Minggu, 25 Maret 2018

Saccharomyces cerevisiae

Jual Culture Saccharomyces cerevisiae

Saccharomyces cerevisiae is a species of yeast. It is believed to have been originally isolated from the skin 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 μm 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. S. cerevisiae is currently the only yeast cell known to have Berkeley bodies present, which are involved in particular secretory pathways. Antibodies against S. cerevisiae are found in 60–70% of patients with Crohn's disease and 10–15% of patients with ulcerative colitis (and 8% of healthy controls).
"Saccharomyces" derives from Latinized Greek and means "sugar-mold" or "sugar-fungus", saccharo (σάκχαρις) being the combining form "sugar" and myces (μύκης, genitive μύκητος) being "fungus". Cerevisiae comes from Latin and means "of beer". Other names for the organism are:
In the 19th century, bread bakers obtained their yeast from beer brewers, and this led to sweet-fermented breads such as the Imperial "Kaisersemmel" roll,[4] which in general lacked the sourness created by the acidification typical of Lactobacillus. However, beer brewers slowly switched from top-fermenting (S. cerevisiae) to bottom-fermenting (S. pastorianus) yeast and this created a shortage of yeast for making bread, so the Vienna Process was developed in 1846.[5] While the innovation is often popularly credited for using steam in baking ovens, leading to a different crust characteristic, it is notable for including procedures for high milling of grains (see Vienna grits[6]), cracking them incrementally instead of mashing them with one pass; as well as better processes for growing and harvesting top-fermenting yeasts, known as press-yeast.
Refinements in microbiology following the work of Louis Pasteur led to more advanced methods of culturing pure strains. In 1879, Great Britain introduced specialized growing vats for the production of S. cerevisiae, and in the United States around the turn of the century centrifuges were used for concentrating the yeast,[7] making modern commercial yeast possible, and turning yeast production into a major industrial endeavor. The slurry yeast made by small bakers and grocery shops became cream yeast, a suspension of live yeast cells in growth medium, and then compressed yeast, the fresh cake yeast that became the standard leaven for bread bakers in much of the Westernized world during the early 20th century.
During World War II, Fleischmann's developed a granulated active dry yeast for the United States armed forces, which did not require refrigeration and had a longer shelf-life and better temperature tolerance than fresh yeast; it is still the standard yeast for US military recipes. The company created yeast that would rise twice as fast, cutting down on baking time. Lesaffre would later create instant yeast in the 1970s, which has gained considerable use and market share at the expense of both fresh and dry yeast in their various applications.
In nature, yeast cells are found primarily on ripe fruits such as grapes (before maturation, grapes are almost free of yeasts).[8] Since S. cerevisiae is not airborne, it requires a vector to move.Queens of social wasps overwintering as adults (Vespa crabro and Polistes spp.) can harbor yeast cells from autumn to spring and transmit them to their progeny.[9] The intestine of Polistesdominula, a social wasp, hosts S. cerevisiae strains as well as S. cerevisiae × S. paradoxus hybrids. Stefanini et al. (2016) showed that the intestine of Polistesdominulafavors the mating of S. cerevisiae strains, both among themselves and with S. paradoxus cells by providing environmental conditions prompting cell sporulation and spores germination.
The optimum temperature for growth of S. cerevisiae is 30–35 °C. Two forms of yeast cells can survive and grow: haploid and diploid. The haploid cells undergo a simple lifecycle of mitosis and growth, and under conditions of high stress will, in general, die. This is the asexual form of the fungus. The diploid cells (the preferential 'form' of yeast) similarly undergo a simple lifecycle of mitosis and growth. The rate at which the mitotic cell cycle progresses often differs substantially between haploid and diploid cells.[11] Under conditions of stress, diploid cells can undergo sporulation, entering meiosis and producing four haploid spores, which can subsequently mate. This is the sexual form of the fungus. Under optimal conditions, yeast cells can double their population every 100 minutes.[12][13] However, growth rates vary enormously both between strains and between environments.[14] Mean replicative lifespan is about 26 cell divisions.
In the wild, recessive deleterious mutations accumulate during long periods of asexual reproduction of diploids, and are purged during selfing: this purging has been termed "genome renewal". 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 nitrogen sources. 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. Concerning organic requirements, most strains of S. cerevisiae require biotin. Indeed, a S. cerevisiae-based growth assay laid the foundation for the isolation, crystallisation, and later structural determination of biotin. Most strains also require pantothenate for full growth. In general, S. cerevisiae is prototrophic for vitamins.
Yeast has two mating types, a and α (alpha), which show primitive aspects of sex differentiation. As in many other eukaryotes, mating leads to genetic recombination, i.e. production of novel combinations of chromosomes. Two haploid yeast cells of opposite mating type can mate to form diploid cells that can either sporulate to form another generation of haploid cells or continue to exist as diploid cells. Mating has been exploited by biologists as a tool to combine genes, plasmids, or proteins at will.
The mating pathway employs a G protein-coupled receptor, G protein, RGS protein, and three-tiered MAPK signaling cascade that is homologous to those found in humans. This feature has been exploited by biologists to investigate basic mechanisms of signal transduction and desensitization. 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 well nourished, 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.

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