4: Working with Yeast

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This lab will introduce you to standard techniques used in microbiology. Very similar techniques are used to culture yeast and bacteria, although the culture conditions are optimized for each organism. In this lab,
you will learn sterile techniques required for maintaining the integrity of yeast strains in the lab, as well as methods for culturing cells and estimating cell numbers.

Objectives

At the end of this lab, students will be able to:

define and describe the relationship between species, strains and genotypes.
use sterile techniques to culture yeast strains on agar plates.
describe the phases of microbial growth.
estimate the density of cells in a liquid culture using spectrophotometry and spot plating.

In this project, you will be working with multiple strains of yeast. Strains are microorganims of the same species that are derived from a single cell and are assumed to possess the same genotype. Laboratory strains have often been derived from other strains by careful planning and experimentation, and their genotypes are at least partially defined. Strains are named according to that laboratory’s conventions, but the strain names are not usually informative about the strain’s genetic composition. Yeast strains are easily cultured in media that contains a carbon source, a nitrogen source, salts, vitamins and essential minerals.

Your success in this lab will depend on your ability to use sterile culture and transfer techniques that will maintain the genetic isolation of your yeast strains. An equally important element in laboratory success is careful bookkeeping, since yeast strains look alike! In this lab, you will learn basic techniques used to culture and quantify microorganisms. You will prepare stock cultures of S. cerevisiae met mutant strains on streak plates. You will also learn how to quantify the number of cells in liquid cultures of S. cerevisiae and S. pombe using spot plates and the spectrophotometer.