Yeast

Yeast are a group of unicellular fungi that reproduce primarily by budding. They are found in soil, on plants, and as part of the normal microbiota of many environments. The term “yeast” refers to a functional group rather than a single taxonomic lineage; the most studied species is Saccharomyces cerevisiae, but many other yeasts occur in genera such as Saccharomyces, Candida, and Pichia. Yeasts vary in size, shape, and metabolic capabilities, but share common traits as eukaryotic, single-celled organisms.

Biology and metabolism: Yeasts are eukaryotes with a nucleus, mitochondria, and a cell wall. The wall is rich in glucans and mannoproteins, with chitin present in some species. Most yeasts are facultative anaerobes: they respire when oxygen is available and ferment sugars to ethanol and carbon dioxide when it is not. This fermentative metabolism underpins many food and beverage processes. Yeasts can utilize a range of carbon sources, including glucose, sucrose, and maltose. In Saccharomyces cerevisiae, high glucose can trigger fermentation even in the presence of oxygen, a phenomenon linked to the Crabtree effect.

Life cycle: Yeasts reproduce mainly by asexual budding, producing daughter cells. Some species also undergo sexual reproduction through spore formation, enabling genetic recombination and the transition between haploid and diploid states.

Applications and health: Yeast-driven fermentation is essential in bread making, brewing, and winemaking, converting sugars into carbon dioxide and alcohol. In industry and research, Saccharomyces cerevisiae serves as a model organism for genetics and cell biology, and is used to produce enzymes, biofuels, and pharmaceuticals. Some yeasts, such as Saccharomyces boulardii, are used as probiotics, while others, including Candida species, can cause infections in immunocompromised individuals.