yeast yeast yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. the first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. they are estimated to constitute 1% of all described fungal species. yeast saccharomyces cerevisiae, a species of yeast cross-sectional labelled diagram of a typical yeast cell scientific classification domain: eukaryota kingdom: fungi phyla and subphyla ascomycota p. p. saccharomycotina (true yeasts) taphrinomycotina p. p. schizosaccharomycetes (fission yeasts) basidiomycota p. p. agaricomycotina p. p. tremellomycetes pucciniomycotina p. p. microbotryomycetes yeasts are unicellular organisms that evolved from multicellular ancestors, with some species having the ability to develop multicellular characteristics by forming strings of connected budding cells known as pseudohyphae or false hyphae. yeast sizes vary greatly, depending on species and environment, typically measuring 3–4 µm in diameter, although some yeasts can grow to 40 µm in size. most yeasts reproduce asexually by mitosis, and many do so by the asymmetric division process known as budding. with their single-celled growth habit, yeasts can be contrasted with molds, which grow hyphae. fungal species that can take both forms (depending on temperature or other conditions) are called dimorphic fungi. the yeast species saccharomyces cerevisiae converts carbohydrates to carbon dioxide and alcohols through the process of fermentation. the products of this reaction have been used in baking and the production of alcoholic beverages for thousands of years. s. cerevisiae is also an important model organism in modern cell biology research, and is one of the most thoroughly studied eukaryotic microorganisms. researchers have cultured it in order to understand the biology of the eukaryotic cell and ultimately human biology in great detail. other species of yeasts, such as candida albicans, are opportunistic pathogens and can cause infections in humans. yeasts have recently been used to generate electricity in microbial fuel cells and to produce ethanol for the biofuel industry. yeasts do not form a single taxonomic or phylogenetic grouping. the term "yeast" is often taken as a synonym for saccharomyces cerevisiae, but the phylogenetic diversity of yeasts is shown by their placement in two separate phyla: the ascomycota and the basidiomycota. the budding yeasts or "true yeasts" are classified in the order saccharomycetales, within the phylum ascomycota. history the word "yeast" comes from old english gist, gyst, and from the indo-european root yes-, meaning "boil", "foam", or "bubble". yeast microbes are probably one of the earliest domesticated organisms. archaeologists digging in egyptian ruins found early grinding stones and baking chambers for yeast-raised bread, as well as drawings of 4,000-year-old bakeries and breweries. vessels studied from several archaeological sites in israel (dating to around 5,000, 3,000 and 2,500 years ago), which were believed to have contained alcoholic beverages (beer and mead), were found to contain yeast colonies that had survived over the millennia, providing the first direct biological evidence of yeast use in early cultures. in 1680, dutch naturalist anton van leeuwenhoek first microscopically observed yeast, but at the time did not consider them to be living organisms, but rather globular structures as researchers were doubtful whether yeasts were algae or fungi. theodor schwann recognized them as fungi in 1837. in 1857, french microbiologist louis pasteur showed that by bubbling oxygen into the yeast broth, cell growth could be increased, but fermentation was inhibited – an observation later called the "pasteur effect". in the paper "mémoire sur la fermentation alcoolique," pasteur proved that alcoholic fermentation was conducted by living yeasts and not by a chemical catalyst. by the late 18th century two yeast strains used in brewing had been identified: saccharomyces cerevisiae (top-fermenting yeast) and s. carlsbergensis (bottom-fermenting yeast). s. cerevisiae has been sold commercially by the dutch for bread-making since 1780; while, around 1800, the germans started producing s. cerevisiae in the form of cream. in 1825, a method was developed to remove the liquid so the yeast could be prepared as solid blocks. the industrial production of yeast blocks was enhanced by the introduction of the filter press in 1867. in 1872, baron max de springer developed a manufacturing process to create granulated yeast, a technique that was used until the first world war. in the united states, naturally occurring airborne yeasts were used almost exclusively until commercial yeast was marketed at the centennial exposition in 1876 in philadelphia, where charles l. fleischmann exhibited the product and a process to use it, as well as serving the resultant baked bread. the mechanical refrigerator (first patented in the 1850s in europe) liberated brewers and winemakers from seasonal constraints for the first time and allowed them to exit cellars and other earthen environments. for john molson, who made his livelihood in montreal prior to the development of the fridge, the brewing season lasted from september through to may. the same seasonal restrictions formerly governed the distiller's art. nutrition and growth yeasts are chemoorganotrophs, as they use organic compounds as a source of energy and do not require sunlight to grow. carbon is obtained mostly from hexose sugars, such as glucose and fructose, or disaccharides such as sucrose and maltose. some species can metabolize pentose sugars such as ribose, alcohols, and organic acids. yeast species either require oxygen for aerobic cellular respiration (obligate aerobes) or are anaerobic, but also have aerobic methods of energy production (facultative anaerobes). unlike bacteria, no known yeast species grow only anaerobically (obligate anaerobes). most yeasts grow best in a neutral or slightly acidic ph environment. yeasts vary in regard to the temperature range in which they grow best. for example, leucosporidium frigidum grows at −2 to 20 °c (28 to 68 °f), saccharomyces telluris at 5 to 35 °c (41 to 95 °f), and candida slooffi at 28 to 45 °c (82 to 113 °f). the cells can survive freezing under certain conditions, with viability decreasing over time. in general, yeasts are grown in the laboratory on solid growth media or in liquid broths. common media used for the cultivation of yeasts include potato dextrose agar or potato dextrose broth, wallerstein laboratories nutrient agar, yeast peptone dextrose agar, and yeast mould agar or broth. home brewers who cultivate yeast frequently use dried malt extract and agar as a solid growth medium. the