Does Mouthwash Kill Bacteria? Data Collection
Classroom Activity to Examine Effect of Mouthwash on Oral Microbes
Nov 4, 2008
The human mouth contains millions of bacteria, most of which do not cause a person any great harm. But the bacteria that naturally exist in the mouth can build up into a biofilm (living layer) called plaque if a person does not practice good oral hygiene.
This experiment is designed to compare the effectiveness of mouthwashes containing alcohol (Listerine) to those that do not contain alcohol (Crest) in reducing the populations of oral bacteria. The experiment also looks at the amount of time that the mouth is rinsed as an experimental variable.
Materials Needed for Each Student:
- 1 Petri dish containing TSY agar
- 3 sterile cotton swabs
- 2 small paper cups
Additional Materials Needed:
- permanent marker for labeling
- mouth wash that does NOT contain alcohol
- mouth wash that does contain alcohol
- biohazard bags to dispose of the swabs
Students with last names starting with A - M, are to use non-alcohol mouthwash (Crest).
Students with last names starting with N - Z, are to use mouthwash containing alcohol (Listerine).
Mouthwash Experiment Procedure
TSY Agar
Each student uses one Petri dish containing Tryptic Soy (TSY) agar. The TSY agar is derived from seaweed and is much like Jell-o in consistency. This is a great material on which to grow bacteria, as it contains moisture, nutrients and buffers that crate an ideal environment for the growth of many types of bacteria.
Labeling Petri Dish
Always label the bottom of a Petri dish (the half containing the agar), because, unlike the top, the bottom half will never become separated from the sample. Students should use a permanent marker, and divide the bottom half of the Petri dish into three sections, like three pieces of pie.
Label the plate with initials and the type of mouthwash assigned; either containing alcohol (Listerine) or no alcohol (Crest).
Taking Bacterial Samples
Petri Dish Section #1
- Label one section of the Petri dish ‘baseline.’
- Take one of the sterile swabs and wipe it the teeth and around within the mouth.
- Once the sample is obtained, wipe the swab over the section of agar labeled ‘baseline’ using a zig-zag pattern that cover most of that area of agar.
Petri Dish Section #2
- Label the next section of the Petri dish ’10 seconds’
- Swish the assigned mouthwash around in mouth for 10 seconds.
- Spit the mouthwash into an empty cup.
- Take one of the sterile swabs and wipe it over the teeth and around within the mouth.
- Once the sample is obtained, wipe the swab over the corresponding section of agar, using the same application pattern as for Section 1.
Petri Dish Section #3
- Label the next section of the Petri dish ’30 seconds’
- Swish the assigned mouthwash around in mouth for 20 more seconds.
- Repeat steps 3 – 5 of Petri Dish Section #2
Incubation and Data Collection
After oral samples are obtained and incubated for 24 hours, the class will examine the number of bacterial colonies and the variety of different types of bacterial colonies growing on the surface of the agar. A colony is made up of million of bacteria that are visible as a dot on the surface of agar in a Petri dish. For a complete description of experimental variables, hypotheses and data analysis, see the article Does Mouthwash Kill Bacteria? Data Analysis.
For a word document of this entire experiment, go to the Virtual Cell Biology Classroom, Prokaryotic Cell Lecture. There will be a link titled Mouthwash Experiment on Controlling Bacteria on that page.
**Note: On 1/10/09 a revised and more effective version of this mouthwash experiment was posted on Suite101 in the article "Does Mouthwash Kill Oral Bacteria?"
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