Saccharomyces cerevisiae | Wikipedia audio article


Saccharomyces cerevisiae () is a species of
yeast. It has been instrumental in winemaking, baking, and brewing since ancient times. 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). S. cerevisiae, a yeast,
have been found to contribute to the smell of bread by Schieberle (1990); proline, and
ornithine present in yeast are precursors of 2‐acetyl‐l‐pyrroline, a roast‐smelling
odorant, in the bread crust.==Etymology==
“Saccharomyces” derives from Latinized Greek and means “sugar-mold” or “sugar-fungus”,
saccharon (σάκχαρον) being the combining form “sugar” and myces (μύκης) being
“fungus”. cerevisiae comes from Latin and means “of beer”. Other names for the organism
are: Brewer’s yeast, though other species are also
used in brewing Ale yeast
Top-fermenting yeast Baker’s yeast
Ragi yeast, in connection to making tapai Budding yeastThis species is also the main
source of nutritional yeast and yeast extract.==History==
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,
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. 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),
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,
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.==Biology=====Ecology===
In nature, yeast cells are found primarily on ripe fruits such as grapes (before maturation,
grapes are almost free of yeasts). 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. The intestine of Polistes dominula, a social wasp, hosts S. cerevisiae
strains as well as S. cerevisiae × S. paradoxus hybrids. Stefanini et al. (2016) showed that
the intestine of Polistes dominula favors 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 (86–95 °F).===Life cycle===
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. 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. However, growth rates vary enormously both between strains and between environments.
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”.===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 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.===Mating===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.===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 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.====Cytokinesis====
Cytokinesis enables budding yeast Saccharomyces cerevisiae to divide into two daughter cells.
S. cerevisiae forms a bud which can grow throughout its cell cycle and later leaves its mother
cell when mitosis has completed.S. cerevisiae is relevant to cell cycle studies because
it divides asymmetrically by using a polarized cell to make two daughters with different
fates and sizes. Similarly, stem cells use asymmetric division for self-renewal and differentiation.=====Timing=====
For many cells, M phase does not happen until S phase is complete. However, for entry into
mitosis in S. cerevisiae this is not true. Cytokinesis begins with the budding process
in late G1 and is not completed until about halfway through the next cycle. The assembly
of the spindle can happen before S phase has finished duplicating the chromosomes. Additionally,
there is a lack of clearly defined G2 in between M and S. Thus, there is a lack of extensive
regulation present in higher eukaryotes.When the daughter emerges, the daughter is two-thirds
the size of the mother. Throughout the process, the mother displays little to no change in
size. The RAM pathway is activated in the daughter cell immediately after cytokinesis
is complete. This pathway makes sure that the daughter has separated properly.=====Actomyosin ring and primary septum formation
=====Two interdependent events drive cytokinesis
in S. cerevisiae. The first event is contractile actomyosin ring (AMR) constriction and the
second event is formation of the primary septum (PS), a chitinous cell wall structure that
can only be formed during cytokinesis. The PS resembles in animals the process of extracellular
matrix remodeling. When the AMR constricts, the PS begins to grow. Disrupting AMR misorients
the PS, suggesting that both have a dependent role. Additionally, disrupting the PS also
leads to disruptions in the AMR, suggesting both the actomyosin ring and primary septum
have an interdependent relationship.The AMR, which is attached to the cell membrane facing
the cytosol, consists of actin and myosin II molecules that coordinate the cells to
split. The ring is thought to play an important role in ingression of the plasma membrane
as a contractile force. Proper coordination and correct positional
assembly of the contractile ring depends on septins, which is the precursor to the septum
ring. These GTPases assemble complexes with other proteins. The septins form a ring at
the site where the bud will be created during late G1. They help promote the formation of
the actin-myosin ring, although this mechanism is unknown. It is suggested they help provide
structural support for other necessary cytokinesis processes. After a bud emerges, the septin
ring forms an hourglass. The septin hourglass and the myosin ring together are the beginning
of the future division site. The septin and AMR complex progress to form
the primary septum consisting of glucans and other chitinous molecules sent by vesicles
from the Golgi body. After AMR constriction is complete, two secondary septums are formed
by glucans. How the AMR ring dissembles remains poorly unknown.Microtubules do not play as
significant a role in cytokinesis compared to the AMR and septum. Disruption of microtubules
did not significantly impair polarized growth. Thus, the AMR and septum formation are the
major drivers of cytokinesis.=====Differences from fission yeast=====
Budding yeast form a bud from the mother cell. This bud grows during the cell cycle and detaches;
fission yeast divide by forming a cell wall Cytokinesis begins at G1 for budding yeast,
while cytokinesis begins at G2 for fission yeast. Fission yeast “select” the midpoint,
whereas budding yeast “select” a bud site During early anaphase the actomyosin ring
and septum continues to develop in budding yeast, in fission yeast during metaphase-anaphase
the actomyosin ring begins to develop==
In biological research=====
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
600 to 300 million years ago, 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-cell organism, S. cerevisiae is
small with a short generation time (doubling time 1.25–2 hours at 30 °C or 86 °F) 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 divides with meiosis, allowing it to be a candidate for sexual genetics research.
