IB Biology HL Yr:2

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

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Research Question:

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


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.



The two types of mouthwash that people can buy
are, cosmetic and therapeutic. People can buy therapeutic mouthwashes in the
store or also by prescription from a doctor. Meanwhile, cosmetic mouthwash relieves
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,
fluoride and peroxide. 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. It is also caused from the buildup
by food debris within the mouth that was not removed due to lack of dental
hygiene. People with halitosis have cavities or gum disease. However, people
that do not have halitosis, also have buildups of bacteria in the mouth, such
as Staphylococcus aureus, Streptococcus pyogenes, Helicobacter pylori,
Streptococcus mutans, and Porphyromonas gingivalis. People have studied the
efficacy of different types of mouthwash and have shown that it provides an
effective control of bacterial growth as well as decreased pain following
dental procedures.

popular type of mouthwash is organic mouthwash, as it only contains natural
ingredients and the human mouth contains beneficial bacteria and the organic
mouthwash only removes the bad bacteria because it does not contain alcohol and
chlorine dioxide which are ingredients in ‘name brand’ mouthwash. 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)

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



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.




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


–       Type
of mouthwash
–       Time


–       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

different brands of mouthwash (different ingredients)

–       permanent

–       Rulers

–       Scotch

–       Agar

–       Incubator

–       Heat


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

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

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
(<25%) Light growth (<25%) Mild growth (25-50%) None (0%) Day 4 (Thursday) Mild Growth (25-49%) Mild Growth (25-49%) Intermediate growth         (50-75%) Light growth (<25%) Day 5 (Friday) Mild Growth (25-49%) Intermediate growth       (50-75%) Intermediate growth         (50-75%) Mild Growth (25-49%)                  Person #1, Day 3:                                                Person #1, Day 5: The Pictures above demonstrate the growth of bacteria at the end of the first trial, which was day #5. With person #1 being on the left and person #4 being on the right. On days 1,3,4,5 I recorded the growth of bacteria on the agar plate and took pictures to show the growth of it on the agar plate. Calculating how much of the colonies of bacteria, filled up on the plate in table #1. We can see that the number of colonies differs in each person due to the meal they ate and the bacteria present in the mouth.   Table 2: The data recorded with the use of mouthwash in the trial, describing the growth of bacteria in the trial without mouthwash, in relation to it filling the agar plate Days 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) Discreet growth (<5%) Light Growth (<20) Medium growth (25-40%) Light Growth (<20) Day 4 (Thursday) Light growth (<20%) Medium growth (25-40%) Medium growth (25-40%) Light Growth (<20) Day 5 (Friday) Medium growth (25-40%) Medium growth (25-40%) Intermediate growth (50-75%) Medium growth     (25-40%)          These pictures were taken on day #3, and starts from person #1 on the left and ends on person #4 on the right. We can see that the three mouthwashes that contain alcohol (1,2,4) reduced the growth of bacteria the most, with person #3 who used nonalcoholic mouthwash, bacteria growth was noticeable and covered at least 50% of the agar plate. These pictures were taken on day #5, and starts from person #1 on the left and ends on person #4 on the right. We can see that at the end of the trial, bacteria growth slowed down even more after being left in an incubator to grow for four days. Except person #3, who used Crest nonalcoholic mouthwash. We can determine from this test that Listerine was the best at reducing the growth of bacteria, with Crest nonalcoholic mouthwash being the worst.           Conclusion: The hypothesis stated in the beginning does support the conclusions that were proposed from my data, The Listerine mouthwash was the most effective in killing the bacteria due to its key ingredient alcohol, while the Crest nonalcoholic mouthwash was the least effective due it not having alcohol. The graphs showed the number of colonies of bacteria present in the agar plate, and it is evident that after day 3, the colonies stopped growing or reduced after being left in the incubator during the trial. The graph also shows that person #3 who did not use alcohol, the colonies of bacteria on that agar plate had a high number of colonies of bacteria growing, and that the mouthwash offered minimal resistance.   One of the limitations of this experiment was that there were not multiple trials and because of that, I was unable in calculating the standard deviation of the colonies of bacteria. Another limitation, is that I did not have access to an incubator or more people to make my trial more accurate. I would have to check on the agar plates at an appropriate time and sometimes, the room would be locked for testing. My experiment was conducted at two different places, one was at home, so I could swab my family members mouth and another was checking the incubator at school, which was only at certain times. Also, some of my family members ate different foods which could've potentially affected my data, such as during work or at the gym.   The agar plates that were used during the trial were purchased from Amazon and were set up to my specifications and included sterile swabs. My first order, the agar plates were damaged and had to request another order be shipped. This delayed my experiment and reduced the number of trials that could be done to one week.   In order to combat these errors if i were to redo the trial again, the way the trials and experiments were conducted would need to be standardized to make the results more reliable. Such as have all the materials and equipment set up before starting the experiment. Overall, the experiment was organized but the delays and low number of trials that occurred could've been eliminated if all material was present before starting and had access to an incubator or a heat lamp in my house.          

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