Mold Control: Toxicology Experiment
Activity Overview
Disciplines Covered: Science & Math
Prep Time: 60-90 minutes
Activity time: 2 class periods, plus
incubation time (about 3-5 days)
Mold Control: Toxicology Experiment
The Dose Makes the Poison
100
LD100
LD50
% Mortality
LOEL
0
LD 50: Lethal Dose 50%
LD 100: Lethal Dose 100%
LOEL: Lowest Observable Effect Level
Mold Control: Toxicology Experiment
Serial Dilution Schematic
Mold Control: Toxicology Experiment
Serial Dilution Preparation
Teacher: Yeast dilutions
Three batches of 10-5 yeast
solution
Students: Cleaners- 100% to 0.0001%
Group A – Lysol Kitchen
Antibacterial Cleaner
Group B – Tilex Mildew Remover
Group C- Borax
Mold Control: Toxicology Experiment
Yeast Serial Dilutions
Mold Control: Toxicology Experiment
Add yeast solution to film
Mold Control: Toxicology Experiment
Lay film cover down
Mold Control: Toxicology Experiment
Press with spreader
Mold Control: Toxicology Experiment
1 ml spread on film
Mold Control: Toxicology Experiment
Incubate in bag
Mold Control: Toxicology Experiment
Petrifilm after Incubation
Mold Control: Toxicology Experiment
Part I: Serial dilutions
Working in pairs, you have 10
minutes to complete this task:
1. Practice using the disposable
pipette with colored water.
2. Create a 10-6 dilution of yeast
a. Mix the 10-5 yeast dilution well.
b. Using a pipette, take 1 ml from 10
yeast dilution.
c. Add this to the 9 ml of water in the
blank tube.
d. Label this new dilution “10-6”
e. Mix well.
Mold Control: Toxicology Experiment
Part II: Petrifilms
You have 5 minutes to complete this
task. Working in pairs:
1. Mix your 10-6 dilution well.
2. Draw 1 ml of your solution into a NEW
disposable pipette.
3. Take one 3M Yeast and Mold petrifilm.
4. Lift the plastic layer.
5. Squeeze out the 1 ml into one large
drop in the middle of the grid
6. Lay the plastic layer back down.
7. Place the spreader over the middle of
the drop.
8. Press down with firm pressure for about
5 seconds.
Mold Control: Toxicology Experiment
Potato Dextrose Agar Plates
• An alternative to 3M Yeast and
Mold Petrifilms are the traditional
agar plates.
• Potato dextrose agar is used
because it is a good medium for
growing yeast.
• The alternative student and
teacher materials are available on
the Hydroville website:
www.hydroville.org
Mold Control: Toxicology Experiment
Yeast solution and plate
Mold Control: Toxicology Experiment
Draw up about 0.5 ml
Mold Control: Toxicology Experiment
Add 4 drops to plate
Mold Control: Toxicology Experiment
Spread with back of spoon
Mold Control: Toxicology Experiment
Liquid on agar
Mold Control: Toxicology Experiment
Incubate upside down
Mold Control: Toxicology Experiment
Agar plate after Incubation
Control
vs.
100% Lysol
Mold Control: Toxicology Experiment
Counting Colonies
Mold Control: Toxicology Experiment
Part III: Data Analysis and
Interpretation
You have 10 minutes to complete
this task.
1. Work together to count the colonies on
the petrifilms for each dilution (Tilex,
Lysol, or Borax).
2. Plot your data on the graphs provided.
3. Find the LOEL and LD50 on the doseresponse curve.
4. Compare answers with other groups to
determine which cleaner is the most
effective.
Mold Control: Toxicology Experiment
Calculations
C= Number of colonies in
control
T= Number of colonies
in treatment
% killed =
C-T x 100
C
Mold Control: Toxicology Experiment
Results
Percent Killed
Dilution
Lysol
Tilex
Borax
0.0001%
2
0
0
0.001%
2
2
0
0.01%
0
56
0
0.10%
34
91
0
1%
61
100
0
10%
91
100
19
100%
100
100
50
Mold Control: Toxicology Experiment
Dose-Response Curve
Percent killed
Mold Control Results
100.0
75.0
Lysol
Tilex
Borax
50.0
25.0
0.0
0.0001
0.01
1
100
Percent Concentration (log scale)
Mold Control: Toxicology Experiment
Student Outcomes
After doing this activity, students will
be able to:
•Develop a dose-response curve.
•Identify the Lowest Observable
Effect Level (LOEL), LD100 and LD50
on the dose-response curve.
•Understand the effect disinfectants
have on yeast growth.
•Understand the purpose of using an
experimental control.