The Effects of the Amount of Sunscreen on Yeast Growth
L. Joch, V. Scott, A. Harkins
July 30, 2013
Abstract
Studies have been done to determine how much sunscreen is necessary to give a
person the proper protection. The purpose of this experiment was to determine the
effects of different amounts of sunscreen on a mutant strain of yeast and a wild strain of
yeast. Three different amounts of SPF 30 sunscreen were applied to saran wrap that
covered YED plates which were smeared with mutant yeast and another set was done
with YED smeared with wild strain yeast. The plates were exposed to the sun for 1 hour
and then incubated at room temperature for 48 hours. The data showed that 0.3 grams
of sunscreen decreased the amount of yeast as compared to the 0.1 gram and
0.01 gram yeast plates. Therefore the data shows that an increased amount of
sunscreen has a greater effect for protection.
Background
If an individual has too much sun exposure there can be negative effects such as skin
cancer. According to the American Cancer Society, it is estimated that there are more
than one million new cases of skin cancer in the United States of America each year.
There are several types of skin cancer: basal and squamosal cell, melanoma and
lymphoma. Basal and squamous cell cancers are found in areas that are typically
exposed to sun such as the forearms, head, neck, etc. and are very treatable.
Melanoma is not as common but is much more severe than squamous cancer.
Lymphoma, another form, is cancer that starts in the cells of the skin but transfers to the
immune system. Lymphoma is considered the deadliest form and more common in
young adults who frequent tanning salons.
However, skin cancer is largely preventable through a broad sun protection program
(EPA, 2013). According to the EPA, it is estimated that 90 percent of non-melanoma
skin cancers and 65 percent of melanoma skin cancers are associated with exposure to
ultraviolet (UV) radiation from the sun. Sunscreen is one of many methods to protect an
individual from UVA and UVB rays. UVA rays penetrate deep into the skin and UVB
rays primarily affect the surface of the skin. Therefore it is important that an individual
uses a broad spectrum sunscreen. The broad spectrum sunscreen will either reflect
and/or absorb UVA and UVB rays. The SPF type and amount of the broad spectrum
sunscreen also has an effect. The EPA recommends that a broad spectrum of SPF 15
(Sun Protection Factor) or higher should be applied 20 minutes prior to going into the
sun so that it absorbs into the skin. Also, the recommended amount to be applied is 1
ounce (28.3 grams) every two hours.
Based on the research and recommendations, an experiment was designed to compare
the effects of different amounts of SPF 30 Broad spectrum sunscreen. The EPA
recommended amount was scaled down so that it would fit to the size of a petri dish.
Another consideration was how much sunscreen an individual would typically use
during a given time period. This amount was measured and then scaled down to fit to
the size of a petri dish. Then a middle range amount was measured so that there were
at least three data points. The goal of the experiment was to determine which amount
of sunscreen had a greater effect on reducing yeast growth which then could show the
amount needed for better protection.
Hypothesis
If the amount of sunscreen increases, the yeast growth decreases.
Materials and Methods
Equipment
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Computer with Internet access
Laboratory journal
Biomedical Sciences Experimental Design handout
How to Write a Scientific Laboratory Research Report handout
Science Laboratory Report rubric
Safety goggles
Latex or nitrile exam gloves
Carolina’s Sunscreen for Yeast and People, Too lab kit:
YED plates
Bottle of sterile water
Sterile, capped, plastic test tubes
1-mL individually-wrapped sterile bulb pipets
5-mL individually-wrapped sterile bulb pipets
Box of sterile toothpicks
Starter plate with UV light-sensitive yeast strain (mutant yeast strain)
Starter plate with wild-type yeast strain
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Aluminum foil squares (approximately 12 cm x 12 cm)
Stop watch
30 ˚C Incubator
Coppertone SPORT High Performance Broad Spectrum SPF 30
Procedure
1. Put on safety gloves and goggles.
2. Working with your partner(s), obtain a WT Yeast Strain starter plate and a Mutant
Yeast Strain starter plate from your teacher. You will have to share these starter
plates with another group.
3. Obtain two sterile, capped, plastic test tubes from your teacher. Label one of the
tubes WT Yeast Strain (wild type yeast). Label the other tube Mutant Yeast
Strain.
4. Pick up a toothpick from the box. Be careful not to touch one end of the
toothpick.
5. Using the clean end of the toothpick, collect a mass of yeast from the starter
plate labeled WT yeast strain. The mass of yeast on your toothpick should be
about 1 mm in diameter.
6. Smear the mass of yeast as far down as you can possibly reach in the test tube
labeled WT Yeast Strain.
7. Obtain a clean toothpick and repeat steps 14-16 using the Mutant Yeast Strain.
8. Obtain a sterile 5 mL pipet from your teacher. Pull out the pipet from the packet
from the bulb end (the side away from the end used to take up liquid). This will
ensure that the pipet remains sterile.
9. Pipet 5 mL of sterile water into each test tube. Make sure to not touch the walls
of the tube with the pipet. You do not want to cross-contaminate your tubes. If
you do accidentally touch the wall of the tube with the pipet, obtain a new pipet
from your teacher before adding water to the second test tube.
