Unicellular organisms are organisms made up of only one cell, or single-cell organisms. Their counterpart is the group of multicellular organisms, made up of many cells. Unicellular organisms are found in all three domains of the tree of life: bacteria, archaea (all single-cell) and eukaryotes (which include plants, animals and fungi). They are historically known as monads.
Types of unicellular fungus include yeasts and moulds, an example of the former being the genus Saccharomyces which is used in the production of bread and brewing beer and an example of the latter being the genus Penicillium which produces the antibiotic Penicillin used to treat bacterial infections. Genera of single-cell fungi can produce asexually or sexually, but most commonly their vegetative growth is by an asexual production process known as budding. A small bud (or bleb), or daughter cell, is formed on the parent cell. Some yeasts however reproduce asexually by binary fission, the original cell splitting into two daughter cells. Saccharomyces cerevisiae in particular reproduces asexually by budding one daughter cell, and this is the specific species used for baking and brewing beer. Schizosaccharomyces pombe on the other hand is a species that reproduces asexually by binary fission, where two identically sized daughter cells are produced.