Saccharomyces cerevisiae

Saccharomyces cerevisiae(/ˌsɛrəˈvɪsi.iː/) (brewer's yeastorbaker's yeast) is a species ofyeast(single-celledfungalmicroorganisms). The species has been instrumental inwinemaking,baking, andbrewingsince ancient times. It is believed to have been originally isolated from the skin ofgrapes.It is one of the most intensively studiedeukaryoticmodel organismsinmolecularandcell biology, much likeEscherichia colias the modelbacterium. It is the microorganism which causes many common types offermentation.S. cerevisiaecells are round to ovoid, 5–10μmin diameter. It reproduces bybudding.
Manyproteinsimportant in human biology were first discovered by studying theirhomologsin yeast; these proteins includecell cycleproteins,signaling proteins, and protein-processingenzymes.S. cerevisiaeis currently the only yeast cell known to haveBerkeley bodiespresent, which are involved in particular secretory pathways.AntibodiesagainstS. cerevisiaeare found in 60–70% of patients withCrohn's diseaseand 10–15% of patients withulcerative colitis, and may be useful as part of a panel of serological markers in differentiating between inflammatory bowel diseases (e.g. between ulcerative colitis and Crohn's disease), their localization and severity.
"Saccharomyces" derives fromLatinizedGreekand means "sugar-mould" or "sugar-fungus",saccharon(σάκχαρον) being the combining form "sugar" andmyces(μύκης) being "fungus".cerevisiaecomes from Latin and means "of beer".Other names for the organism are:
This species is also the main source ofnutritional yeastandyeast extract.
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 ofLactobacillus. However, beer brewers slowly switched from top-fermenting (S. cerevisiae) to bottom-fermenting (S. pastorianus) yeast. TheVienna Processwas 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 ofLouis Pasteurled to more advanced methods of culturing pure strains. In 1879, Great Britain introduced specialized growing vats for the production ofS. cerevisiae, and in the United States around the turn of the 20th century centrifuges were used for concentrating the yeast,turning yeast production into a major industrial process which simplified its distribution, reduced unit costs and contributed to the commercialization and commoditization of bread and beer. Fresh "cake yeast" became the standard leaven for bread bakers in much of the Western world during the early 20th century.
DuringWorld War II,Fleischmann'sdeveloped agranulatedactive 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.Lesaffrewould 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).S. cerevisiaecan also be found year-round in the bark ofoak trees.SinceS. cerevisiaeis not airborne, it requires a vector to move.
Queens of social wasps overwintering as adults (Vespa crabroandPolistesspp.) can harbor yeast cells from autumn to spring and transmit them to their progeny.The intestine ofPolistes dominula, a social wasp, hostsS. cerevisiaestrains as well asS. cerevisiae×S. paradoxushybrids. Stefanini et al. (2016) showed that the intestine ofPolistes dominulafavors the mating ofS. cerevisiaestrains, both among themselves and withS. paradoxuscells by providing environmental conditions prompting cellsporulationand spores germination.
The optimum temperature for growth ofS. cerevisiaeis 30–35 °C (86–95 °F).
Two forms of yeast cells can survive and grow:haploidanddiploid. The haploid cells undergo a simplelifecycleofmitosisand growth, and under conditions of high stress will, in general, die. This is theasexualform of the fungus. The diploid cells (the preferential 'form' of yeast) similarly undergo a simple lifecycle of mitosis andgrowth. The rate at which the mitotic cell cycle progresses often differs substantially between haploid and diploid cells.Under conditions ofstress, diploid cells can undergosporulation, enteringmeiosisand producing four haploidspores, which can subsequently mate. This is thesexualform of thefungus. Under optimal conditions, yeast cells can double their population every 100 minutes.However, growth rates vary enormously between strains and between environments.Meanreplicativelifespan is about 26 cell divisions.
In the wild, recessive deleterious mutations accumulate during long periods ofasexual reproductionof diploids, and are purged duringselfing: this purging has been termed "genome renewal".
AllstrainsofS. cerevisiaecan growaerobicallyonglucose,maltose,andtrehaloseand fail to grow onlactoseandcellobiose. However, growth on othersugarsis variable.Galactoseandfructoseare 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 onsucroseand trehalose.
All strains can useammoniaandureaas the solenitrogensource, but cannot usenitrate, since they lack the ability to reduce them toammoniumions. They can also use mostamino acids, smallpeptides, and nitrogen bases as nitrogen sources.Histidine,glycine,cystine, andlysineare, however, not readily used.S. cerevisiaedoes not excreteproteases, so extracellular protein cannot be metabolized.
Yeastsalso have a requirement forphosphorus, which is assimilated as a dihydrogen phosphate ion, andsulfur, which can be assimilated as asulfateion or as organic sulfur compounds such as theamino acidsmethionine and cysteine. Some metals, likemagnesium,iron,calcium, andzinc, are also required for good growth of the yeast.
