Yeasts or yeasts are unicellular fungi that multiply by sprouting or dividing (splitting). Propagation by sprouting led to the synonymous term sprout fungi , although not all yeasts multiply by sprouting and on the other hand there are also hyphae-forming fungi whose hyphae grow when sprouting ( e.g. Candida ). Most yeasts belong to the department of the hose mushrooms at (Ascomycota). However, development stages of other fungi are also referred to as yeasts. Examples of mushroom yeasts (Basidiomycota) are the sprouting stages of the various naked basidia species ( Exobasidium ), certain developmental stages of many smut fungi or even optionally human pathogenic fungi such as Malassezia furfur .
History and meaning
Yeasts are one of the most important microorganisms of commercial importance that have always served mankind. Already in the early advanced civilizations of the Middle East , the alcoholic drinks wine and beer as well as bread were made with the help of yeast, without fully understanding the connections.
The fact that yeast ( ancient Greek ζύμη zyme , Latin fermentum ) is useful for making beer was already known in ancient times . In addition, yeasts were used as raising agents - Pliny the Elder described the production or breeding of yeast for this purpose in his Naturalis historia .
Louis Pasteur described in his work Études sur la bière in 1876 that yeast consists of microorganisms and that the presence of these organisms is of essential importance for the fermentation process . Pasteur said that without yeast there would be no fermentation and the presence of other organisms (wild yeast or bacteria ) would disturb the fermentation behavior, resulting in spoiled beers or wines. Eduard Buchner received the Nobel Prize for Chemistry in 1907 for his studies on " alcoholic fermentation without yeast cells " and another work on cell-free fermentation at the Berlin Agricultural University .
Yeasts are used in the production of beer, wine ( wine yeast ), spirits, food and a variety of biochemical and therapeutic substances. Some yeasts cause food and feed spoilage, others are of medicinal importance.
Yeasts play an important role as model organisms in biology , as they can easily be cultivated, genetically modified and examined in the laboratory. They are among the smallest eukaryotic organisms. Since they are eukaryotes , their similarity to higher organisms is significantly greater than that of bacteria .
As eukaryotes , yeasts are generally much larger than most bacteria and have typical cell structures of eukaryotes: complex membrane structures , chromosomes and a large number of organelles including the mitochondria and the endoplasmic reticulum , structures that are not found in prokaryotes (bacteria and archaea ) .
About 700 types of yeast are known today with over 5,000 strains, but only a few have been accurately described. There is currently no binding defining definition for yeasts, because the properties of some well-known yeasts, such as reproduction by cell division, are not common to all yeasts and are not unique to them.
Most yeasts are facultatively anaerobic , meaning they do not depend on oxygen . If oxygen is available, you can use it for an oxidative energy metabolism (aerobic respiration ): You can oxidize various sugars to carbon dioxide and water . In the absence of oxygen, however, many yeasts can only break down the sugars into low-molecular substances, for example into ethanol and carbon dioxide (e.g. in alcoholic fermentation ). Sugar oxidation under aerobic conditions provides more energy than fermentation. Therefore the rate of mass growth and the rate of cell division are much higher with oxidative sugar breakdown than with fermentation.
Yeasts use a wide range of carbohydrates. However, no species has yet been described that can use all of the naturally occurring sugars. Some examples: The top-fermenting strains of the yeast Saccharomyces cerevisiae can use glucose , fructose , mannose , galactose , sucrose , maltose , maltotriose and raffinose . The closely related species Saccharomyces diastaticus and the bottom-fermenting strains of Saccharomyces cerevisiae (formerly regarded as species S. uvarum or S. carlsbergensis ) also use dextrins and melobiosis . However, Saccharomyces cerevisiae and their relatives cannot use pentoses such as ribose , xylose and arabinose , nor can cellobiose , lactose , inulin and cellulose .
