Yeast is a unicellular eukaryotic fungus and there are currently 1500 species of yeast. Some species like Candida albicans are opportunistic pathogens, which may cause infections in humans, while some species, such as Saccharomyces cerevisiae, are useful to humans.
What is S. cerevisiae?
S. cerevisiae, popularly known as baker’s yeast or brewer’s yeast, is a species of yeast that is considered to have been originally isolated from the skin of grapes. It is one of the most-studied single cellular eukaryotic organisms. S. cerevisiae cells are ovoid in shape, around 5-10um in diameter. It can reproduce by both sexual and asexual means (budding). This species of yeast loves to feed on sugars like glucose, maltose, and trehalose. They can produce energy (ATP) by aerobic respiration and can switch to anaerobic respiration in the absence of oxygen. In anaerobic respiration, it undergoes fermentation and produces alcohol and CO2 by feeding on sugars like sucrose and trehalose. This property of S. cerevisiae of fermenting sugars makes it a key ingredient in the baking and wine-making industry.
What is alcoholic fermentation?
Alcoholic fermentation (ethanol fermentation) is performed by Saccharomyces cerevisiae and some fishes (goldfish & carp). Under anaerobic conditions, S. cerevisiae consumes sugars like glucose, fructose, and sucrose to produce energy, ethanol and carbon dioxide are released as by-products in the process.
Role of S. cerevisiae in biological research
Just like E. coli, S. cerevisiae (baker’s or brewing yeast) has been used as a popular model for biological research. It is considered to be the first unicellular eukaryotic organism to have its whole genome sequenced in 1996 by Goffeau et al.
Features of S. cerevisiae that make it a popular model in research studies
- It is a simple unicellular species of yeast with membrane-bound cell organelles just like any other eukaryotic organism. It has a genome size of approximately 12 Mbp. Like humans, it has its genetic material (DNA) packaged into chromosomes.
- It can tolerate a wide range of environmental conditions.
- Like a eukaryotic organism, it has membrane-bound cell organelles like mitochondria, Golgi apparatus, nucleus, etc. Therefore, it can be a great model to study certain mitochondrial-related disorders and also helps to understand biological pathways in humans.
- It is only after studying the yeast that scientists have found that genes are translated into proteins, and each protein has a specific function in the body.
- Genetic manipulations can be easily done on this single-celled eukaryotic organism as compared to other complex model organisms like mice and zebrafish.
- Around 23 % of genes involved in human diseases are orthologues to the yeast genome. Mutating these genes can easily help researchers find out the functions of these genes in humans.
- Many new drugs have been discovered by using S. cerevisiae as a model.
- Their cell division process is similar to that of humans, which makes them a perfect model for cancer research.
- S. cerevisiae can be easily grown in laboratories as it has simple nutritional requirements. It can be grown in a liquid or agar medium, containing glucose or dextrose as a carbon source and some salts as a nitrogen source.
- Cell division in yeast is almost similar to that of humans. Genes responsible for cell division in humans have orthologues in yeast genomes.
- It can act as a powerful tool to have a better knowledge about human diseases.
- Under Saccharomyces Genome Deletion Project, around 6000 genes of S. cerevisiae were mutated to find out the precise function associated with a particular gene.
S. cerevisiae as a model in research studies
S. cerevisiae is one of the most commonly used single-celled eukaryotic model organisms. It has been used in many research studies like cell cycle, apoptosis, gene regulation, regulation of gene expression, and neurodegenerative diseases. It was used as a model organism by Leland H. Hartwell, an American scientist who shared the 2001 Nobel Prize in Physiology or Medicine with Paul Nurse and Tim Huntin for discovering protein molecules that control the division (duplication) of cells, in studying the link between genes involved in cell division and cancer. He chose S. cerevisiae as it was simple, unicellular, and easy to manipulate. He found out that genes involved in cell division in yeast were also found in humans. With his studies, he proved that mutation in these cell division cycle controlling genes can cause cancer in humans.
Examples of S. cerevisiae as a model in research studies
In Neurodegenerative Studies
Neurodegenerative diseases like Parkinson’s, Alzheimer’s, and Huntington’s mainly occur due to protein misfolding, which ultimately leads to the aggregation of these toxic proteins in the central nervous system. Using S. cerevisiae as a research model, it was found that in Parkinson’s there is a mutation in α –synuclein that causes misfolding of these protein molecules. Misfolded α -synuclein protein molecules aggregate in the neurons as ‘Lewy bodies’ and cause damage to the neurons. This finding helps the scientist explore S. cerevisiae as a tool to discover drugs that could reverse the effect of α -synuclein toxicity.
In Cancer Research
Genes like MSH 2 (MutS homolog 2) and MLH 1 (MutL homolog 1) are found in both S. cerevisiae and humans. Mutations of these genes in yeast have shown the role of these genes in colon cancer. MSH 2 is a tumour suppressor gene and the MLH1 gene helps in DNA repair. Mutation in these two genes can lead to hereditary non-polyposis colorectal cancer in humans.
