Gunjan in the mouthwash trial will show resistance

 Gunjan PatelIB Biology HL Yr:2

The Effects of mouthwash on the human mouth and how it affects the production of bacteria

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Research Question:

How does mouthwash affect the growth of bacteria in the human mouth?


This internal assessment aims to determine how variations of mouthwash affect the production of bacteria in the human mouth. This is going to be achieved by using sterile cotton swabs to swab the mouths (teeth and gum lining) of five individuals and then placing it in a zig-zag pattern on a petri dish lined with agar. It will be measured by using different types of mouthwashes, such as organic, different brands and different contents in the mouthwash.  It will attempt to determine if bacteria in the mouthwash trial will show resistance to the mouthwash during the week by recording the growth daily in a qualitative measure.



Cosmetic and therapeutic mouthwashes exist for people around the world. People can buy therapeutic mouthwashes in the store or can be modified via prescription Meanwhile, cosmetic mouthwash may relieve bad breath for a short period of time and in place of it, will be a refreshing taste but it does not have any active ingredients while therapeutic does. Ingredients in a therapeutic mouthwash may include: cetylpyridinium chloride (which controls plaque and gingivitis growth), fluoride (which is proven to prevent tooth decay) and peroxide (which is used in whitening mouthwashes and toothpaste). Mouthwash is also helpful because it can access places in the mouth where a toothbrush cannot. One of the most common uses of therapeutic mouthwash is to treat halitosis, which means bad breath, which is secondary to the bacterial buildup in the mouth that ultimately causes inflammation and gives off noxious odors.  It is also caused from the buildup by food debris within the mouth that was not removed due to lack of dental hygiene.  Many people with halitosis have cavities or gum disease. However, even in the absence of halitosis, essentially every person has buildups of bacteria in the mouth, including Staphylococcus aureus, Streptococcus pyogenes, Helicobacter pylori, Streptococcus mutans (main one that causes disease), and Porphyromonas gingivalis. Studies have been performed regarding the efficacy of different types of mouthwash and have shown to provide effective control of bacterial growth as well as decreased pain following dental procedures due to the inhibition of the body’s natural inflammatory response to bacteria in the mouth.

Another popular type of mouthwash is organic mouthwash, as it only contains natural ingredients and the human mouth contains beneficial bacteria and organic mouthwash only removes the hazardous bacteria because it does not contain alcohol and chlorine dioxide. Xylitol, oils, and plant-based extracts are all ingredients in a typical organic mouthwash. While Listerine is the most popular ‘name brand’ mouthwash, and it contains alcohol which its role is to reduce hydrogen bonds in protein structures, which causes denaturation. 


Personal engagement:

I chose to conduct this experiment because I have always been curious why each mouthwash says that they are number one. I wanted to find a way to prove which mouthwash is the best at reducing the count of colonies of bacteria and to do this I had four members of my family swab their mouths at the beginning of the week and put it in four agar plates to let it grow throughout the week. Each day after that, I assigned each member of my family a specific mouthwash and at the same time making sure each person ate the same type of food to keep my data accurate. They would use the mouthwash once at the end of each day. And at the end of the week, I would swab their mouths and put it on an agar plate to let the bacteria grow.


The mouthwashes that were used and the person who used the mouthwash are listed below:

–       Listerine Antiseptic Mouthwash containing alcohol (Person #1)

–       Crest Scope Mouthwash (Person #2)

–       Crest Pro-health CPC Antiplaque Mouthwash (alcohol-free) (Person #3)

–       Thera Breath Oral Rinse (Person #4)

o   This mouthwash is clinically tested and uses OXYD-8 to attack only harmful bacteria and eliminate the bad taste in your mouth.



This experiment is to determine how variations of mouthwash affect the growth of bacteria, and it is expected that, out of the list of the used mouthwashes in my experiment, Listerine and Scope brands will have the least number of bacteria grown. This is due too that they are the only ones that contain alcohol as an active ingredient, which is a known antiseptic that is used worldwide as a disinfectant in multiple places such as a hospital. Furthermore, the “organic” mouthwashes that are recommended by dentists, contain natural/herbal products. The data on their efficacy are lacking compared to the “name brand” products. Bacteria that are exposed to mouthwash daily, will develop resistance and will determine if the bacteria show any signs of resistance for one week. I hypothesize, that the Listerine antiseptic mouthwashes will be the most effective in inhibiting the growth of bacteria with the organic “herbal” brand being the least effective.



Dependent Variable:

–       The amount of bacteria present
–       Changes due to mouthwash use
–       Number of colonies

Independent Variable:

–       Type of mouthwash
–       Time

Controlled Variable:

–       Temperature where petri dishes are stored 
–       Light conditions where petri dishes are stored
–       Size of Petri Dish
–       Type of Agar
–       Time between both trials
–       Trial without mouthwash



–       (10) Pre-poured sterile LB-agar plates

–       (10) 6-inch sterile cotton swabs 

–       four different brands of mouthwash (different ingredients)

–       permanent markers

–       Rulers

–       Scotch tape

–       Agar plates

–       Incubator

–       Heat lamps


1.     Obtain eight agar plates and sterile swabs from Amazon and get a heat lamp in the ready position.

2.     Using a sterile cotton swab, swab the inside of four individual’s mouths and make sure to swab the front teeth along with the gums while twirling the swab in a circular motion

a.     Ensure that the individuals are eating the same cuisine until the end of the trial

3.     After swabbing for individuals, remove the lid of the agar plates and carefully twirl the cotton swab across the plate in a semi zig-zag pattern, and then label each agar plate with the name of the individual (also mouthwash when repeating mouthwash)

4.     Close the lid of the agar plates and slowly place them upside down and move the plates to under the heat lamp.

5.     Leave the agar plates under the heat lamp for 48 hours and after time has passed, remove the plates from the lamp and take a picture of the plates to document the growth

6.     Put the agar plates on a table and near the lamp and put it back under the heat lamp for 4 days. Continue to take pictures every 24 hours

7.     Obtain four different types of mouthwashes, such as Listerine antiseptic, Crest non-alcoholic, Crest antiseptic, and organic mouthwash, to determine which mouthwash is most effective in killing bacteria.

8.     After the mouths have been swabbed (step 2), assign a mouthwash to each individual (different ingredients, such as organic/non-alcoholic). And after dinner every day, each individual will need to use the mouthwash they have been assigned

9.     After four days have passed, repeat steps 2-6 but with using mouthwash and also using an incubator to let the plates with agar grow on it.

Data and Analysis:

Table 1: The data recorded, describing the growth of bacteria in the trial without mouthwash, in relation to it filling the agar plate


Agar plate, person #1

Agar plate, person #2

Agar plate, person #3

Agar plate, person #4 

Day 1 (Monday)

None (0%)

None (0%)

None (0%)

None (0%)

Day 3 (Wednesday)

Light growth (


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