or flavours. the general method for making yeast extract for food products such as vegemite and marmite on a commercial scale is heat autolysis, i.e. to add salt to a suspension of yeast, making the solution hypertonic, which leads to the cells' shrivelling up. this triggers autolysis, wherein the yeast's digestive enzymes break their own proteins down into simpler compounds, a process of self-destruction. the dying yeast cells are then heated to complete their breakdown, after which the husks (yeast with thick cell walls that would give poor texture) are removed. yeast autolysates are used in vegemite and promite (australia); marmite (the united kingdom); the unrelated marmite (new zealand); vitam-r (germany); and cenovis (switzerland). nutritional yeast flakes are yellow in colournutritional yeast, which is whole dried, deactivated yeast cells, usually s. cerevisiae. usually in the form of yellow flake or powder, its nutty and umami flavor makes it a vegan substitute for cheese powder. another popular use is as a topping for popcorn. it can also be used in mashed and fried potatoes, as well as in scrambled eggs. it comes in the form of flakes, or as a yellow powder similar in texture to cornmeal. in australia, it is sometimes sold as "savoury yeast flakes". both types of yeast foods above are rich in b-complex vitamins (besides vitamin b12 unless fortified), making them an attractive nutritional supplement to vegans. the same vitamins are also found in some yeast-fermented products mentioned above, such as kvass. nutritional yeast in particular is naturally low in fat and sodium and a source of protein and vitamins as well as other minerals and cofactors required for growth. many brands of nutritional yeast and yeast extract spreads, though not all, are fortified with vitamin b12, which is produced separately by bacteria. in 1920, the fleischmann yeast company began to promote yeast cakes in a "yeast for health" campaign. they initially emphasized yeast as a source of vitamins, good for skin and digestion. their later advertising claimed a much broader range of health benefits, and was censured as misleading by the federal trade commission. the fad for yeast cakes lasted until the late 1930s. probiotics some probiotic supplements use the yeast s. boulardii to maintain and restore the natural flora in the gastrointestinal tract. s. boulardii has been shown to reduce the symptoms of acute diarrhea, reduce the chance of infection by clostridium difficile (often identified simply as c. difficile or c. diff), reduce bowel movements in diarrhea-predominant ibs patients, and reduce the incidence of antibiotic-, traveler's-, and hiv/aids-associated diarrheas. aquarium hobby yeast is often used by aquarium hobbyists to generate carbon dioxide (co2) to nourish plants in planted aquaria. co2 levels from yeast are more difficult to regulate than those from pressurized co2 systems. however, the low cost of yeast makes it a widely used alternative. scientific research diagram showing a yeast cell several yeasts, in particular s. cerevisiae and s. pombe, have been widely used in genetics and cell biology, largely because they are simple eukaryotic cells, serving as a model for all eukaryotes, including humans, for the study of fundamental cellular processes such as the cell cycle, dna replication, recombination, cell division, and metabolism. also, yeasts are easily manipulated and cultured in the laboratory, which has allowed for the development of powerful standard techniques, such as yeast two-hybrid, synthetic genetic array analysis, and tetrad analysis. many proteins important in human biology were first discovered by studying their homologues in yeast; these proteins include cell cycle proteins, signaling proteins, and protein-processing enzymes. on 24 april 1996, s. cerevisiae was announced to be the first eukaryote to have its genome, consisting of 12 million base pairs, fully sequenced as part of the genome project. at the time, it was the most complex organism to have its full genome sequenced, and the work of seven years and the involvement of more than 100 laboratories to accomplish. the second yeast species to have its genome sequenced was schizosaccharomyces pombe, which was completed in 2002. it was the sixth eukaryotic genome sequenced and consists of 13.8 million base pairs. as of 2014, over 50 yeast species have had their genomes sequenced and published. genomic and functional gene annotation of the two major yeast models can be accessed via their respective model organism databases: sgd and pombase. genetically engineered biofactories various yeast species have been genetically engineered to efficiently produce various drugs, a technique called metabolic engineering. s. cerevisiae is easy to genetically engineer; its physiology, metabolism and genetics are well known, and it is amenable for use in harsh industrial conditions. a wide variety of chemical in different classes can be produced by engineered yeast, including phenolics, isoprenoids, alkaloids, and polyketides. about 20% of biopharmaceuticals are produced in s. cerevisiae, including insulin, vaccines for hepatitis, and human serum albumin. pathogenic yeasts main article: fungal infection gram stain of candida albicans from a vaginal swab. the small oval chlamydospores are 2–4 µm in diameter. a photomicrograph of candida albicans showing hyphal outgrowth and other morphological characteristics some species of yeast are opportunistic pathogens that can cause infection in people with compromised immune systems. cryptococcus neoformans and cryptococcus gattii are significant pathogens of immunocompromised people. they are the species primarily responsible for cryptococcosis, a fungal infection that occurs in about one million hiv/aids patients, causing over 600,000 deaths annually. the cells of these yeast are surrounded by a rigid polysaccharide capsule, which helps to prevent them from being recognised and engulfed by white blood cells in the human body. yeasts of the genus candida, another group of opportunistic pathogens, cause oral and vaginal infections in humans, known as candidiasis. candida is commonly found as a commensal yeast in the mucous membranes of humans and other warm-blooded animals. however, sometimes these same strains can become pathogenic. the yeast cells sprout a hyphal outgrowth, which locally penetrates the mucosal membrane, causing irritation and shedding of the tissues. a book from the 1980s listed the pathogenic yeasts of candidiasis in probable descending order of virulence for humans as: c. albicans, c. tropicalis, c. stellatoidea, c. glabrata, c. krusei, c. parapsilosis, c. guilliermondii, c. viswanathii, c. lusitaniae, and rhodotorula mucilaginosa. candida glabrata is the second most common candida pathogen after c. albicans, causing infections of the urogenital tract, and of the bloodstream (candidemia). c. auris has been more recently identified. food spoilage yeasts are able to grow in foods with a low ph (5.0 or lower) and in the presence of sugars, organic acids, and other easily metabolized carbon sources. during their growth, yeasts metabolize some food components and produce metabolic end products. this causes the physical, chemical, and sensible properties of a food to change, and the food is spoiled. the growth of yeast within food products is often seen on their surfaces, as in cheeses or meats, or by the fermentation of sugars in beverages, such as juices, and semiliquid products, such as syrups and jams. the yeast of the genus zygosaccharomyces have had a long history as spoilage yeasts within the food industry. this is mainly because these species can grow in the presence of high sucrose, ethanol, acetic acid, sorbic acid, benzoic acid, and sulfur dioxide concentrations, representing some of the commonly used food preservation methods. methylene blue is used to test for the presence of live yeast cells. in oenology, the major spoilage yeast is brettanomyces bruxellensis. candida blankii has been detected in iberian ham and meat. symbiosis an indian study of seven bee species and nine plant species found 45 yeast species from 16 genera colonise the nectaries of flowers and honey stomachs of bees. most were members of the genus candida; the most common species in honey bee stomachs was dekkera intermedia, while the most common species colonising flower nectaries was candida blankii. although the mechanics are not fully understood, it was found that a. indica flowers more if candida blankii is present. in another example, spathaspora passalidarum, found in the digestive tract of scarab beetles, aids the digestion of plant cells by fermenting xylose.