For routine culture, scientists usually use rich media that supply all the nutrients that cells need to grow. The individual components of rich media are often undefined. For example, yeast are commonly grown in a medium known as YPD, which is simple and inexpensive to prepare. The “Y” in YPD refers to a yeast extract, which contains the water-soluble compounds generated when yeast are forced to self-digest. (Those of you who have visited Australia may have encoun- tered yeast extract in the popular spread, Marmite.) The “P” refers to peptone, a mixture of peptides and amino acids prepared by digesting animal protein with proteases. The “D” refers to dextrose, or glucose, which is the favored carbon source of yeast (Sherman, 2002).
Because YPD is composed largely of crude extracts, its composition may show significant batch-to-batch variation. This variation is rarely a problem, because YPD contains more than enough essential nutrients to satisfy the metabolic requirements of cells. Many experiments, however, require media with a defined composition. To meet this need, the yeast community has developed a variety of synthetic media. Individual components of the synthetic media may be manipulated to suit the needs of an experiment. (Later in the semester, we will use defined media to select for particular genotypes.)
Yeast can be grown in liquid cultures or on the surface of plates containing solid media. Agar is usually used to solidify liquid growth media when preparing plates. Strains are typically maintained on agar plates for routine use. Cells grow in colonies on plates. The cells in a colony are genetically very similar, if not identical, because they are derived from the same progenitor cell. Most yeast strains can be stored on plates in the refrigerator for several months with minimal loss of viability.