ABSTRACT
Although the term yeast is used extensively in scientific literature, it does not represent a taxonomic designation which can be rigorously defined. Historically, the word itself originated from ancient words describing the visible changes occurring in fermenting liquids. During the 19th century when the biological basis for the alcoholic fermentation became firmly established, the organisms responsible (such as Saccharomyces cereviae) were described as single, hyaline, round to oval budding cells capable of forming ascospores. After Pasteur related the fermentative process to the ability of yeasts to grow in the absence of air, it was assumed that all yeasts could grow anaerobically. As the years passed, other organisms were discovered which were similar but not identical in morphological and physiological properties and the definition for yeast was expanded to include them. For example, although most yeast cells are hyaline some such as Rhodotorula produce red or yellow carotenoid pigments. Budding, too, is a characteristic not common to all yeasts; species of the genus Schizosaccharomyces multiply exclusively by fission. Species of several yeast genera such as Endomycopsis and Rhodosporidium are capable of forming true mycelium. Even fermentation is not a universal characteristic since many species are incapable of anaerobic growth. Many yeasts are apparently unable to form ascospores or other kinds of sexually produced spores and are, therefore, assigned to the Fungi Imperfecti. Some budding yeasts produce spores which are forcibly discharged from the tips of sterigmata. Although these so-called ballistospores resemble basidiospores, they do not result from a sexual process. Recently, other budding yeasts (Rhodosporidium and Leucosporidium) have been shown to produce sexual spores known as sporidia; these yeasts have life cycles similar to the Ustilaginales of the Basidiomycetes.
