Manyproteins important in human biology were first discovered by studying theirhomologs in yeast; these proteins includecell cycle proteins,signaling proteins, and protein-processingenzymes.S. cerevisiae is currently the only yeast cell known to haveBerkeley bodies present, which are involved in particular secretory pathways.Antibodies againstS. cerevisiae are found in 60–70% of patients withCrohn's disease and 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.[2]
"Saccharomyces" derives fromLatinizedGreek and means "sugar-mould" or "sugar-fungus",saccharon (σάκχαρον) being the combining form "sugar" andmyces (μύκης) being "fungus".[3][4]cerevisiae comes from Latin and means "of beer".[5] 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,[7]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 Process was developed in 1846.[8]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[9]),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.[10]
Refinements in microbiology following the work ofLouis Pasteur led 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,[11]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.[12]
DuringWorld War II,Fleischmann's developed agranulated 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.[citation needed]
In nature, yeast cells are found primarily on ripe fruits such as grapes (before maturation, grapes are almost free of yeasts).[13]S. cerevisiae can also be found year-round in the bark ofoak trees.[14] SinceS. cerevisiae is not airborne, it requires a vector to move.[15]
Queens of social wasps overwintering as adults (Vespa crabro andPolistes spp.) can harbor yeast cells from autumn to spring and transmit them to their progeny.[16] The intestine ofPolistes dominula, a social wasp, hostsS. cerevisiae strains as well asS. cerevisiae ×S. paradoxus hybrids. Stefanini et al. (2016) showed that the intestine ofPolistes dominula favors the mating ofS. cerevisiae strains, both among themselves and withS. paradoxus cells by providing environmental conditions prompting cellsporulation and spores germination.[17]
The optimum temperature for growth ofS. cerevisiae is 30–35 °C (86–95 °F).[16]
Two forms of yeast cells can survive and grow:haploid anddiploid. The haploid cells undergo a simplelifecycle ofmitosis and growth, and under conditions of high stress will, in general, die. This is theasexual form 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.[18] Under conditions ofstress, diploid cells can undergosporulation, enteringmeiosis and producing four haploidspores, which can subsequently mate. This is thesexual form of thefungus. Under optimal conditions, yeast cells can double their population every 100 minutes.[19][20] However, growth rates vary enormously between strains and between environments.[21] Meanreplicative lifespan is about 26 cell divisions.[22][23]
In the wild, recessive deleterious mutations accumulate during long periods ofasexual reproduction of diploids, and are purged duringselfing: this purging has been termed "genome renewal".[24][25]
Allstrains ofS. cerevisiae can growaerobically onglucose,maltose,[26] andtrehalose[27] and fail to grow onlactose andcellobiose. However, growth on othersugars is variable.Galactose andfructose 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 onsucrose and trehalose.[citation needed]
All strains can useammonia andurea as the solenitrogen source, 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, andlysine are, however, not readily used.S. cerevisiae does not excreteproteases, so extracellular protein cannot be metabolized.
Yeasts also have a requirement forphosphorus, which is assimilated as a dihydrogen phosphate ion, andsulfur, which can be assimilated as asulfate ion or as organic sulfur compounds such as theamino acids methionine and cysteine. Some metals, likemagnesium,iron,calcium, andzinc, are also required for good growth of the yeast.
Concerning organic requirements, most strains ofS. cerevisiae requirebiotin.[28] Indeed, aS. cerevisiae-based growth assay laid the foundation for the isolation, crystallization, and later structural determination of biotin. Most strains also requirepantothenate 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.[29] As in many other eukaryotes, mating leads togenetic recombination, i.e. production of novel combinations of chromosomes. Twohaploid yeast cells of opposite mating type can mate to formdiploid cells that can eithersporulate 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.[citation needed]
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.[citation needed]
Cytokinesis enables budding yeastSaccharomyces 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.[30]
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.[31]
For many cells, M phase does not happen until S phase is complete. However, for entry into mitosis inS. 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.[30] 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.[30]
When the daughter emerges, the daughter is two-thirds the size of the mother.[32] Throughout the process, the mother displays little to no change in size.[33] The RAM pathway is activated in the daughter cell immediately after cytokinesis is complete. This pathway makes sure that the daughter has separated properly.[32]
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.[32] 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.[34][33]
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.[30] The ring is thought to play an important role in ingression of the plasma membrane as a contractile force.[citation needed]
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.[30] 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.[35]
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.[36] After AMR constriction is complete, two secondary septums are formed by glucans. How the AMR ring dissembles remains poorly unknown.[31]
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.[37] Thus, the AMR and septum formation are the major drivers of cytokinesis.[citation needed]
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[30]
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[38]
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[38]