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 knockout 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.===In the study of aging===
S. cerevisiae has been highly studied as a model organism to better understand aging
for more than five decades and has contributed to the identification of more mammalian genes
affecting aging than any other model organism. Some of the topics studied using yeast are
calorie restriction, as well as in genes and cellular pathways involved in senescence.
The two most common methods of measuring aging in yeast are Replicative Life Span, which
measures the number of times a cell divides, and Chronological Life Span, which measures
how long a cell can survive in a non-dividing stasis state. Limiting the amount of glucose
or amino acids in the growth medium has been shown to increase RLS and CLS in yeast as
well as other organisms. At first, this was thought to increase RLS by up-regulating the
sir2 enzyme, however it was later discovered that this effect is independent of sir2. Over-expression
of the genes sir2 and fob1 has been shown to increase RLS by preventing the accumulation
of extrachromosomal rDNA circles, which are thought to be one of the causes of senescence
in yeast. The effects of dietary restriction may be the result of a decreased signaling
in the TOR cellular pathway. This pathway modulates the cell’s response to nutrients,
and mutations that decrease TOR activity were found to increase CLS and RLS. This has also
been shown to be the case in other animals. A yeast mutant lacking the genes sch9 and
ras2 has recently been shown to have a tenfold increase in chronological lifespan under conditions
of calorie restriction and is the largest increase achieved in any organism.Mother cells
give rise to progeny buds by mitotic divisions, but undergo replicative aging over successive
generations and ultimately die. However, when a mother cell undergoes meiosis and gametogenesis,
lifespan is reset. The replicative potential of gametes (spores) formed by aged cells is
the same as gametes formed by young cells, indicating that age-associated damage is removed
by meiosis from aged mother cells. This observation suggests that during meiosis removal of age-associated
damages leads to rejuvenation. However, the nature of these damages remains to be established.
During starvation of non-replicating S. cerevisiae cells, reactive oxygen species increase leading
to the accumulation of DNA damages such as apurinic/apyrimidinic sites and double-strand
breaks. Also in non-replicating cells the ability to repair endognous double-strand
breaks declines during chronological aging.===Meiosis, recombination and DNA repair
===S. cerevisiae reproduces by mitosis as diploid
cells when nutrients are abundant. However, when starved, these cells undergo meiosis
to form haploid spores.Evidence from studies of S. cerevisiae bear on the adaptive function
of meiosis and recombination. Mutations defective in genes essential for meiotic and mitotic
recombination in S. cerevisiae cause increased sensitivity to radiation or DNA damaging chemicals.
For instance, gene rad52 is required for both meiotic recombination and mitotic recombination.
