Does Mouthwash Kill Bacteria? Data Analysis

Class Experiment to Examine Effect of Mouth Wash on Oral Microbes

Nov 4, 2008

After completing a classroom experiment in which students take oral samples to see if mouthwash reduces the number and variety of bacteria, this is how data are examined.

This classroom activity is designed to compare effectiveness of different mouthwashes, one containing alcohol (Listerine) and one that does not contain alcohol (Crest), in reducing the populations of oral bacteria. The experimental design also allows for the assessment of rinse time as a factor in mouthwash effectiveness.

After oral samples are obtained and incubated for 24 hours, the class then examines 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 millions of bacteria that are visible as dots on the surface of agar in a Petri dish.

What Are Dependent and Independent Variables?

Most experiments are designed to establish a cause and effect relationship between two variables. The independent variable is the factor whose effect is tested in the experiment. Its magnitude is purposely varied or manipulated by the experimenter. The dependent variable is the factor measured by the investigator. Its magnitude depends on the independent variable. In order to effectively measure the effect of the independent variable on the dependent variable, all other variables must be held constant for the duration of the experiment. These variables are referred to as controlled variables.

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. For a complete description of the experimental setup see the article Does Mouthwash Kill Bacteria? Data Collection.

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.

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.

**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?"

The copyright of the article Does Mouthwash Kill Bacteria? Data Analysis in Microbiology is owned by Tami Port. Permission to republish Does Mouthwash Kill Bacteria? Data Analysis in print or online must be granted by the author in writing.

Jan 10, 2009 10:02 AM

Tami Port :

I have used this experiment in class once. The results weren't all that clear, as there are many variables that are not well controlled (if student rinses entire mouth, where student swabs in mouth for oral sample...to name just a few. I have a revised and improved experimental procedure in a new Suite 101 article at: http://bacteriology.suite101.com/article.cfm/does_mouthwash_kill_oral_bacte ria.