candidiasis is typically caused by Candida albicans. In addition to being the causative agent in vaginal yeast infections Candida is also a cause of diaper rash and thrush of the mouth and throat.


EM image of S. cerevisiae Credit: UC Berkeley
File:Pombe cerevisiae.jpg
Fission (S. pombe) and budding (S. cerevisiae) yeasts Credit: Susan L. Forsberg, Nat Rev Gen 2:659

Yeast reproduce asexually by an asymmetric division process called budding (eg. Saccharomyces), by a symmetric division process called fission (eg. Schizosaccharomyces), or they can grow as simple irregular filaments (mycelium). In budding, a small bud emerges from the surface of the parent cell and enlarges until it is almost the size of the parent, while in fission the rod shaped cell grows at the cell's tips and then divides in half to produce two daughter cells of equal size. Yeast can also reproduce sexually, and most do so my forming asci, which contain up to eight haploid ascospores. These ascospores may fuse with adjoining nuclei and multiply through vegetative division or, as with certain yeast, fuse with other ascospores.


Commercial Applications

Saccharomyces cerevisiae, one of the most well-known and commercially significant species of yeast, , has long been used as a leavening agent in baking. S. cerevisiae, commonly known as baker’s yeast, ferment sugars present in dough to carbon dioxide and ethanol. The carbon dioxide becomes trapped in small bubbles in the dough, which causes the dough to rise.
Wild yeast and acid-generating bacteria are used in making sourdough bread
Sourdough bread is not produced with baker's yeast, rather a combination of wild yeast (often Candida milleri) and an acid-generating bacteria (Lactobacillus sanfranciscensis sp. nov). The C. milleri strengthens the gluten and the L. sanfrancisco ferments the maltose.


File:Hansen-emil-christian.jpeg
Dr. Emil Christian Hansen, 1842-1909

Saccharomyces cerevisiae and other yeast species have also long been used to ferment the sugars of rice, wheat, barley, and corn to produce alcoholic beverages such as beer and wine. There are two major classes of beer brewing yeast: ale yeast (top-fermenting type) and lager yeast (bottom-fermenting type). Ales are produced by S. cerevisiae, the same yeast used to make bread. These yeast rise to the surface during fermentation, and hence are called top-fermenting yeast. Top-fermenting yeasts are used to brew ales, porters, stouts, and wheat beers. Lagers, on the other hand, are produced by Saccharomyces pastorianus. This yeast species was formerly known as Saccharomyces carlsbergensis, named after the Carlsberg Brewery in Copenhagen where it was first isolated in pure culture by Dr. Emil Christian Hansen in 1883. Lager yeast are best used at lower temperatures and grow slower than ale yeast. As a result, they produce less surface foam and typically settle to the bottom of the fermenter, and are thus called bottom-type fermenters. Some of the lager styles made from bottom-fermenting yeasts include Pilsners, Märzen, Bocks, and American malt liquors. In modern beer brewing many of the original top fermentation strains have been modified to be bottom fermenters.


Bunches of grapes on vines at Trinity Hill vineyard in Hawkes Bay NZ

Yeast produce wine by fermenting sugars present in grape juice (must) into ethanol. The fermentation of wine can be initiated by naturally occurring yeast present in the vineyards, but many wineries choose to add a pure yeast culture to dominate and control the fermentation. The bubbles in champagne and sparkling wines are produced by a secondary fermentation, typically in the bottle, which traps the carbon dioxide. Carbon dioxide produced in wine production is released as a by-product. One yeast cell can ferment approximately its own weight of glucose per hour. Under optimal conditions S. cerevisiae can produce up to 18 percent, by volume, ethanol with 15 to 16 percent being the norm. The sulfur dioxide present in commercially produced wine is actually added just after the grapes are crushed to kill the naturally present bacteria, mold, and yeast.


The in-depth knowledge of S. cerevisiae and it's ability to be metabolically engineered has made it an important organism in the production of specialty chemicals, such as biofuels, industrial lubricants, detergents, and biopharmaceuticals.

Yeast as a Model Organism

Yeast, particularly S. cerevisiae, became a model organism for studying cell biology and genetics because it is a single-celled eukaryote that is fairly easy to grow and manipulate genetically. In addition, the basic cellular mechanics of replication, recombination, cell division and metabolism are generally conserved between it and more complex eukaryotes, including humans. As a result yeast are one of the most thoroughly researched eukaryotic organisms. In 1996, S. cerevisiae was the first eukaryote to have its genome completely sequenced. Several significant scientific discoveries have been made through S. cerevisiae research, including some which have led to Nobel Prizes. These include an award to Drs. Leland Hartwell, Tim Hunt, and Paul Nurse in 2001 for their discovery of genes involved in regulating the cell cycle, as well as a 2009 award to Drs. Elizabeth Blackburn, Carol Greider, and Jack Szostak for their work showing how telomeres and the enzyme telomerase protect the ends of chromosomes from degradation.

Resources

  • Saccharomyces Genome Database (SGD) provides comprehensive integrated biological information for the budding yeast Saccharomyces cerevisiae along with search and analysis tools to explore these data.
  • PomBase, a comprehensive database for the fission yeast Schizosaccharomyces pombe, providing structural and functional annotation, literature curation and access to large-scale data sets.
  • Fungal Genomes Central, a portal to information and resources about fungi and fungal sequencing projects from NCBI and the fungi research community.
  • An extensive list of yeast-related resources on topics ranging from general yeast information to nucleic acids, genomes and proteins, expression data, localization, phenotypes and more.

Suggested Reading

  • YeastBook. (2011) A comprehensive compendium of reviews that presents the current state of knowledge of the molecular biology, cellular biology, and genetics of the yeast Saccharomyces cerevisiae, Genetics
  • The Life of Yeasts. (1978) H.J. Phaff, M.W. Miller, and E. M. Mrak, Harvard University Press, Cambridge, Massachussetts.
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