What type of microorganism is yeast?

Yeast is a fungus. Unlike the molds many people associate with fungi, most yeasts grow mainly as single cells and reproduce by budding or, in some species, by fission.

Yeasts are eukaryotic microorganisms, so their cells contain a nucleus and membrane-bound organelles. At least 1,500 yeast species are recognized, and they occur in two major fungal phyla: Ascomycota and Basidiomycota. The word “yeast” therefore describes a growth form rather than one single evolutionary group.

Saccharomyces cerevisiae is the familiar baker’s and brewer’s yeast. It consumes sugars and produces carbon dioxide and alcohol through fermentation; carbon dioxide makes bread rise, while alcohol contributes to beer and wine. Yeast has been used in baking and brewing for thousands of years, and Antonie van Leeuwenhoek observed yeast cells microscopically in 1680.

A common mix-up is treating yeast as a bacterium because both can be microscopic and useful in food production. Yeast is fundamentally different: it is a fungus with a complex eukaryotic cell structure.