S. cerevisiae has developed as amodel organism because it scores favorably on a number of criteria.
As a single-cell organism,S. cerevisiae is small with a short generation time (doubling time 1.25–2 hours[40] at 30 °C or 86 °F) and can be easilycultured. 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 betransformed allowing for either the addition of new genes or deletion throughhomologous recombination. Furthermore, the ability to growS. cerevisiae as a haploid simplifies the creation ofgene knockout strains.
As aneukaryote,S. cerevisiae shares the complex internal cell structure of plants and animals without the high percentage ofnon-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.
For more than five decadesS. cerevisiae has 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.[41] 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.[41] Limiting the amount of glucose or amino acids in thegrowth medium has been shown to increase RLS and CLS in yeast as well as other organisms.[42] 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.[42] The effects of dietary restriction may be the result of a decreased signaling in the TOR cellular pathway.[41] This pathway modulates the cell's response to nutrients, and mutations that decrease TOR activity were found to increase CLS and RLS.[41][42] This has also been shown to be the case in other animals.[41][42] A yeast mutant lacking the genesSch9 andRas2 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.[43][44]
Mother cells give rise to progeny buds by mitotic divisions, but undergo replicativeaging over successive generations and ultimately die. However, when a mother cell undergoesmeiosis andgametogenesis,lifespan is reset.[45] 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. cerevisiae cells,reactive oxygen species increase leading to the accumulation ofDNA damages such as apurinic/apyrimidinic sites and double-strand breaks.[46] Also in non-replicating cells the ability torepair endogenous double-strand breaks declines during chronologicalaging.[47]
S. cerevisiae reproduces by mitosis as diploid cells when nutrients are abundant. However, when starved, these cells undergo meiosis to form haploid spores.[48]
Evidence from studies ofS. cerevisiae bear on the adaptive function of meiosis andrecombination.Mutations defective in genes essential for meiotic and mitotic recombination inS. cerevisiae cause increased sensitivity toradiation orDNA damaging chemicals.[49][50] For instance, generad52 is required for both meiotic recombination[51] and mitotic recombination.[52]Rad52 mutants have increased sensitivity to killing byX-rays,Methyl methanesulfonate and the DNA cross-linking agent8-methoxypsoralen-plus-UVA, and show reduced meiotic recombination.[50][51][53] These findings suggest thatrecombination repair during meiosis and mitosis is needed for repair of the different damages caused by these agents.
Ruderfer et al.[49] (2006) analyzed the ancestry of naturalS. cerevisiae strains and concluded thatoutcrossing 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 ofopposite mating type MATa and MATα come into contact. Ruderfer et al.[49] 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 fromoutcrossing is inconsistent with the idea that production ofgenetic 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,[54] since this benefit is realized during each meiosis, whether or not out-crossing occurs.
S. cerevisiae was the first eukaryoticgenome to be completely sequenced.[55] The genome sequence was released to thepublic domain on April 24, 1996. Since then, regular updates have been maintained at theSaccharomyces Genome Database. Thisdatabase is a highly annotated and cross-referenced database for yeast researchers. Another importantS. cerevisiae database is maintained by the Munich Information Center for Protein Sequences (MIPS). Further information is located at theYeastract curated repository.[56]
TheS. cerevisiae genome is composed of about 12,156,677base pairs and 6,275genes, compactly organized on 16 chromosomes.[55] Only about 5,800 of these genes are believed to be functional. It is estimated at least 31% of yeast genes havehomologs in the human genome.[57] 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.[58]
Systematic gene names for Baker's yeast
Example gene name
YGL118W
Y
the Y indicates this is a yeast gene
G
chromosome on which the gene is located (chromosome 1 = A etc.)
L
left or right arm of the chromosome
118
sequence number of the gene/ORF on this arm, starting at the centromere
W
whether the coding sequence is on the Watson or Crick strand
Examples:
YBR134C (aka SUP45 encodingeRF1, a translation termination factor) is located on the right arm of chromosome 2 and is the 134thopen 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 ofDNA 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.
