Saccharomyces cerevisiae

Saccharomyces cerevisiae is a species of yeast. It is perhaps the most useful yeast, having been instrumental to baking and brewing since ancient times. It is believed that it was originally isolated from the skins of grapes (one can see the yeast as a component of the thin white film on the skins of some dark-colored fruits such as plums; it exists among the waxes of the cuticle). It is one of the most intensively studied eukaryotic model organisms in molecular and cell biology, much like Escherichia coli as the model bacterium. It is the microorganism behind the most common type of fermentation. S. cerevisiae cells are round to ovoid, 5–10 micrometres in diameter. It reproduces by a division process known as budding.

Many proteins important in human biology were first discovered by studying their homologs in yeast; these proteins include cell cycle proteins, signaling proteins, and protein-processing enzymes. The petite mutation in S. cerevisiae is of particular interest.

Antibodies against S. cerevisiae are found in 60–70% of patients with Crohn's disease and 10–15% of patients with ulcerative colitis.

Nomenclature
"Saccharomyces" derives from Latinized Greek and means "sugar mold" or "sugar fungus", saccharo- being the combining form "sugar-" and myces being "fungus". Cerevisiae comes from Latin and means "of beer". Other names for the organism are:
 * S. cerevisiae short form of the scientific name
 * Brewer's yeast, though other species are also used in brewing
 * Ale yeast
 * Top-fermenting yeast
 * Baker's yeast
 * Budding yeast

This species is also the main source of nutritional yeast and yeast extract.

Life cycle
There are two forms in which yeast cells can survive and grow: haploid and diploid. The haploid cells undergo a simple life cycle of mitosis and growth, and under conditions of high stress will, in general, die. The diploid cells (the preferential 'form' of yeast) similarly undergo a simple life cycle of mitosis and growth, but under conditions of stress can undergo sporulation, entering meiosis and producing a variety of haploid spores, which can proceed on to mate.

Nutritional requirements
All strains of S. cerevisiae can grow aerobically on glucose, maltose, and trehalose and fail to grow on lactose and cellobiose. However, growth on other sugars is variable. Galactose and fructose are shown to be two of the best fermenting sugars. The ability of yeasts to use different sugars can differ depending on whether they are grown aerobically or anaerobically. Some strains cannot grow anaerobically on sucrose and trehalose.

All strains can use ammonia and urea as the sole nitrogen source, but cannot use nitrate, since they lack the ability to reduce them to ammonium ions. They can also use most amino acids, small peptides, and nitrogen bases as a nitrogen source. Histidine, glycine, cystine, and lysine are, however, not readily used. S. cerevisiae does not excrete proteases, so extracellular protein cannot be metabolized.

Yeasts also have a requirement for phosphorus, which is assimilated as a dihydrogen phosphate ion, and sulfur, which can be assimilated as a sulfate ion or as organic sulfur compounds such as the amino acids methionine and cysteine. Some metals, like magnesium, iron, calcium, and zinc, are also required for good growth of the yeast.

Mating
Yeast has two mating types, a and α (alpha), which show primitive aspects of sex differentiation, and are, hence, of great interest. For more information on the biological importance of these two cell types, where they come from (from a molecular biology point of view), and details of the process of mating type switching, see Mating of yeast.

Cell cycle
Growth in yeast is synchronised with the growth of the bud, which reaches the size of the mature cell by the time it separates from the parent cell. In rapidly growing yeast cultures, all the cells can be seen to have buds, since bud formation occupies the whole cell cycle. Both mother and daughter cells can initiate bud formation before cell separation has occurred. In yeast cultures growing more slowly, cells lacking buds can be seen, and bud formation only occupies a part of the cell cycle. The cell cycle in yeast normally consists of the following stages – G1, S, G2, and M – which are the normal stages of mitosis.

A model organism
When researchers look for an organism to use in their studies, they look for several traits. Among these are size, generation time, accessibility, manipulation, genetics, conservation of mechanisms, and potential economic benefit. The yeast species S. pombe and S. cerevisiae are both well studied; these two species diverged approximately, and are significant tools in the study of DNA damage and repair mechanisms.

S. cerevisiae has developed as a model organism because it scores favorably on a number of these criteria.
 * As a single celled organism S. cerevisiae is small with a short generation time (doubling time 1.25–2 hours at 30 C) and can be easily cultured. These are all positive characteristics in that they allow for the swift production and maintenance of multiple specimen lines at low cost.
 * S. cerevisiae can be transformed allowing for either the addition of new genes or deletion through homologous recombination. Furthermore, the ability to grow S. cerevisiae as a haploid simplifies the creation of gene knockouts strains.
 * As a eukaryote, S. cerevisiae shares the complex internal cell structure of plants and animals without the high percentage of non-coding DNA that can confound research in higher eukaryotes.
 * S. cerevisiae research is a strong economic driver, at least initially, as a result of its established use in industry.

Genome sequencing
S. cerevisiae was the first eukaryotic genome that was completely sequenced. The genome sequence was released in the public domain on April 24, 1996. Since then, regular updates have been maintained at the Saccharomyces Genome Database. This database is a highly annotated and cross-referenced database for yeast researchers. Another important S. cerevisiae database is maintained by the Munich Information Center for Protein Sequences (MIPS). The genome is composed of about 12,156,677 base pairs and 6,275 genes, compactly organized on 16 chromosomes. Only about 5,800 of these are believed to be true functional genes. Yeast is estimated to share about 23% of its genome with that of humans.

Other tools in yeast research
The availability of the S. cerevisiae genome sequence and the complete set of deletion mutants has further enhanced the power of S. cerevisiae as a model for understanding the regulation of eukaryotic cells. A project underway to analyze the genetic interactions of all double deletion mutants through synthetic genetic array analysis will take this research one step further.

Approaches that can be applied in many different fields of biological and medicinal science have been developed by yeast scientists. These include yeast two-hybrid for studying protein interactions and tetrad analysis.

Astrobiology
Among other microorganisms, a sample of living S. cerevisiae will be included in the Living Interplanetary Flight Experiment, which would complete a three-year interplanetary round-trip in a small capsule aboard the Russian Fobos-Grunt spacecraft, to be launched in late 2011. The goal is to test whether selected organisms can survive a few years in deep space by flying them through interplanetary space. The experiment will test one aspect of transpermia, the hypothesis that life could survive space travel, if protected inside rocks blasted by impact off one planet to land on another.

Brewing
Saccharomyces cerevisiae is used in brewing beer, when it is sometimes called a top-fermenting or top-cropping yeast. It is so called because during the fermentation process its hydrophobic surface causes the flocs to adhere to CO2 and rise to the top of the fermentation vessel. Top-fermenting yeasts are fermented at higher temperatures than lager yeasts, and the resulting beers have a different flavor than the same beverage fermented with a lager yeast. "Fruity esters" may be formed if the yeast undergoes temperatures near 21 C, or if the fermentation temperature of the beverage fluctuates during the process. Lager yeast normally ferments at a temperature of approximately 5 C, where Saccharomyces cerevisiae becomes dormant. Lager yeast can be fermented at a higher temperature to create a beer style known as "steam beer".

Uses in aquaria
Owing to the high cost of commercial CO2 cylinder systems, CO2 injection by yeast is one of the most popular DIY approaches followed by aquaculturists for providing CO2 to underwater aquatic plants. The yeast culture is, in general, maintained in plastic bottles, and typical systems provide one bubble every 3–7 seconds. Various approaches have been devised to allow proper absorption of the gas into the water.