Yeast strains, transformed or untransformed, can be maintained as colonies on solid media at 4° and restreaking every 2 to 4 weeks. Alternatively, strains may be stored at -80 indefinitely. This is preferable in that it reduces the likelihood of accumulating spontaneous mutations.
To make frozen stocks of yeast strains: -Use sterile technique and sterile solutions throughout this method.-
Grow a starter culture at 30 with shaking (250 rpm) until it reaches saturation.
In a 1.8 ml cryotube, mix 0.5 ml of the saturated culture with 0.5 ml of YPD containing 20% glycerol.
Flash freeze the tube in liquid nitrogen and store at -80.
To use, chip out a few pieces of the frozen stock using a sterile pipette tip or sterile toothpick and streak onto a plate containing the appropriate solid media.
Allow the yeast to grow at 30 until colonies appear (2-6 days).