The availability of theS. cerevisiae genome sequence and a set of deletion mutants covering 90% of the yeast genome[59] has further enhanced the power ofS. cerevisiae as a model for understanding the regulation of eukaryotic cells. A project underway to analyze the genetic interactions of all double-deletion mutants throughsynthetic 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[update] 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".[60] This model was made from 5.4 million two-gene comparisons in which a doublegene knockout for each combination of the genes studied was performed. The effect of the double knockout on thefitness 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.[60]
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.[61] 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.[60]
Approaches that can be applied in many different fields of biological and medicinal science have been developed by yeast scientists. These includeyeast two-hybrid for studyingprotein interactions andtetrad analysis. Other resources, include a gene deletion library including ~4,700 viable haploid single gene deletion strains. AGFP fusion strain library used to study protein localisation and aTAP tag library used to purify protein from yeast cell extracts.[citation needed]
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. cerevisiae genome from scratch that is more stable than the wild type. In the synthetic genome alltransposons,repetitive elements and manyintrons are removed, all UAGstop codons are replaced with UAA, andtransfer RNA genes are moved to a novelneochromosome. As of March 2017[update], 6 of the 16 chromosomes have been synthesized and tested. No significant fitness defects have been found.[63]
All 16 chromosomes can be fused into one single chromosome by successive end-to-end chromosome fusions andcentromere deletions. The single-chromosome and wild-type yeast cells have nearly identicaltranscriptomes and similar phenotypes. The giant single chromosome can support cell life, although this strain shows reduced growth across environments, competitiveness,gamete production and viability.[64]
Among other microorganisms, a sample of livingS. cerevisiae was included in theLiving Interplanetary Flight Experiment, which would have completed a three-year interplanetary round-trip in a small capsule aboard the RussianFobos-Grunt spacecraft, launched in late 2011.[65][66] The goal was to test whether selectedorganisms could survive a few years indeep space by flying them through interplanetary space. The experiment would have tested one aspect oftranspermia, the hypothesis thatlife could survive space travel, if protected inside rocks blasted by impact off one planet to land on another.[65][66][67] 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. cerevisiae isBioSentinel. (see:List of microorganisms tested in outer space)
Saccharomyces cerevisiae is used in brewing beer, when it is sometimes called atop-fermenting or top-cropping yeast. It is so called because during the fermentation process its hydrophobic surface causes theflocs to adhere to CO2 and 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 cerevisiae becomes dormant. A variant yeast known asSaccharomyces cerevisiae var.diastaticus is a beer spoiler which can cause secondary fermentations in packaged products.[68]
In May 2013, theOregon legislature madeS. cerevisiae theofficial state microbe in recognition of the impact craft beer brewing has had on the state economy and the state's identity.[69]
S. cerevisiae is used in baking; the carbon dioxide generated by the fermentation is used as aleavening 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 thebarm or yeast-filled foam from brewingale from the brewers (producing thebarm cake); today, brewing and baking yeast strains are somewhat different.[citation needed]
Saccharomyces cerevisiae is 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 CO2 cylinder systems,CO2 injection by yeast is one of the most popularDIY 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.[70]
This articleis missing information about non-boulardii (CBS 5926) strains, such as CNCM I-3856: might be a good idea to search the EMA database. Please expand the article to include this information. Further details may exist on thetalk page.(January 2022)
Several clinical and experimental studies have shown thatS. cerevisiae var. boulardii is, to lesser or greater extent, useful for prevention or treatment of several gastrointestinal diseases.[71] Moderate quality evidence has shownS. cerevisiae var. boulardii reduces risk of antibiotic-associated diarrhoea both in adults[72][71][73] and in children[72][71] and to reduce risk of adverse effects ofHelicobacter pylori eradication therapy.[74][71][73] There is some evidence to support efficacy ofS. cerevisiae var. boulardii in prevention (but not treatment) of traveler's diarrhoea[71][73] and, at least as an adjunct medication, in treatment of acute diarrhoea in adults and children and of persistent diarrhoea in children.[71] It may also reduce symptoms of allergic rhinitis.[75]