In Aging Studies
Many studies have been done on S. cerevisiae to understand the genes and pathways involved in aging. In yeast, two types of methods have been discovered to measure aging: RLS (replicative life span) and CLS (chronological life span). It was seen that by reducing the amount of glucose and amino acids in the medium we can increase the RLS and CLS in yeast. It was shown that over-expressing the SIR2 AND FOB1 genes increase the RLS in yeast. Two yeast mutants were created in which sch9 and ras2 genes were knocked out by genetic engineering. Mutations in these two genes have shown a tenfold increase in the chronological life span of yeast.
In DNA Repair Research Studies
S. cerevisiae has also been used in research studies of DNA repair mechanisms. It was shown that mutation in the rad52 gene, which is responsible for DNA double-stranded break repair and in homologous recombination, can make yeast more susceptible to die because of X-rays and other chemical agents. This shows that recombination during mitosis and meiosis is very crucial for the repair of damaged DNA.
We all are aware of the fact that viruses are a major threat to humans, plants, and animals. Many researchers are trying to find a cure against deadly viruses like HIV, HCV, etc. Being a eukaryote, S. cerevisiae has been proved to be a fruitful model in virus research. Virologists are exploiting S. cerevisiae as a model to understand the pathogenesis of viruses. HIV is still a major threat to human life. Research studies based on yeast have shown that 3 HIV proteins: Vpr (viral protein R), PR (protease), Rev (regulator of expression of virion protein) play an important role in its pathogenesis. It was observed that viral protein Vpr causes suppression of the immune system by killing CD4+ T cells. Studies in S. cerevisiae have shown that PR is involved in cell growth arrest and causes loss of membrane integrity, which leads to cell lysis.
In Studying Apoptosis
S. cerevisiae has been used as a popular model for studying gene regulations and apoptosis. The fact that yeast and humans share common machinery and regulators responsible for cell death has opened new possibilities of exploring yeast as a tool to investigate cell apoptotic pathways. Apoptosis is a programmed cell death that occurs as a normal part of cell growth and development.
Commercial applications of S. cerevisiae
In Food Industry
The ability of S. cerevisiae (brewer’s yeast) to ferment sugars and convert them into alcohol (ethanol) and CO2 has made it a key ingredient in the bakery and wine-making industries. Fermentation was first discovered by Louis Pasteur. Ethanol produced as a by-product in fermentation has been used commercially in alcohol formation and in making biofuels. Carbon dioxide released as a result of fermentation act as a leavening agent. A leavening agent can be any substance that helps in the expansion of dough or batter.
S. cerevisiae (baker’s yeast) is a key ingredient in making bread and other bakery products, such as cake, cookies, muffins, etc. Bread is formed by converting fermentable sugars into alcohol and carbon dioxide (CO2). The dough for making bread consists of flour, water, yeast, and salt. Amylase present in the flour breaks down the starch (amylase or amylopectin) and releases maltose and sucrose. Baker’s yeast present in the dough contains maltase, which breaks the maltose into glucose, which undergoes aerobic respiration to produce two molecules of pyruvic acid. Soon, the yeast runs out of oxygen and switches to anaerobic respiration for energy. In anaerobic respiration, it breaks down the pyruvic acid into CO2 and ethanol. These CO2 molecules get trapped in the gluten molecules of flour, resulting in the fluffiness of the dough. The dough is then heated, which kills the yeast and evaporates the ethanol.
In Alcoholic Beverages
We all are aware of the fact that nowadays alcohol has become a part of human civilization. Do you know that alcohol is a yeast product? S. cerevisiae can convert sugars (glucose, fructose, and sucrose) present in fruits and grains into ethyl alcohol and CO2 through alcoholic fermentation. Alcoholic fermentation is a biotechnological process that can be achieved by certain bacteria and yeast. Starchy crops like barley, wheat, rice, sugarcane, or maize can act as raw materials for alcohol production. Under anaerobic conditions, pyruvic acid first converts into acetaldehyde, which is then converted into ethyl alcohol and CO2. Some bacterial species like Zymomonas mobilis can also perform alcoholic fermentation.
In Wine Industry
Wines are formed through the fermentation of grapes (Vitis vinifera). Winemaking involves two main biotechnological processes. The first is alcoholic fermentation and the other is malolactic fermentation. In alcoholic fermentation, the sugar present in fruits like grapes is converted into ethanol and CO2. After alcoholic fermentation, most red wines undergo malolactic fermentation by lactic acid bacteria, Oenococcus oeni. This process converts the harsh tasting malic acid into creamier lactic acid and also raises the pH. This is necessary for the deacidification and flavour modification of the wine.
S. cerevisiae (brewer’s yeast) is most commonly used in breweries. It is also a rich source of chromium. Beer is considered one of the most commonly consumed alcoholic beverages. It is made by fermenting malted cereals such as barley, wheat, corn, etc. Brewer’s yeast can be classified into two categories based on their position in the fermenter, ale yeast and larger yeast. Ale yeast present at the top of the fermenter requires a temperature of around 60 to 72 F, and with an increase in the temperature, ale yeast can experience a sudden surge in ester production, which leads to intense fruitiness in the beer. Larger yeast thrives in colder temperatures (42 to 52 F); therefore, they are present at the bottom of the fermenter. At low temperatures, larger yeast metabolizes ester present in the beer, thus turning the beer into less fruity and more delicate. Beer is formed by mixing wort with brewer’s yeast. Wort is produced by soaking malted grains like barley, wheat, and corn in water. In the presence of oxygen, yeast, after consuming sugars, starts dividing by asexual reproduction (budding). After the depletion of oxygen, it switches to anaerobic respiration and starts fermentation of sugars to produce energy.