Rad52 mutants have increased sensitivity to killing by X-rays, Methyl methanesulfonate
and the DNA cross-linking agent 8-methoxypsoralen-plus-UVA, and show reduced meiotic recombination. These
findings suggest that recombination repair during meiosis and mitosis is needed for repair
of the different damages caused by these agents. Ruderfer et al. (2006) analyzed the ancestry
of natural S. cerevisiae strains and concluded that outcrossing occurs only about once every
50,000 cell divisions. Thus, it appears that in nature, mating is likely most often between
closely related yeast cells. Mating occurs when haploid cells of opposite mating type
MATa and MATα come into contact. Ruderfer et al. pointed out that such contacts are
frequent between closely related yeast cells for two reasons. The first is that cells of
opposite mating type are present together in the same ascus, the sac that contains the
cells directly produced by a single meiosis, and these cells can mate with each other.
The second reason is that haploid cells of one mating type, upon cell division, often
produce cells of the opposite mating type with which they can mate. The relative rarity
in nature of meiotic events that result from outcrossing is inconsistent with the idea
that production of genetic variation is the main selective force maintaining meiosis in
this organism. However, this finding is consistent with the alternative idea that the main selective
force maintaining meiosis is enhanced recombinational repair of DNA damage, since this benefit is
realized during each meiosis, whether or not out-crossing occurs.===Genome sequencing===
S. cerevisiae was the first eukaryotic genome to be completely sequenced. The genome sequence
was released to 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 S. cerevisiae
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 genes are believed to be functional. It is
estimated at least 31% of yeast genes have homologs in the human genome. Yeast genes
are classified using gene symbols (such as sch9) or systematic names. In the latter case
the 16 chromosomes of yeast are represented by the letters A to P, then the gene is further
classified by a sequence number on the left or right arm of the chromosome, and a letter
showing which of the two DNA strands contains its coding sequence.
Examples: YBR134C (aka SUP45 encoding eRF1, a translation
termination factor) is located on the right arm of chromosome 2 and is the 134th open
reading frame (ORF) on that arm, starting from the centromere. The coding sequence is
on the Crick strand of the DNA. YDL102W (aka POL3 encoding a subunit of DNA
polymerase delta) is located on the left arm of chromosome 4; it is the 102nd ORF from
the centromere and codes from the Watson strand of the DNA.===Gene function and interactions===
The availability of the S. cerevisiae genome sequence and a set of deletion mutants covering
90% of the yeast genome 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. The goal is to form a functional map of the cell’s
processes. As of 2010 a model of genetic interactions
is most comprehensive yet to be constructed, containing “the interaction profiles for ~75%
of all genes in the Budding yeast”. This model was made from 5.4 million two-gene comparisons
in which a double gene knockout for each combination of the genes studied was performed. The effect
of the double knockout on the fitness of the cell was compared to the expected fitness.
Expected fitness is determined from the sum of the results on fitness of single-gene knockouts
for each compared gene. When there is a change in fitness from what is expected, the genes
are presumed to interact with each other. This was tested by comparing the results to
what was previously known. For example, the genes Par32, Ecm30, and Ubp15 had similar
interaction profiles to genes involved in the Gap1-sorting module cellular process.
Consistent with the results, these genes, when knocked out, disrupted that process,
confirming that they are part of it.From this, 170,000 gene interactions were found and genes
with similar interaction patterns were grouped together. Genes with similar genetic interaction
profiles tend to be part of the same pathway or biological process. This information was
used to construct a global network of gene interactions organized by function. This network
can be used to predict the function of uncharacterized genes based on the functions of genes they
are grouped with.===Other tools in yeast research===
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. Other resources, include a gene deletion library including
~4,700 viable haploid single gene deletion strains. A GFP fusion strain library used
to study protein localisation and a TAP tag library used to purify protein from yeast
cell extracts.===Synthetic yeast genome project===
The international Synthetic Yeast Genome Project (Sc2.0 or Saccharomyces cerevisiae version
2.0) aims to build an entirely designer, customizable, synthetic S. cerevisiae genome from scratch
that is more stable than the wild type. In the synthetic genome all transposons, repetitive
elements and many introns are removed, all UAG stop codons are replaced with UAA, and
transfer RNA genes are moved to a novel neochromosome. As of March 2017, 6 of the 16 chromosomes
have been synthesized and tested. No significant fitness defects have been found.===Astrobiology===
Among other microorganisms, a sample of living S. cerevisiae was included in the Living Interplanetary
