Saccharomycetales

an order of unicellular fungi (yeasts and various chiefly parasitic molds) of the class Ascomycetes. They reproduce, as a rule, by building. Saccharomycetales cells are usually oval, measuring 8-10 microns long and 2-7 microns wide. In sexual reproduction, usually two externally similar, less often different, cells come together and copulate. Their contents fuse giving rise to a sac (ascus) in which so-called ascospores are formed. In species of the genus Saccharomyces, the ascus originates parthenogenetically from a single cell without the fusion of cells and contains one to four ascospores. Copulation of the ascospores or of the cells germinated from them occurs. The subsequent cycle of development takes place in the diploid phase. Heterogamy with the formation of one ascospore is characteristic of the Saccharomycetales of the genus Debaryomyces, and isogamy with the formation of four to six ascospores is characteristic of the genus Zygosaccharomyces. The Saccharomycetales of the genus Schizosaccharomyces reproduce mostly by meiosis. When growing under unfavorable conditions, the cells copulate in pairs, forming outgrowths that meet one another into which the nuclei of both cells proceed, fusing into a diploid nucleus which subsequently divides three times into eight nuclei for the future ascospores. Artificial hybridization of the Saccharomycetales was shown to be possible. Unicellular fungi that reproduce by budding but do not form ascospores are often regarded as the Saccharomycetales. They should be called yeastlike fungi and included in the assemblage Fungi imperfecti.

The Saccharomycetales are widely distributed in nature, especially where saccharine substances exist (berries, fruits, nectar of flowers, dairy products, and so forth). Because some Saccharomycetales can induce alcoholic fermentation of sugars, they are used in baking, wine-making, beer brewing, the production of alcohol and glycerol, the dairy industry, and the production of ergosterol (provitamin D₂), nucleic acids, and so forth. The Saccharomycetales contain high-quality protein, carbohydrates, and an abundance of B-complex vitamins.

Liquid brewer’s yeast improves gastric and pancreatic secretion, stimulates the absorption of food in the intestine, and increases the resistance of the organism to infections. Yeast is prescribed for a variety of anemias, diabetes, furunculosis, ulcers, neuralgia, and so forth and to increase the protein content of the diet. Dried medicinal yeast is also available in tablets. The Saccharomycetales and yeastlike fungi can appear in pathogenic forms that cause such diseases in man and animals as blastomycosis and candidiasis.