- Saccharomyces cerevisiae
- ( Baker 's yeast, top-fermenting brewer's yeast ) is a budding yeast and was first isolated in 1888 by Emil Christian Hansen . It is propagated as a pure cultivation of top-fermenting strains, predominantly aerobically in nutrient solutions, and is sold as dry yeast or compressed yeast . When beer is brewed, top-fermented stems rise during fermentation and float on the substrate . Bottom-fermenting strains, on the other hand, sink towards the end of the main fermentation.
- Saccharomyces carlsbergensis , Saccharomyces uvarum and others
- Bottom- fermenting yeasts ferment more types of sugar , even at lower temperatures. They are used to make lager beer .
- Candida albicans
- colonizes mucous membranes, skin and digestive tract as a saprophyte and is found in three quarters of all people. Only triggers diseases under certain circumstances ("weakness parasite").
- Saccharomyces boulardii
- is used to treat diarrhea
- Brettanomyces bruxellensis
- Pest yeast in must and wine, which causes the so-called “horse sweat” aroma. On the other hand, it is used to make the Belgian Lambic beer .
- Pichia pastoris
- Used in biotechnological processes to produce proteins.
- Malassezia furfur
- Among other things, it is responsible for increased dandruff formation in humans, especially on the scalp.
Yeasts are used in a wide variety of biotechnological processes. The best known is the production of beverages containing ethanol , such as beer or wine (and other alcoholic beverages ) as well as the ethanol itself. Sugar yeast ( Saccharomyces ) in particular are used for bread ("white bakery", yeast dough ) and beer production. For the production of yeast, see section 5. Production in the article Baker's yeast .
If the liquid to be fermented contains pectin , methanol is produced during fermentation , which in the human body is broken down into methanal ( formaldehyde ) and subsequently into methanoic acid ( formic acid ) and can lead to blindness .
Although the taxonomy (biological systematic classification) of yeast is controversial, at least 1,000 separate strains of Saccharomyces are defined. Industry focuses more on the properties of individual tribes than on overall taxonomic classifications. For the taxonomy "insignificant" differences between strains such as top or bottom fermentation as well as temperature optima can be of decisive importance in the technical application. Classic yeast cultivation appears difficult, since most of the industrial strains are polyploid or aneuploid and consequently do not have a haploid - diploid life cycle . These strains are therefore genetically more stable, but hardly offer suitable reproductive activities for the use of classic breeding methods. However, techniques involving spheroplast formation and recombinant DNA lead to the generation of other yeast strains with industrial potential.
The total mass of yeast produced today, including those from brewing , winemaking and food production , is millions of tons annually. Although yeasts of the Saccharomyces cerevisiae species are the most important form of economic importance, there are numerous “exotic” yeast species with other potential uses for technical applications. Most Saccharomyces yeasts are generally recognized worldwide as safe in terms of food law (GRAS - Generally Recognized As Safe ) and produce two very important primary metabolic products, ethanol and carbon dioxide . These and other yeasts are made available through collections for yeasts, some of which are state-run and some of which are privately organized, the yeast banks ; Examples are the Weihenstephan yeast bank in Germany or the National Collection of Yeast Cultures in Great Britain.
Ethanol is used as drinking alcohol, as a fuel and as a solvent . The use of carbon dioxide ranges from leavening dough, additives to beverages, production of hops extract to use in greenhouse cultures. There are also other important applications of yeasts themselves. Yeast extracts are used to flavor foods and, as a source of nucleotides, are an important component of breast milk substitutes. Yeasts serve as a source of vitamin B for humans and animals . Sterile yeast extracts serve as components of nutrient media for the cultivation of fungi in enzyme production or for the production of bacteria for probiotics and ensiling additives .
The structure of the cell wall of some Saccharomyces species can be specifically controlled via the rearing environment (fermentation process, nutrition), which makes these organisms very popular in the biotechnology industry. The lattice-like glucan fraction of the cell wall of some strains has been shown to have toxin-binding properties. Defined mannanoproteins enable the fight against pathogenic bacteria or serve as oral “promoters” of vaccines and medications, applications that could also be of interest for animal nutrition. The well-described nutrient synthesis of yeast allows the production of amino acids and organically bound trace elements for human and animal nutrition. The use of genetic engineering has led to numerous other important uses of yeast, including strains that, through genetic modification, produce non-yeast-typical proteins and peptides such as interferon , human serum albumin or insulin .