Concerning organic requirements, most strains ofS. cerevisiaerequirebiotin.Indeed, aS. cerevisiae-based growth assay laid the foundation for the isolation, crystallization, and later structural determination of biotin. Most strains also requirepantothenatefor full growth. In general,S. cerevisiaeis prototrophic for vitamins.
Yeast has two mating types,aand α (alpha), which show primitive aspects of sex differentiation.As in many other eukaryotes, mating leads togenetic recombination, i.e. production of novel combinations of chromosomes. Twohaploidyeast cells of opposite mating type can mate to formdiploidcells that can eithersporulateto 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 aG protein-coupled receptor,G protein,RGS protein, and three-tieredMAPKsignaling cascade that is homologous to those found in humans. This feature has been exploited by biologists to investigate basic mechanisms ofsignal transductionanddesensitization.
Growth in yeast is synchronized with the growth of thebud, which reaches the size of the mature cell by the time it separates from the parent cell. In well nourished, rapidly growing yeastcultures, all the cells have buds, since bud formation occupies the wholecell 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.
Cytokinesisenables budding yeastSaccharomyces cerevisiaeto divide into two daughter cells.S. cerevisiaeforms a bud which can grow throughout its cell cycle and later leaves its mother cell when mitosis has completed.
S. cerevisiaeis 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 cellsuse asymmetric division for self-renewal and differentiation.
For many cells, M phase does not happen until S phase is complete. However, for entry into mitosis inS. cerevisiaethis 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.
Two interdependent events drive cytokinesis inS. cerevisiae. The first event is contractileactomyosin ring(AMR) constriction and the second event is formation of the primaryseptum(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.
When researchers look for an organism to use in their studies, they look for several traits. Among these are size,short generation time, accessibility, ease of manipulation, genetics,conservation of mechanisms,and potential economic benefit.The yeast speciesSchizosaccharomyces pombeandS. cerevisiaeare both well studied; these two species diverged approximately600 to 300million years ago, and are significant tools in the study ofDNA damageandrepair mechanisms.
S. cerevisiaehas developed as amodel organismbecause it scores favorably on a number of criteria.
For more than five decadesS. cerevisiaehas been studied as a model organism to better understand aging and has contributed to the identification of more mammalian genes affecting aging than any other model organism.Some of the topics studied using yeast arecalorie restriction, as well as in genes and cellular pathways involved insenescence. The two most common methods of measuring aging in yeast are Replicative Life Span (RLS), which measures the number of times a cell divides, and Chronological Life Span (CLS), which measures how long a cell can survive in a non-dividing stasis state.Limiting the amount of glucose or amino acids in thegrowth mediumhas 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 ofsir2. Over-expression of the genes sir2 and fob1 has been shown to increase RLS by preventing the accumulation ofextrachromosomal 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 genesSch9andRas2has 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 replicativeagingover successive generations and ultimately die. However, when a mother cell undergoesmeiosisandgametogenesis,lifespanis reset.The replicative potential ofgametes(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 torejuvenation. However, the nature of these damages remains to be established.
During starvation of non-replicatingS. cerevisiaecells,reactive oxygen speciesincrease leading to the accumulation ofDNA damagessuch as apurinic/apyrimidinic sites and double-strand breaks.Also in non-replicating cells the ability torepairendogenous double-strand breaks declines during chronologicalaging.
S. cerevisiaereproduces by mitosis as diploid cells when nutrients are abundant. However, when starved, these cells undergo meiosis to form haploid spores.
Evidence from studies ofS. cerevisiaebear on the adaptive function of meiosis andrecombination.Mutationsdefective in genes essential for meiotic and mitotic recombination inS. cerevisiaecause increased sensitivity toradiationorDNA damaging chemicals.For instance, generad52is required for both meiotic recombinationand mitotic recombination.Rad52mutants have increased sensitivity to killing byX-rays,Methyl methanesulfonateand the DNA cross-linking agent8-methoxypsoralen-plus-UVA, and show reduced meiotic recombination.These findings suggest thatrecombination repairduring meiosis and mitosis is needed for repair of the different damages caused by these agents.
Ruderfer et al.(2006) analyzed the ancestry of naturalS. cerevisiaestrains and concluded thatoutcrossingoccurs 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 ofopposite mating typeMATa 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 sameascus, 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 fromoutcrossingis inconsistent with the idea that production ofgenetic variationis 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.
S. cerevisiaewas the first eukaryoticgenometo be completely sequenced.The genome sequence was released to thepublic domainon April 24, 1996. Since then, regular updates have been maintained at theSaccharomycesGenome Database. Thisdatabaseis a highly annotated and cross-referenced database for yeast researchers. Another importantS. cerevisiaedatabase is maintained by the Munich Information Center for Protein Sequences (MIPS). Further information is located at theYeastractcurated repository.