Administration ofS. cerevisiae var. boulardii is considered generally safe.[73] In clinical trials it was well tolerated by patients, and adverse effects rate was similar to that in control groups (i. e. groups withplacebo or no treatment).[72] No case ofS. cerevisiae var. boulardii fungemia has been reported during clinical trials.[73]
In clinical practice, however, cases offungemia, caused byS. cerevisiae var. boulardii are reported.[73][71] Patients withcompromised immunity or those with central vascular catheters are at special risk. Some researchers have recommended avoiding use ofS. cerevisiae var. boulardii as treatment in such patients.[73] Others suggest only that caution must be exercised with its use in risk group patients.[71]
Saccharomyces cerevisiae is proven to be anopportunistic human pathogen, though of relatively lowvirulence.[76] Despite widespread use of this microorganism at home and in industry, contact with it very rarely leads to infection.[77]Saccharomyces cerevisiae was found in the skin, oral cavity, oropharinx, duodenal mucosa, digestive tract, and vagina of healthy humans[78] (one review found it to be reported for 6% of samples from human intestine[79]). Some specialists considerS. cerevisiae to be a part of thenormal microbiota of the gastrointestinal tract, the respiratory tract, and the vagina of humans,[80] while others believe that the species cannot be called a truecommensal because it originates in food.[79][81] Presence ofS. cerevisiae in the human digestive system may be rather transient;[81] 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.[79][77]
Under certain circumstances, such asdegraded immunity,Saccharomyces cerevisiae can cause infection in humans.[77][76] 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.[76] Cases of infection oforal cavity andpharynx caused byS. cerevisiae are also known.[76]
OccasionallySaccharomyces cerevisiae causesinvasive infections (i. e. gets into the bloodstream or other normally sterile body fluid or into a deep site tissue, such aslungs,liver orspleen) that can gosystemic (involve multiple organs). Such conditions are life-threatening.[76][81] More than 30% cases ofS. cerevisiae invasive infections lead to death even if treated.[81]S. cerevisiae invasive infections, however, are much rarer than invasive infections caused byCandida albicans[76][82] even in patients weakened by cancer.[82]S. cerevisiae causes 1% to 3.6%nosocomial cases offungemia.[81] A comprehensive review ofS. cerevisiae invasive infection cases found all patients to have at least one predisposing condition.[81]
Saccharomyces cerevisiae may enter the bloodstream or get to other deep sites of the body by translocation fromoral orenteralmucosa or through contamination of intravascular catheters (e. g.central venous catheters).[80] Intravascular catheters, antibiotic therapy and compromised immunity are major predisposing factors forS. cerevisiae invasive infection.[81]
A number of cases offungemia were caused by intentional ingestion of livingS. cerevisiae cultures for dietary or therapeutic reasons, including use ofSaccharomyces boulardii (a strain ofS. cerevisiae which is used as aprobiotic for treatment of certain forms ofdiarrhea).[76][81]Saccharomyces boulardii causes about 40% cases of invasiveSaccharomyces infections[81] and is more likely (in comparison to otherS. cerevisiae strains) to cause invasive infection in humans without general problems with immunity,[81] though such adverse effect is very rare relative toSaccharomyces boulardii therapeutic administration.[83]
S. boulardii may contaminate intravascular catheters through hands of medical personnel involved in administering probiotic preparations ofS. boulardii to patients.[81]
A case was reported when anodule wassurgically excised from a lung of a man employed in baking business, and examination of the tissue revealed presence ofSaccharomyces cerevisiae. Inhalation ofdry baking yeast powder is supposed to be the source of infection in this case.[84][81]
Not all strains ofSaccharomyces cerevisiae are 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).[78] Some industrial strains are also capable of growing above 37 °C.[76]European Food Safety Authority (as of 2017) requires that allS. 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.[85]
The ability to grow at elevated temperatures is an important factor for strain's virulence but not the sole one.[78]
Other traits that are usually believed to be associated with virulence are: ability to produce certain enzymes such asproteinase[76] andphospholipase,[78] invasive growth[78] (i.e. growth with intrusion into the nutrient medium), ability to adhere to mammalian cells,[78] ability to survive in the presence ofhydrogen peroxide[78] (that is used bymacrophages to kill foreign microorganisms in the body) and other abilities allowing the yeast to resist or influence immune response of the host body.[78] Ability to form branching chains of cells, known aspseudohyphae is also sometimes said to be associated with virulence,[76][78] though some research suggests that this trait may be common to both virulent and non-virulent strains ofSaccharomyces cerevisiae.[78]
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