10. Shake or vortex the tubes until the yeast cells are completely resuspended in the
water (the water should look slightly cloudy).
11. Obtain eight YED plates.
12. Label 4 plates WT Yeast Strain and 4 plates plate Mutant Yeast Strain. Also label
both plates with your and your partner’s names. Label each set of plates with the
following: control, 0.01g, 0.1g, and 0.3g.
13. Use the 1 mL pipet and pipet 250 μL of each yeast suspension onto the
appropriately labeled plate. Use a different pipet for each yeast strain. Close the
plates as soon as the yeast suspension has been added.
14. Spread the yeast cells onto the plates. Do not use a swirling motion, as this will
only run the beads along the edge of the plate.
15. Let the plates sit until the excess liquid has soaked into the agar.
16. Remove the lid from one of the plates and quickly cover the plate with plastic
wrap. Make sure the plastic wrap is pulled taut and does not touch the surface of
the plate.
o Note: The plastic wrap is used instead of the petri dish cover
because the petri dish cover might contain a pigment that absorbs
some components of sunlight and will interfere with this experiment.
17. Mass two sets of the following: 0.01 g SPF30 sunscreen, 0.1g SPF 30
sunscreen, and 0.3 g SPF 30 sunscreen.
18. Wrap one wild strain plate and one mutated strain with saran wrap and set off to
the side, this will serve as the control.
19. Get clean gloves before each of the following steps.
20. Apply 0.01 grams of Coppertone SPF30 to the saran wrap of one wild and one
mutated agar plate.
21. Apply .1 grams of Coppertone SPF30 to the saran wrap of one wild and one
mutated agar plate.
22. Apply .3 grams of Coppertone SPF 30 to the saran wrap of one wild and one
mutated agar plate.
23. Place all the plates in a sunny place for 1 hour.
24. After 1 hour collect the plates, wrap the plates in aluminum foil, and incubate for
48 hours at room temperature.
25. Observe and write down results using percent growth covering the plate.
Results
The following are the results for the mutated yeast strain plates: Control plate had 5%,
the 0.3 g sunscreen plate had 60% growth, the 0.1 g sunscreen plate had 80% growth,
and the 0.01 g sunscreen plate had 95% growth. The following results were observed
for the wild type strain plates: Control plate had 20% growth, the 0.3 g plate had 50%
growth, 0.1 g sunscreen plate had 80% growth, and 0.01 g sunscreen plate had 95%
growth. Figure 1 shows the growth for each of the plates.
Figure 1 – Sunscreen Experimental Plates
Discussion
One immediate observation was that the wild strain had 20% growth Also, there was a
30% difference between the 0.3 gram and the 0.1 gram plates and a 15% growth
difference between. The wild strain plates should have more than 20% growth since it
was the wild type strain which is able to repair itself. There was evidence of different
amounts of yeast growth on the different plates with the 0.3 gram plate having a lower
percentage of yeast growth. However, there was not a substantial growth difference
between the mutant strain plates as expected which is shown by the 20% difference
between the 0.3 gram and 0.1 gram and the 35% difference between the 0.3 gram and
0.01 gram. When comparing the wild strain and the mutant strain there was not a
significant difference with the effects of the sunscreen. Yet, the results do show that a
larger amount of sunscreen does have an effect like the recommendations by the EPA.
There are some possible sources of error that could have affected the results of the
experiment. There could have been an inconsistent quantity of sunscreen used on each
dish which would have affect where the yeast would grow. The amount of condensation
accumulated on the saran wrap could have combined with the sunscreen and
potentially evaporated, taking the sunscreen with it, so the sunscreen would not have
been as effective. It also rained for some period of time before the dishes were
retrieved. Some of the growth could have been bacteria and not yeast thus increasing
the amount of growth on the plates. The varying amounts of sunscreen that was put on
each dish could have been different depending on if the rain washed it away or
not. Some researcher error in the spreading of the sunscreen could have made a
difference in how the yeast responded. Inadequate sunlight could have contributed to a
lack of growth since the yeast like warmth to grow and flourish.
Some suggested improvements to eliminate errors would be to do the lab on a sunny
day so there are more UVA and UVB rays. A sunny day would also reduce the chance
of contamination of the plates and sunscreen being washed away due to the rain. Also,
the time could be increased to greater than one hour so the yeast has a longer
exposure time. Finally, there was one set of plates for each strain that was used. In a
good experimental design multiple trials would be done so that any outliers can be
eliminated.
Conclusion
The plates treated with a greater amount of sunscreen reduced the amount of growth of
the yeast.
Citations
Skin Cancer. (2013). In American Cancer Society. Retrieved July 30, 2013, from
http://www.cancer.org/cancer/skincancer/index
Sunscreen: The Burning Facts. (2006, September). In United States Environmental
Protection Agency. Retrieved July 29, 2013, from
http://www.epa.gov/sunwise/doc/sunscreen.pdf