A collective name for those fungi which possess, under normal conditions of growth, a vegetative body (thallus) consisting, at least in part, of simple, single cells. The cells making up the thallus occur in pairs, in groups of three, or in straight or branched chains consisting of as many as 12 or more cells. Vegetative reproduction is characterized by budding or fission. Sexual reproduction also occurs in yeast, and is differentiated from that of other fungi by sexual states that are not enclosed in a fruiting body. Yeasts are a phylogenetically diverse group of organisms that occur in two divisions of fungi (Ascomycotina and Basidiomycotina) and 100 genera. The 700 or more species that have been described possibly represent only 1% of the species in nature, so the majority of the yeasts have yet to be discovered. Yeast plays a large part in industrial fermentation processes such as the production of industrial enzymes and chemicals, food products, industrial ethanol, and malt beverage and wine; in diseases of humans, animals and plants; in food spoilage; and as a model of molecular genetics. Genetic engineering Medical mycology

The shape and size of the individual cells of some species vary slightly, but in other species the cell morphology is extremely heterogeneous. The shape of yeast cells may be spherical, globose, ellipsoidal, elongate to cylindrical with rounded ends, more or less rectangular, pear-shaped, apiculate or lemon-shaped, ogival or pointed at one end, or tetrahedral. The diameter of a spherical cell may vary from 2 to 10 micrometers. The length of cylindrical cells is often 20–30 μm and, in some cases, even greater.

The asexual multiplication of yeast cells occurs by a budding process, by the formation of cross walls or fission, and sometimes by a combination of these two processes. Yeast buds are sometimes called blastospores or blastoconidia. When yeast reproduces by a fission mechanism, the resulting cells are termed arthrospores or arthroconidia.

Yeasts are categorized into two groups, based on their methods of sexual reproduction: the ascomycetous (Division Ascomycotina) and basidiomycetous (Division Basidiomycotina) yeasts.

The sexual spores of the ascomycetous yeasts are termed ascospores, which are formed in simple structures, often a vegetative cell. Such asci are called naked asci because of the absence of an ascocarp, which is a more complex fruiting body found in the higher Ascomycetes. If the vegetative cells are diploid, a cell may transform directly into an ascus after the 2n nucleus undergoes a reduction or meiotic division. Ascomycota

Certain yeasts have been shown to be heterothallic; that is, sporulation occurs when strains of opposite mating type (usually indicated by “a” and α) are mixed on sporulation media. However, some strains may be homothallic (self-fertile), and reduction division and karyogamy (fusion of two haploid nuclei) take place during formation of the sexual spore. Yeasts that produce sporogenous cells represent the teleomorphic form of the life cycle. In cases, in which sexual cycles are unknown, the yeast represents the asexual or anamorphic form. A species of yeast may be originally discovered in the anamorphic form and named accordingly; subsequently, the sexual state may be found and a name applied to represent the teleomorph. Consequently, the anamorphic and teleomorphic names will differ.

Basidiospores and teliospores are the sexual spores that are produced in the three classes of basidiomycetous yeasts: Urediniomycetes, Hymenomycetes, and Ustilaginomycetes. Sexual reproduction and life cycle in these yeasts is typical of other basidiomycetes in that it can include both unifactorial (bipolar) and bifactorial (tetrapolar) mating systems. Basidiomycota

Some yeasts have the ability to carry out an alcoholic fermentation. Other yeasts lack this property. In addition to the fermentative type of metabolism, fermentative yeasts as a rule have a respiratory type of metabolism, whereas nonfermentative yeasts have only a respiratory, or oxidative, metabolism. Both reactions produce energy, with respiration producing by far the most, which is used in part for synthetic reactions, such as assimilation and growth. Part is lost as heat. In addition, small or sometimes large amounts of by-products are formed, including organic acids, esters, aldehydes, glycerol, and higher alcohols. When a fermenting yeast culture is aerated, fermentation is suppressed and respiration increases. This phenomenon is called the Pasteur effect. Fermentation

Yeasts are ubiquitous in nature. They exist on plants and animals; in waters, sediments, and soils; and in terrestrial, aquatic, and marine habitats. Yeasts require oxygen for growth and reproduction; therefore they do not inhabit anaerobic environments such as anoxic sediments. Many species have highly specific habitats, whereas others are found on a variety of substrates in nature.