Advantages of yeast as "expression platforms"
Yeasts consist of a large number of very different organisms and not only of the baker's or brewer's yeast Saccharomyces cerevisiae, which is generally known from baking and brewing .
Yeasts are ideal systems for the production of foreign proteins. As eukaryotes , they are able to glycosylate proteins , so they are able to attach sugar chains to proteins: many proteins are glycoproteins . They are also able to secrete these glycoproteins into the surrounding nutrient medium - the intestinal bacterium E. coli , for example, cannot. The proteins produced in yeast are thus identical or very similar to the proteins of animals or humans.
The first “expression platform” (“protein factory”) based on a type of yeast used the already mentioned baker's yeast. However, there are more than 800 different types of yeast with very different properties. In contrast to baker's yeast, some of them are not limited to glucose as a carbon source for their growth, but can use a variety of different substrates. Various of these yeasts are used - like baker's yeast - for the genetic engineering of proteins.
Arxula adeninivorans ( Blastobotrys adeninivorans )
Arxula adeninivorans is a dimorphic yeast species (it grows in yeast form below a temperature of 42 ° C, above this temperature in filamentous form). It can grow on very different energy and carbon sources andassimilate nitrate . It was used for the production of different proteins. Genetically modified strains were used to produce biodegradable plastic or biosensors for measuring estrogens in environmental samples.
Candida boidinii is a methylotrophic yeast species (i.e.,capable ofgrowth with methanol oxidation as the energy source and methanol as the carbon source). Like other methylotrophic yeast species (see below Hansenula polymorpha and Pichia pastoris ) it offers an excellent platform for the production of foreign proteins. Productivities of many grams per liter of culture have been described for them.
Hansenula polymorpha ( Pichia angusta )
Hansenula polymorpha is a methylotrophic yeast species (see Candida boidinii ). It can also grow on a variety of other substrates, is a thermo-tolerant microorganism, and can assimilate nitrate. Among other things, it was used for the production of hepatitis B vaccines , insulin and interferon-alpha2a for the treatment of hepatitis C , as well as for the production of various technical enzymes.
Kluyveromyces lactis (previously Saccharomyces lactis ; secondary crop form Candida sphaerica ) is a type of yeast related to the better-known yeast Kluyveromyces marxianus (Candida kefyr) , which is used, among other things, for the production of kefir . It can grow with the help of various sugars such as glucose in particular . As a special feature it can in the milk and whey contained lactose to lactic acid ferment . Among other things, after genetic engineering, it was used for the production of chymosin , rennet and for curdling the milk in cheese production. The production of the chymosin takes place in large fermenters on a scale of several tens of thousands of liters.
Pichia pastoris ( Komagataella phaffii ) is another methylotrophic yeast species (see Candida boidinii and Hansenula polymorpha ). Various elements are available as kits for this “platform” ; it is used worldwide in universities and academic institutions for protein production. More recently, strains have been developed that make the complex sugar chains of human proteins completely authentic (yeast sugar chains in yeast proteins are usually similar, but not entirely identical).
The term “yeast” denotes a collective term, but is often only used for this type of yeast, the traditional baker's or brewer's yeast Saccharomyces cerevisiae , because this is the original meaning of the word “yeast”. Saccharomyces cerevisiae was and is used, among other things, for the production of technical enzymes , but also of active pharmaceutical ingredients such as insulin and hepatitis B vaccines.
Yarrowia lipolytica is a dimorphic yeast species (cf. Arxula adeninivorans ) which, like other species already described, can grow on different substrates. It has great potential for industrial applications, but there is still no commercially available genetic engineering product that has been produced with the help of this yeast.