fungicide cycloheximide is sometimes added to yeast growth media to inhibit the growth of saccharomyces yeasts and select for wild/indigenous yeast species. this will change the yeast process. the appearance of a white, thready yeast, commonly known as kahm yeast, is often a byproduct of the lactofermentation (or pickling) of certain vegetables. it is usually the result of exposure to air. although harmless, it can give pickled vegetables a bad flavor and must be removed regularly during fermentation. ecology yeasts are very common in the environment, and are often isolated from sugar-rich materials. examples include naturally occurring yeasts on the skins of fruits and berries (such as grapes, apples, or peaches), and exudates from plants (such as plant saps or cacti). some yeasts are found in association with soil and insects. yeasts from the soil and from the skins of fruits and berries have been shown to dominate fungal succession during fruit decay. the ecological function and biodiversity of yeasts are relatively unknown compared to those of other microorganisms. yeasts, including candida albicans, rhodotorula rubra, torulopsis and trichosporon cutaneum, have been found living in between people's toes as part of their skin flora. yeasts are also present in the gut flora of mammals and some insects and even deep-sea environments host an array of yeasts. an indian study of seven bee species and nine plant species found 45 species from 16 genera colonize the nectaries of flowers and honey stomachs of bees. most were members of the genus candida; the most common species in honey stomachs was dekkera intermedia and in flower nectaries, candida blankii. yeast colonising nectaries of the stinking hellebore have been found to raise the temperature of the flower, which may aid in attracting pollinators by increasing the evaporation of volatile organic compounds. a black yeast has been recorded as a partner in a complex relationship between ants, their mutualistic fungus, a fungal parasite of the fungus and a bacterium that kills the parasite. the yeast has a negative effect on the bacteria that normally produce antibiotics to kill the parasite, so may affect the ants' health by allowing the parasite to spread. certain strains of some species of yeasts produce proteins called yeast killer toxins that allow them to eliminate competing strains. (see main article on killer yeast.) this can cause problems for winemaking but could potentially also be used to advantage by using killer toxin-producing strains to make the wine. yeast killer toxins may also have medical applications in treating yeast infections (see "pathogenic yeasts" section below). marine yeasts, defined as the yeasts that are isolated from marine environments, are able to grow better on a medium prepared using seawater rather than freshwater. the first marine yeasts were isolated by bernhard fischer in 1894 from the atlantic ocean, and those were identified as torula sp. and mycoderma sp. following this discovery, various other marine yeasts have been isolated from around the world from different sources, including seawater, seaweeds, marine fish and mammals. among these isolates, some marine yeasts originated from terrestrial habitats (grouped as facultative marine yeast), which were brought to and survived in marine environments. the other marine yeasts were grouped as obligate or indigenous marine yeasts, which are confined to marine habitats. however, no sufficient evidence has been found to explain the indispensability of seawater for obligate marine yeasts. it has been reported that marine yeasts are able to produce many bioactive substances, such as amino acids, glucans, glutathione, toxins, enzymes, phytase, and vitamins with potential applications in the food, pharmaceutical, cosmetic, and chemical industries as well as for marine culture and environmental protection. marine yeast was successfully used to produce bioethanol using seawater-based media which will potentially reduce the water footprint of bioethanol. reproduction the yeast cell's life cycle: buddingconjugationspore yeasts, like all fungi, may have asexual and sexual reproductive cycles. the most common mode of vegetative growth in yeast is asexual reproduction by budding, where a small bud (also known as a bleb or daughter cell) is formed on the parent cell. the nucleus of the parent cell splits into a daughter nucleus and migrates into the daughter cell. the bud then continues to grow until it separates from the parent cell, forming a new cell. the daughter cell produced during the budding process is generally smaller than the mother cell. some yeasts, including schizosaccharomyces pombe, reproduce by fission instead of budding, and thereby creating two identically sized daughter cells. in general, under high-stress conditions such as nutrient starvation, haploid cells will die; under the same conditions, however, diploid cells can undergo sporulation, entering sexual reproduction (meiosis) and producing a variety of haploid spores, which can go on to mate (conjugate), reforming the diploid. the haploid fission yeast schizosaccharomyces pombe is a facultative sexual microorganism that can undergo mating when nutrients are limited. exposure of s. pombe to hydrogen peroxide, an agent that causes oxidative stress leading to oxidative dna damage, strongly induces mating and the formation of meiotic spores. the budding yeast saccharomyces cerevisiae reproduces by mitosis as diploid cells when nutrients are abundant, but when starved, this yeast undergoes meiosis to form haploid spores. haploid cells may then reproduce asexually by mitosis. katz ezov et al. presented evidence that in natural s. cerevisiae populations clonal reproduction and selfing (in the form of intratetrad mating) predominate. in nature, the mating of haploid cells to form diploid cells is most often between members of the same clonal population and out-crossing is uncommon. analysis of the ancestry of natural s. cerevisiae strains led to the conclusion that out-crossing occurs only about once every 50,000 cell divisions. these observations suggest that the possible long-term benefits of outcrossing (e.g. generation of diversity) are likely to be insufficient for generally maintaining sex from one generation to the next. rather, a short-term benefit, such as recombinational repair during meiosis, may be the key to the maintenance of sex in s. cerevisiae. some pucciniomycete yeasts, in particular species of sporidiobolus and sporobolomyces, produce aerially dispersed, asexual ballistoconidia. uses the useful physiological properties of yeast have led to their use in the field of biotechnology. fermentation