The term “yeast” does not refer to any recognized taxonomic name but instead describes fungi that are unicellular and usually reproduce asexually by budding. The term yeast is also used, more specifically, for those species in the genus Saccharomyces that are used to leaven bread and ferment alcoholic beverages.
Among mycologists, there is some disagreement over what should be called a yeast. Many mycologists use the term to describe any fungus that has a unicellular budding format any time in its life. They often use the term “monomorphic” to describe those that are always unicellular and the term “dimorphic” to describe those that can have both unicellular and filamentous growth.
Others, however, reserve the name yeast for those species that are permanently unicellular and use the term “yeastlike” to describe those fungi that can alternate between mycelial and unicellular forms. Because some species that have traditionally been called yeasts have later been shown to have a mycelial form, the former broader definition will be used here.
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Taxonomy
Yeasts are found in all three major fungal phyla, Zygomycota, Ascomycota, and Basidiomycota, but the vast majority are ascomycetes. As in many fungi, the placement of some species in the proper phylum is made difficult by the lack of data on sexual reproduction.
In others, the sexually reproducing, or telomorph, formand the asexually reproducing, or anamorph, form have been assigned different names. In addition, dimorphic fungi were often assigned different names for their yeast and mycelial phases.
Mucor indicus (synonymous with Mucor rouxii) is a zygomycote yeast. Basidiomycete genera include Filobasidiella (anamorph: Cryptococcus), Rhodospiridium, and Ustilago. Some ascomycote genera are Saccharomyces, Candida, Blastomyces, and Ajellomyces (anamorph: Histoplasma).
Reproduction
The most common reproductive mechanism seen in yeasts is budding. During this asexual process the nucleus divides, and a small section of the original cell containing one of the new nuclei begins to bulge from the original cell. The cell and the bud begin to separate by the formation of a new cell wall called, at this stage, the cell plate.
The bud grows and, usually, separates from the original cell. In some species, buds do not separate and, after they have grown, may produce buds of their own. This pattern leads to a connected group of cells produced by sequential budding referred to as a pseudomycelium.
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| budding Yeast Cell (Saccharomyces Cerevisiae) |
Yeasts may bud new cells from any part of the original cell (called multilateral budding) or from just the tips of the cell (called polar budding). The release of a bud often leaves a bud scar, and the scarred area is usually not able to produce another bud.
Other methods of reproduction include fission, in which the original cell divides equally, and the production of various kinds of spores occurs both asexually and sexually.
Cells
Like all fungi, yeasts are eukaryotic organisms that can exist in haploid, diploid, and dikaryotic (two haploid nuclei per cell) states. Unlike filamentous fungi, in which the zygote is the only diploid cell, ascomycote yeasts such as Saccharomyces can have a prolonged diploid state after the haploid nuclei of the dikaryote fuse.
Cell components of yeasts are quite similar to those of filamentous fungi. One exception is that monomorphic yeasts have much lower levels of chitin in their cell walls, and the small amount that is present is found mainly in the bud scars.
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Uses
Both brewing and bread-making, which use various Saccharomyces species, have existed for millennia. Four-thousand-year-old tomb paintings in Egypt depict both, but only since the mid-1800’s has the involvement of yeast in these processes been studied.
In both, complex carbohydrates are converted to glucose, and the yeast ferments glucose, producing ethyl alcohol and carbon dioxide. Saccharomyces cerevisiae is the most common baker’s yeast, although Candida milleri is important in the production of sourdough breads.
In beer brewing, the bottom-fermenting Saccharomyces carlsbergensis, which tolerates cold (10 degrees Celsius) is the most commonly used yeast. Wines, a few beers, and most ales use Saccharomyces cerevisiae, a top fermenter that requires higher temperatures (20-25 degrees Celsius).
Yeasts are also important in the development of flavor and texture in certain cheeses. Production of Limburger, Camembert, Brie, and Swiss cheeses all rely on yeast fermentation. Because of their high nutrient content, yeasts themselves are important foods and food additives.
Yeasts have also been used in the production of various industrial chemicals and biochemicals, including glycerol, ethanol, B vitamins, and polysaccharides. With the advent of modern genetic techniques, yeasts are being engineered to produce many other useful products.
Pathogens
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| Candida albicans |
Many yeasts can cause disease in plants or animals. Histoplasma capsulatum, Blastomyces dermatiditis, and Cryptococcus neoformans are all dimorphic fungi that can cause systemic infection in humans when the fungi are in the yeast form.
Candida albicans, also dimorphic, is an opportunistic human pathogen which is pathogenic in its filamentous form. Most Candida infections, such as vaginal yeast infections and thrush, are superficial, but systemic infections can occur in immuno-compromised individuals.
Pneumocystis carini, which causes respiratory infections in patients with acquired immunodeficiency syndrome, was originally classified as a protist but is now thought to be a dimorphic fungus. The dimorphic genera Ustilago and Taphrina both contain many plant pathogens.