TheS. cerevisiaegenome is composed of about 12,156,677base pairsand 6,275genes, 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 havehomologsin 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:
The availability of theS. cerevisiaegenome sequence and a set of deletion mutants covering 90% of the yeast genomehas further enhanced the power ofS. cerevisiaeas a model for understanding the regulation of eukaryotic cells. A project underway to analyze the genetic interactions of all double-deletion mutants throughsynthetic genetic arrayanalysis will take this research one step further. The goal is to form a functional map of the cell's processes.
As of 2010a 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 doublegene knockoutfor each combination of the genes studied was performed. The effect of the double knockout on thefitnessof 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.
Approaches that can be applied in many different fields of biological and medicinal science have been developed by yeast scientists. These includeyeast two-hybridfor studyingprotein interactionsandtetrad analysis. Other resources, include a gene deletion library including ~4,700 viable haploid single gene deletion strains. AGFP fusion strain libraryused to study protein localisation and aTAP tag libraryused to purify protein from yeast cell extracts.
Stanford University'syeast deletion projectcreatedknockout mutationsof every gene in theS. cerevisiaegenome to determine their function.
The yeast genome is highly accessible to manipulation, hence it is an excellent model for genome engineering.
The international Synthetic Yeast Genome Project (Sc2.0 orSaccharomyces cerevisiae version 2.0) aims to build an entirely designer, customizable, syntheticS. cerevisiaegenome from scratch that is more stable than the wild type. In the synthetic genome alltransposons,repetitive elementsand manyintronsare removed, all UAGstop codonsare replaced with UAA, andtransfer RNAgenes are moved to a novelneochromosome. As of March 2017, 6 of the 16 chromosomes have been synthesized and tested. No significant fitness defects have been found.
All 16 chromosomes can be fused into one single chromosome by successive end-to-end chromosome fusions andcentromeredeletions. The single-chromosome and wild-type yeast cells have nearly identicaltranscriptomesand similar phenotypes. The giant single chromosome can support cell life, although this strain shows reduced growth across environments, competitiveness,gameteproduction and viability.
Among other microorganisms, a sample of livingS. cerevisiaewas included in theLiving Interplanetary Flight Experiment, which would have completed a three-year interplanetary round-trip in a small capsule aboard the RussianFobos-Gruntspacecraft, launched in late 2011.The goal was to test whether selectedorganismscould survive a few years indeep spaceby flying them through interplanetary space. The experiment would have tested one aspect oftranspermia, the hypothesis thatlifecould 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 usingS. cerevisiaeisBioSentinel. (see:List of microorganisms tested in outer space)
Saccharomyces cerevisiaeis used in brewing beer, when it is sometimes called atop-fermentingor top-cropping yeast. It is so called because during the fermentation process its hydrophobic surface causes theflocsto adhere to CO2and rise to the top of the fermentation vessel. Top-fermenting yeasts are fermented at higher temperatures than the lager yeastSaccharomyces pastorianus, and the resulting beers have a different flavor from 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) or 278 k, whereSaccharomyces cerevisiaebecomes dormant. A variant yeast known asSaccharomyces cerevisiaevar.diastaticusis a beer spoiler which can cause secondary fermentations in packaged products.
In May 2013, theOregonlegislature madeS. cerevisiaetheofficial state microbein recognition of the impact craft beer brewing has had on the state economy and the state's identity.
S. cerevisiaeis used in baking; the carbon dioxide generated by the fermentation is used as aleavening agentin bread and other baked goods. Historically, this use was closely linked to the brewing industry's use of yeast, as bakers took or bought thebarmor yeast-filled foam from brewingalefrom the brewers (producing thebarm cake); today, brewing and baking yeast strains are somewhat different.
Saccharomyces cerevisiaeis the main source of nutritional yeast, which is sold commercially as a food product. It is popular with vegans and vegetarians as an ingredient in cheese substitutes, or as a general food additive as a source of vitamins and minerals, especially amino acids and B-complex vitamins.
Owing to the high cost of commercial CO2cylinder systems,CO2injectionby yeast is one of the most popularDIYapproaches followed by aquaculturists for providing CO2to 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.
Saccharomyces cerevisiaeis used as aprobioticin humans and animals. The strainSaccharomyces cerevisiae var. boulardiiis industrially manufactured and used clinically as a medication.
