Yeasts are unicellular fungi that grow as solitary, oval to spherical cells typically measuring 3–10 µm in diameter. They reproduce asexually by budding or binary fission and, depending on the species, may also form sexual spores. Yeasts occur in the Ascomycota and Basidiomycota phyla and are found in diverse habitats such as soil, plant surfaces, animal mucosa and fermented foods.
Structure, reproduction and diversity
The cell wall of a yeast consists mainly of β-1,3-glucan, mannoproteins and chitin, providing rigidity and shape. In budding yeasts such as Saccharomyces cerevisiae and Candida albicans, a daughter cell emerges from a small bud on the mother cell; repeated budding leaves characteristic scars. Fission yeasts like Schizosaccharomyces pombe divide symmetrically by medial septation. Under nutrient limitation, some yeasts undergo meiosis and form ascospores or basidiospores within sacs. Many species can extend chains of elongated budding cells called pseudohyphae, especially under invasive conditions. Yeasts exhibit metabolic versatility, fermenting sugars to ethanol and carbon dioxide under anaerobic or high‑sugar conditions, and switching to aerobic respiration when oxygen is available. Their capacity to assimilate diverse carbon sources and tolerate osmotic stress underlies their ecological success.
Industrial and clinical significance
Yeasts have long been exploited for food and beverage production. S. cerevisiae ferments dough and wort, producing carbon dioxide and ethanol that leaven bread and brew beer and wine. It also serves as a model organism for eukaryotic genetics and biotechnology. Species such as Pichia pastoris and Komagataella phaffii are used for heterologous protein expression. Not all yeasts are beneficial: C. albicans and related species are opportunistic pathogens of humans, causing mucosal and invasive candidiasis, particularly in immunocompromised individuals. Cryptococcus neoformans, an encapsulated basidiomycetous yeast, causes meningitis. Identification of yeasts relies on colony morphology, germ tube formation, carbohydrate assimilation profiles and molecular sequencing. Effective management of pathogenic yeasts requires understanding their dimorphic transitions and resistance mechanisms. Unicellular yeasts bridge fundamental and applied microbiology through their roles in fermentation, basic research and opportunistic infections. Their simple growth form belies a remarkable diversity of reproductive strategies and ecological niches. Related Terms: Fungi, Saccharomyces cerevisiae, Candida, Budding, Fermentation