Saccharomyces cerevisiae, commonly known as baker's yeast, is an important microbe in baked good and alcohol production. Its ability to ferment sugar is utilized in two different manners. In alcohol production, this microbe ferments the sugars of rice, wheat, barley, grapes, and corn into ethanol to produce beer, wines, and champagne. In the production of baked goods, S. cerevisiae ferments the sugars present in the dough, which releases carbon dioxide; this, in turn, causes the dough to rise. S. cerevisiae is considered the most commercially significant yeast.

Additionally, this yeast is also the favored eukaryotic microbe for genetic engineering. Not only is the entire genome sequenced, a significant advantage to scientists utilizing a microbial system for molecular genetic research, but S. cerevisiae's mechanisms of cellular replication, recombination, division, and metabolism have been highly conserved throughout the higher eukaryotes. As a single-celled organism, it is easy to grow and is well characterized biochemically.

S. cerevisiae may reproduce asexually by budding at the pole of the mature cell (see figure). During budding, the cell wall softens and a bud forms which will eventually become the new cell. The nucleus of the mature cell undergoes mitosis, and one of the nuclei then moves to the bud. When the bud reaches the appropriate size, a cell wall forms and the two cells separate.

During the organism's sexual reproduction cycle, haploid cells are able to mate to produce diploid progeny. These progeny then undergo meiosis to produce haploid cells. Both the diploid and haploid forms of this microbe can exist stably in nature.

S. cerevisiae has also been utilized successfully to express foreign proteins. One important example of this is its ability to express the surface antigen of the hepatitis B virus. As this virus cannot be grown in vitro, obtaining preparations of the surface antigen for vaccine use has been difficult. Fortunately, the viral gene coding for the immunogenic surface antigen has been cloned and expressed in S. cerevisiae and is currently being used in the hepatitis B vaccine.

Figure 1: Scanning electron micrograph of a S. cerevisiae haploid cell (green) that has produced at least six daughters as shown by the polar contiguous array of circular bud scars (blue). It is important to note that the colors are simulated to highlight specific areas of the cells. These are not natural colors for yeast.

Additional resource:
http://genome-www.stanford.edu/Saccharomyces/ - Saccharomyces cerevisiae Genome Database

References
1. Dale, J. W. 1999. Molecular genetics of bacteria, 3rd ed. John Wiley and Sons, Chichester, England.
2. Doyle, M. P., L. R. Beuchat, and T. J. Montville. 1997. Food microbiology: fundamentals and frontiers. ASM Press, Washington, D.C.
2. Nichol, D. S. T. 1998. An introduction to genetic engineering. Cambridge University Press, Cambridge, England.

Legend written by:
Kristen Catlin-LeBaron
American Society for Microbiology
Washington, D.C. 20036
klebaron@asmusa.org