Comparison of the different yeasts
The various types of yeast differ considerably in certain product developments. In addition, the so-called wild types must first be transformed into “genetic protein factories”. Suitable yeast strains having to using a vector (concretely, using a plasmid ) transformed be. Such a plasmid contains all the necessary genetic elements for recognizing a transformed strain and the genetic guidance for the production of the desired protein. These elements are briefly summarized below:
- A selection marker that is necessary to distinguish a transformed strain from non-transformed strains - this can be achieved, for example, by a genetic element that enables a defective strain to grow back in media in which an indispensable substance is used which the strain itself can no longer produce due to its defect, such as a certain amino acid .
- Certain elements in order to multiply the plasmids after ingestion or to incorporate them into a specific location on the yeast chromosome ( ARS and / or rDNA sequence).
- A DNA segment that is responsible for the synthesis of the desired protein, a so-called expression cassette . Such a “cassette” consists of a sequence of regulatory sections: first of all it contains a promoter that controls to what extent and under what circumstances a subsequent sequence is read (transcription of the mRNA ) and thus how much and under what circumstances protein is produced becomes.
This means that the following sequence is variable according to the substance to be produced. For example, it can define the amino acid sequence for insulin, hepatitis B surface antigens or interferon. The expression cassette is bounded by a subsequent terminator sequence, through which a correct termination of the transcription takes place. The promoter elements for the control of transcription come from very active genes of the individual yeast species, in Hansenula polymorpha from genes of the methanol metabolism. They are strong and can also be regulated by adding certain carbon sources to a culture medium. Most of the promoters, like the ones just mentioned, only function in a single system, namely that from which they come.
It has been found that the different types of yeast are very different in their ability to produce certain proteins. There are differences in processing and modification and in general in productivity. Since they differ, it cannot be ruled out that a yeast specified at the beginning of a process and product development is not at all or only imperfectly able to produce the desired substance. This in turn can have costly and time consuming consequences. It therefore makes sense to check several types of yeast at the same time for their ability to produce a certain protein at the beginning of a development. For this purpose, a vector system was developed that is functional in all yeasts examined so far. It has a modular structure and contains a “universal” target sequence that is present in an identical sequence in all yeasts (the rDNA ). Inside the expression cassette it contains a promoter which is active in all yeasts.
Yeast in animal nutrition
In addition to the use of dead beer or brewer's yeast as a highly available protein source, specific strains of Saccharomyces cerevisiae have been used as probiotics in animal nutrition for around 20 years . The triumphant advance of this application form, especially in the ruminant sector, goes back to an important observation from the brewing industry: To stabilize the finished green beer, brewers use a small amount of yeast in the process of "puffing", which consumes residual oxygen. In this context, the British brewing scientist James Hough described in 1965 an unusually high oxygen-consuming activity in the strain S. cerevisiae NCYC 1026 . His student, the Irish brewing engineer Pearse Lyons , used this observation for the first time commercially in 1980 to stabilize the anaerobic state in the rumen of cows. Today, the use of live yeast cultures in the feeding of ruminants and horses is standard worldwide.
Other effects relevant to animal performance and health can be traced back to the environment-defining and bacteria-stimulating properties of the living yeast. Various fiber-degrading and lactate-degrading bacteria react to the presence of yeasts by increasing their metabolism and their reproductive activities. The properties used are again specific for individual Saccharomyces strains. Also known are strains with opposite effects, such as the stimulation of lactate builders.
The search for future applications for yeast in animal nutrition is focused on the production of natural hemicellulases and cellulases for the production of higher-quality proteins and individual amino acids from inexpensive raw materials such as rice husks or by-products from the alcohol industry. Other areas include the generation of peptides for balanced young animal feeding in the sense of “ideal proteins” and the use of yeast protein as a basis for chelating drugs and trace elements. The cultivation and production of yeasts of the desired type requires a lot of know-how, but it is very versatile and, above all, very safe. Saccharomyces cerevisiae and their relatives will therefore be with mankind for a long time to come.
- Pure yeast
- Yeast production
- Yeast bank
- Yeast two hybrid system
- Arxula adeninivorans
- Nutritional yeast
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