Flight Experiment, which would have completed a three-year interplanetary round-trip in
a small capsule aboard the Russian Fobos-Grunt spacecraft, launched in late 2011. The goal
was to test whether selected organisms could survive a few years in deep space by flying
them through interplanetary space. The experiment would have tested 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. Fobos-Grunt’s mission ended unsuccessfully,
however, when it failed to escape low Earth orbit. The spacecraft along with its instruments
fell into the Pacific Ocean in an uncontrolled re-entry on January 15, 2012. The next planned
exposure mission in deep space using S. cerevisiae is BioSentinel. (see: List of microorganisms
tested in outer space)==In commercial applications=====
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 the lager yeast Saccharomyces pastorianus, 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 (70 °F), or if the fermentation
temperature of the beverage fluctuates during the process. Lager yeast normally ferments
at a temperature of approximately 5 °C (41 °F), where Saccharomyces cerevisiae becomes
dormant. A variant yeast known as Saccharomyces cerevisiae var. diastaticus is a beer spoiler
which can cause secondary fermentations in packaged products.In May 2013, the Oregon
legislature made S. cerevisiae the official state microbe in recognition of the impact
craft beer brewing has had on the state economy and the state’s identity.===Baking===S. cerevisiae is used in baking; the carbon
dioxide generated by the fermentation is used as a leavening agent in bread and other baked
goods. Historically, this use was closely linked to the brewing industry’s use of yeast,
as bakers took or bought the barm or yeast-filled foam from brewing ale from the brewers (producing
the barm cake); today, brewing and baking yeast strains are somewhat different.===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.==Direct use in medicine==
Saccharomyces cerevisiae is used as a probiotic in humans and animals. Especially, a strain
Saccharomyces cerevisiae var. boulardii is industrially manufactured and clinically used
as a medication. Several clinical and experimental studies
have shown that Saccharomyces cerevisiae var. boulardii is, to lesser or greater extent,
useful for prevention or treatment of several gastrointestinal diseases. Moderate quality
evidence shown Saccharomyces cerevisiae var. boulardii to reduce risk of antibiotic-associated
diarrhea both in adults and in children and to reduce risk of adverse effects of Helicobacter
pylori eradication therapy. Also some limited evidence support efficacy of Saccharomyces
cerevisiae var. boulardii in prevention (but not treatment) of traveler’s diarrhea and,
at least as an adjunct medication, in treatment of acute diarrhea in adults and children and
of persistent diarrhea in children.Administration of S. cerevisiae var. boulardii is considered
generally safe. In clinical trials it was well tolerated by patients, and adverse effects
rate was similar to that in control groups (i. e. groups with placebo or no treatment).
No case of S. cerevisiae var. boulardii fungemia has been reported during clinical trials.In
clinical practice, however, cases of fungemia, caused by Saccharomyces cerevisiae var. boulardii
are reported. Patients with compromised immunity or those with central vascular catheters are
at especial risk. Some researchers have recommended not to use Saccharomyces cerevisiae var. boulardii
for treatment of such patients. Others suggest only that caution must be exercised with its
use in risk group patients.==A human pathogen==
Saccharomyces cerevisiae is proven to be an opportunistic human pathogen, though of relatively
low virulence. Despite widespread use of this microorganism at home and in industry, contact
with it very rarely leads to infection.Saccharomyces cerevisiae was found in the skin, oral cavity,
oropharinx, duodenal mucosa, digestive tract and vagina of healthy humans (one review found
it to be reported for 6% of samples from human intestine). Some specialists consider S. cerevisiae
to be a part of normal microbiota of the gastrointestinal tract, the respiratory tract and the vagina
of humans while others believe that the species cannot be called a true commensal and originates
in food. Presence of S. cerevisiae in human digestive system may be rather transient,
for example experiments show that in the case of oral administration to healthy individuals
it is eliminated from the intestine within 5 days after the end of administration.Under
certain circumstances, however, such as degraded immunity, Saccharomyces cerevisiae can cause
infection in humans. Studies show that it causes 0.45-1.06% of the cases of yeast-induced
vaginitis. In some cases women suffering from S. cerevisiae-induced vaginal infection were
intimate partners of bakers, and the strain was found to be the same that their partners
used for baking. As of 1999, no cases of S. cerevisiae-induced vaginitis in women, who
worked in bakeries themselves, were reported in scientific literature. Some cases were
linked by researchers to the use of the yeast in home baking. Cases of infection of oral
cavity and pharynx caused by S. cerevisiae are also known.===Invasive and systemic infections===
Occasionally Saccharomyces cerevisiae causes invasive infections (i. e. gets into the bloodstream
or other normally sterile body fluid or into a deep site tissue, such as lungs, liver or
spleen) that can go systemic (involve multiple organs). Such conditions are life-threatening.
