Classroom Science Experiment: Does Mouthwash Kill Bacteria?

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
1. Label one section of the Petri dish 'baseline.'
2. Take one of the sterile swabs and wipe it the teeth and around within the mouth.
3. 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
1. Label the next section of the Petri dish '10 seconds.'
2. Swish the assigned mouthwash around in mouth for 10 seconds.
3. Spit the mouthwash into an empty cup.
4. Take one of the sterile swabs and wipe it over the teeth and around within the mouth.
5. 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
1. Label the next section of the Petri dish '30 seconds.'
2. Swish the assigned mouthwash around in mouth for 20 more seconds.
3. Repeat steps 3 - 5 of Petri Dish Section #2.

Mouthwash Experiment Variables, Questions and Hypotheses

After collecting the oral samples, there are several questions that the students should answer to ensure that they understand the scientific method of this experiment.

Experimental Variables

What Is Being Manipulated? There are really two questions being investigated in this experiment; two independent variables that are manipulated. What are they?

What Will Be Measured? When the plates are examined after incubation, the class will be comparing the different sections of the plate as well as different types of mouthwash. What are these are the dependent variables.

Questions and Hypotheses

What Scientific Questions Are Being Asked Though This Experiment? Hint: One question relates to each of the independent variables.

What Are the Hypotheses? The students need to make two hypotheses, each based on one of the scientific questions being asked. A hypothesis is a prediction, an educated guess of what the results will bear. It is not necessary for all students to have same hypothesis.

Recording Results of Mouthwash Experiment

After incubation of the plates for at least 24 hours, the class will compare the number of bacteria colonies in each section, and the variety (number of different types of colonies that do not look the same). Students will compare the differences between their own 3 sections as well as the differences that may result from using different kinds of mouthwash.