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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 Prep*

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

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