Yeast

Yeast is the name for fungi that live as single, rounded cells rather than as threads. Yeasts turn up in several different fungal groups, so the word describes a way of growing rather than one branch of the family tree. Most reproduce asexually by budding: a small bulge forms on the parent cell, receives a copy of the nucleus after mitosis and then pinches off. Some fungi, called dimorphic fungi, switch between a yeast form and a thread-like mold form depending on temperature or other conditions.

Yeasts are facultative anaerobes. With oxygen they respire aerobically, and without it they ferment sugars into ethanol and carbon dioxide. That is how Saccharomyces cerevisiae, the baker's and brewer's yeast, makes dough rise and turns the sugars of grain and fruit into beer and wine.

S. cerevisiae is also one of biology's leading model organisms. It is a eukaryote that grows fast, is easy to modify genetically and processes proteins much as human cells do, so many human genes were first understood through their yeast versions. Other yeasts, such as Candida albicans, live harmlessly on and in the body but can cause thrush and other infections when they overgrow.