Several clinical and experimental studies have shown thatS. cerevisiae var. boulardiiis, to lesser or greater extent, useful for prevention or treatment of several gastrointestinal diseases.Moderate quality evidence has shownS. cerevisiae var. boulardiireduces risk of antibiotic-associated diarrhoea both in adultsand in childrenand to reduce risk of adverse effects ofHelicobacter pylorieradication therapy.There is some evidence to support efficacy ofS. cerevisiae var. boulardiiin prevention (but not treatment) of traveler's diarrhoeaand, at least as an adjunct medication, in treatment of acute diarrhoea in adults and children and of persistent diarrhoea in children.It may also reduce symptoms of  allergic rhinitis.
Administration ofS. cerevisiae var. boulardiiis 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 withplaceboor no treatment).No case ofS. cerevisiae var. boulardiifungemia has been reported during clinical trials.
In clinical practice, however, cases offungemia, caused byS. cerevisiae var. boulardiiare reported.Patients withcompromised immunityor those with central vascular catheters are at special risk. Some researchers have recommended avoiding use ofS. cerevisiae var. boulardiias treatment in such patients.Others suggest only that caution must be exercised with its use in risk group patients.
Saccharomyces cerevisiaeis proven to be anopportunistic human pathogen, though of relatively lowvirulence.Despite widespread use of this microorganism at home and in industry, contact with it very rarely leads to infection.Saccharomyces cerevisiaewas 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 considerS. cerevisiaeto be a part of thenormal microbiotaof the gastrointestinal tract, the respiratory tract, and the vagina of humans,while others believe that the species cannot be called a truecommensalbecause it originates in food.Presence ofS. cerevisiaein the 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, such asdegraded immunity,Saccharomyces cerevisiaecan cause infection in humans.Studies show that it causes 0.45–1.06% of the cases ofyeast-induced vaginitis. In some cases, women suffering fromS. cerevisiae-induced vaginal infection were intimate partners of bakers, and the strain was found to be the same that their partnersused for baking. As of 1999, no cases ofS. 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 oforal cavityandpharynxcaused byS. cerevisiaeare also known.
OccasionallySaccharomyces cerevisiaecausesinvasive infections(i. e. gets into the bloodstream or other normally sterile body fluid or into a deep site tissue, such aslungs,liverorspleen) that can gosystemic(involve multiple organs). Such conditions are life-threatening.More than 30% cases ofS. cerevisiaeinvasive infections lead to death even if treated.S. cerevisiaeinvasive infections, however, are much rarer than invasive infections caused byCandida albicanseven in patients weakened by cancer.S. cerevisiaecauses 1% to 3.6%nosocomialcases offungemia.A comprehensive review ofS. cerevisiaeinvasive infection cases found all patients to have at least one predisposing condition.
Saccharomyces cerevisiaemay enter the bloodstream or get to other deep sites of the body by translocation fromoralorenteralmucosaor through contamination of intravascular catheters (e. g.central venous catheters).Intravascular catheters, antibiotic therapy and compromised immunity are major predisposing factors forS. cerevisiaeinvasive infection.
A number of cases offungemiawere caused by intentional ingestion of livingS. cerevisiaecultures for dietary or therapeutic reasons, including use ofSaccharomyces boulardii(a strain ofS. cerevisiaewhich is used as aprobioticfor treatment of certain forms ofdiarrhea).Saccharomyces boulardiicauses about 40% cases of invasiveSaccharomycesinfectionsand is more likely (in comparison to otherS. cerevisiaestrains) to cause invasive infection in humans without general problems with immunity,though such adverse effect is very rare relative toSaccharomyces boulardiitherapeutic administration.
S. boulardiimay contaminate intravascular catheters through hands of medical personnel involved in administering probiotic preparations ofS. boulardiito patients.
Systemic infection usually occurs in patients who have theirimmunity compromiseddue to severe illness (HIV/AIDS,leukemia, other forms ofcancer) or certain medical procedures (bone marrow transplantation,abdominal surgery).
A case was reported when anodulewassurgically excisedfrom a lung of a man employed in baking business, and examination of the tissue revealed presence ofSaccharomyces cerevisiae. Inhalation ofdry baking yeast powderis supposed to be the source of infection in this case.
Not all strains ofSaccharomyces cerevisiaeare equally virulent towards humans. Most environmental strains are not capable of growing at temperatures above 35 °C (i. e. at temperatures of living body of humans and othermammalian). Virulent strains, however, are capable of growing at least above 37 °C  and often up to 39 °C (rarely up to 42 °C).Some industrial strains are also capable of growing above 37 °C.European Food Safety Authority(as of 2017) requires that allS. cerevisiaestrains 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 asproteinaseandphospholipase,invasive growth(i.e. growth with intrusion into the nutrient medium), ability to adhere to mammalian cells,ability to survive in the presence ofhydrogen peroxide(that is used bymacrophagesto 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 aspseudohyphaeis also sometimes said to be associated with virulence,though some research suggests that this trait may be common to both virulent and non-virulent strains ofSaccharomyces cerevisiae.
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