More than 30% cases of S. cerevisiae invasive infections lead to death even if treated.
S. cerevisiae invasive infections, however, are much rarer than invasive infections caused
by Candida albicans even in patients weakened by cancer. S. cerevisiae causes 1% to 3.6%
nosocomial cases of fungemia. A comprehensive review of S. cerevisiae invasive infection
cases found all patients to have at least one predisposing condition.Saccharomyces cerevisiae
may enter the bloodstream or get to other deep sites of the body by translocation from
oral or enteral mucosa or through contamination of intravascular catheters (e. g. central
venous catheters). Intravascular catheters, antibiotic therapy and compromised immunity
are major predisposing factors for S. cerevisiae invasive infection.A number of cases of fungemia
were caused by intentional ingestion of living S. cerevisiae cultures for dietary or therapeutic
reasons, including use of Saccharomyces boulardii (a strain of S. cerevisiae which is used as
a probiotic for treatment of certain forms of diarrhea). Saccharomices boulardii causes
about 40% cases of invasive Saccharomyces infections and is more likely (in comparison
to other S. cerevisiae strains) to cause invasive infection in humans without general problems
with immunity, though such adverse effect is very rare relative to Saccharomices boulardii
therapeutic administration.S. boulardii may contaminate intravascular catheters through
hands of medical personnel involved in administering probiotic preparations of S. boulardii to
patients.Systemic infection usually occurs in patients who have their immunity compromised
due to severe illness (HIV/AIDS, leukemia, other forms of cancer) or certain medical
procedures (bone marrow transplantation, abdominal surgery).A case was reported when a nodule
was surgically excised from a lung of a man embloyed in baking business, and examination
of the tissue revealed presence of Saccharomyces cerevisiae. Inhalation of dry baking yeast
powder is supposed to be the source of infection in this case.===Virulence of different strains===
Not all strains of Saccharomyces cerevisiae are equally virulent towards humans. Most
environmental strains are not capable to grow at temperatures above 35 °C (i. e. at temperatures
of living body of humans and other mammalian). Virulent strains, however, are capable to
grow at least above 37 °C and often up to 39 °C (rarely up to 42 °C). Some industrial
strains are also capable to grow above 37 °C. European Food Safety Authority (as of
2017) requires that all S. cerevisiae strains capable of growth above 37 °C that are added
to the food or feed chain in viable form must, as to be qualified presumably safe, show no
resistance to antimycotic drugs used for treatment of yeast infections.The ability to grow at
elevated temperatures is an important factor for strain’s virulence but not the sole one.Other
traits that are usually believed to be associated with virulence are: ability to produce certain
enzymes such as proteinase and phospholipase, invasive growth (i.e. growth with intrusion
into the nutrient medium), ability to adhere to mammalian cells, ability to survive in
the presence of hydrogen peroxide (that is used by macrophages to kill foreign microorganisms
in the body) and other abilities allowing the yeast to resist or influence immune response
of the host body. Ability to form branching chains of cells, known as pseudohyphae is
also sometimes sait to be associated with virulence, though some research suggests that
this trait may be common to both virulent and non-virulent strains of Saccharomyces
cerevisiae.==See also==
Saccharomyces cerevisiae extracts: Vegemite, Marmite, Cenovis, Guinness Yeast Extract,
mannan oligosaccharides, pgg-glucan, zymosan Saccharomyces cerevisiae boulardii (Saccharomyces
boulardii) Category:Saccharomyces cerevisiae genes

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