In Chocolate Production
Chocolate is one of the most popular food items consumed by all age groups. It is also used in making cakes, bakery, and puddings. Chocolate is a plant product and is made from seeds of Theobroma cocoa tree. Cocoa seeds are actually extremely bitter in taste and only after fermentation do they develop the flavour. In cocoa fermentation, the cocoa seeds are left intact with their pulp for several days. Yeast (S. cerevisiae) starts fermenting the sugars present in the pulp that gives the chocolate its amazing flavour and aroma. Cocoa fermentation not only improves the flavour of chocolate but also removes the tannin present in the seeds, which is responsible for the bitterness in cocoa seeds.
In Coffee Production
If you are a coffee lover, then you should thank S. cerevisiae for the peculiar aroma and flavour of the coffee. This species of yeast has been used as a starter in the dry fermentation of coffee beans. S. cerevisiae produces an enzyme called pectin methyl esterases that has pectinolytic activity. The pectinolytic activity of this enzyme helps in removing the mucilage from the coffee beans. S. cerevisiae strains used in the dry fermentation of coffee beans have special effects on the sensorial properties of coffee.
In Biofuel (bioethanol) Production
Ethanol produced by fermentation of sugars by S. cerevisiae can be used as fuel. In 1826, an American inventor named Samuel Morey used ethanol and turpentine as a biofuel in boats. Ethanol is colourless alcohol, which is made from the fermentation of biomass material. In the United States, food grains with high sugar content like sugarcane, barley, sorghum, etc., are used as feedstocks for making ethanol, and the country uses ethanol as a biofuel by blending 10% ethanol with gasoline to form “gasohol.”
Benefits of using biofuel over petroleum
- Ethanol with gasoline has much higher octane number than petroleum. Octane number is the measure of fuel’s ability to tolerate compression and resist knocking of engine.
- It has a higher flame speed and increased heat of vaporization.
- It is Less toxic than petroleum.
- Biofuels are biodegradable, hence they cause less air pollution.
- They are much cheaper than petroleum and diesel.
Biofuel categories based on the substrates used for fermentation
First Generation Biofuel
Crops like sugarcane, corn, rice, and wheat act as a substrate for ethanol fermentation by S. cerevisiae.
Second Generation Biofuel
The substrate used for fermentation in second-generation biofuels includes lignocellulosic biomass such as wood, straw, crop or food waste, etc.
Third Generation Biofuel
Substrate for third-generation biofuels includes algal biomass.
In Pharmaceutical Industry
Being a unicellular eukaryotic organism, it has the properties of both bacteria and humans. Like bacteria, it can replicate fast, and like any other eukaryotic organism, it can produce protein in a properly folded form. The protein formed by S. cerevisiae undergoes the process of glycosylation, disulfide bond formation, and acylation. All these properties make it a key ingredient in pharmaceutical industries to produce proteins for humans. Proteins produced in S. cerevisiae are secreted outside, which can be easily purified. S. cerevisiae has now become a part of the million-dollar biopharmaceutical industry; the production of insulin by S. cerevisiae in 1987 is one such example of biopharmaceutical products. Some more examples of biopharmaceuticals produced by S. cerevisiae are Insulin, human serum albumin, hepatitis vaccine, etc.
What are biopharmaceuticals?
Bio pharmaceuticals are medicines or any other medical product, produced by or extracted from biological sources like living cells or micro-organisms.
Advantages of using S.cerevisiae in biopharmaceuticals’ production
- Being a eukaryote it ensures the proper folding of human proteins.
- Protein is excreted intracellularly, which reduces the cost of downstream purification.
Nutritional Use of S. cerevisiae
S. cerevisiae is a great source of many nutrients like chromium, potassium, zinc, vitamin B1 (thiamine), vitamin B6 (Pyridoxine), and Vitamin B2 (riboflavin). It has all the nine essential amino acids required by our body. Nutritional yeast has a cheesy and nutty taste. It has gained popularity among people who are vegans or are allergic to soy and gluten. Nutritional yeast contains chromium, which helps in lowering the blood sugar levels in diabetic patients. It also has a fiber called beta-glucan, which helps in reducing cholesterol, thus it is beneficial for heart patients.
It has been commercially used in aquariums for providing CO2 (carbon dioxide) to aquatic plants. To cut down the cost of CO2 cylinders, many aquaculturists are now using CO2 injection. These injections contain a mixture of yeast, sugar, and water. Yeast, present in the injection, feeds on sugar, and it releases carbon dioxide as a by-product.
In Probiotic Drinks
S. boulardii, one of the strains of S. cerevisiae, has been commercially used as probiotics. It is used to prevent diarrhea and inflammatory bowel disease.