YEAST POPULATION LAB

The growth of populations and the maximum population size in a habitat are affected by the availability of nutrients, physical conditions in the habitat, and biotic interactions. In this lab we will follow the growth of yeast populations in a habitat with differing conditions. Food concentrations for the yeast will be varied by changing the amount or type of sugar in the growth medium. We will attempt to assess the effect of varying sugar concentration/type on the percent growth of the yeast population (PG %) / day.

Direct counts of the yeast will be impossible, so we will estimate population size with a spectrophotometer. A spectrophotometer is a machine which shines light through a sample to measure its density. The greater the density (and hence number) of cells in the sample the higher amount of light absorbed causing a higher absorbance reading on the spectrophotometer. Each group will monitor the growth of their cultures over five days and the cultures will be kept incubated at room temperature.

SETUP PROCEDURE (per group):

1.  Obtain approval of your experimental procedure from your instructor. Don’t forget a control!

2.  All tubes used in this lab contain 25 ml of sterile Sabouraud Dextrose broth.

3.  5 drops of yeast suspension should be added to each tube used in the experiment.

4.  The amount and type of sugar used in the tubes will vary from group to group.

MEASUREMENT PROCEDURE:

1. Make sure the spectrophotometer is set to read at 550 nm. Insert a control tube (blank) into the spectrophotometer and calibrate the spectrophotometer (spec) so that the absorbance reads 0 using this tube. If the medium in the control tube appears cloudy (which may indicate bacterial growth) and gives a high absorbance reading, discard and use another control tube.

2. Immediately before reading any tube, finger vortex the tube so that spinning column of medium reaches the bottom of the tube for several seconds. This is critical! The yeast cells are heavy and will tend to sink to the bottom of the tube, so you must finger vortex the tubes to resuspend them: otherwise, your spec readings will be erroneously low.

3. Read the absorbance for the tube. Record the tube number and absorbance.

4. Repeat steps 2 and 3 of the procedure for each tube.