# Comparing concentrations of media

Success Goal

Conduct a simple comparison of yeast growth between a low percent and high percent YPD media.

## Requirements​

• At least two available Pioreactors
• Dry baker's yeast
• YPD broth mix, or any other media mix

## Introduction​

The most accessible introductory experiment you can conduct with basic supplies is comparing concentrations of media. In this particular example, we will use YPD media and grow yeast cultures, but any media/culture you have readily available will work!

YPD is a common complete media used to propagate yeast cultures, containing peptone and sugars nexcessary for yeast growth.

Now consider creating two YPD media compositions: one with 1% YPD w/v, and one with 5% YPD w/v. We can easily infer that a culture grown in 5% YPD will flourish more than one grown in 1% but let's expore our hypothesis further!

Discuss with your students the following ideas:

• How would both cultures start off? Will one have a faster rate than another? Why?
• Will one reach a higher growth rate? Why or why not?
• How long will it take for the culture in 5% YPD to slow in growth? Compared to the culture in 1% YPD?

## Experiment​

We created 1% YPD by adding 0.15 grams of YPD broth powder to 15 mL:

$\text {w/v \%}= \frac {\text{mass}}{\text {volume}}$
$\text {mass} = {0.01} \times {15 \:\text {mL}}$
$\text {mass} = 0.15 \:\text {grams}$

Using the same logic, 5% YPD was created by adding 0.75 grams to 15 mL.

We inoculated both with 100 uL of a yeast slurry using a micropipette. If this is unavailable, a dropped will work as well!

### Results​

As expected, growth diminished earlier for the culture inoculated in 1% YPD:

## Recommendations​

Consider lower or higher bounds for your media compositions. How much sooner will yeast stop growing in 0.5% YPD? Can we test when yeast growth is halted by YPD concentration versus reaching a peak turbidity?

## Detailed procedure​

1. Prepare a sterile stock of two different concentrations of your media of choice.
2. Inoculate each vial with a very small amount of your microorganism, using best practices to avoid other contamination.
• In our example, a yeast stock solution can be made by diluting a small amount of yeast in 15mL of YPD stock media, then a drop of this stock solution can be added to your vials.
3. Wipe the vials and place them in the Pioreactors.
4. Visit pioreactor.local and start a new experiment.