All Science Fair Projects

TITLE: Effectiveness of Sunscreens Against Ultra Violet Light Experiment

INTRODUCTION: 

Because it is unethical to use humans to conduct scientific experiments, we will
use Escherichia coli, a type of bacteria, as a substitute for human skin in
today’s experiment. This is called a model. We will test the efficacy of
sunscreen to block UV light on bacteria, and then we may use that information to
consider the efficacy of the same sunscreen on human skin cells. 

HYPOTHESIS: 

A hypothesis can be one of:
- do sunscreens with higher SPF values provide greater protection
- do different brands of sunscreen provide greater protection
- is duration of exposure for different brands a factor, etc). 

EXPERIMENT DESIGN:

This is a two- week experiment. During the first week you will formulate your
question and hypothesis, inoculate Petri dishes with bacteria, and then cover
the dishes with Saran wrap that has a covering of sunscreen.

You will then expose your samples to UV light. During the second week you will
perform a colony count of those E. coli that survived exposure to the
UV light. This is necessary to calculate the percent survival of the bacteria.

Escherichia coli is not a harmful bacterium, however, you should wash your hands
before and after handling E. coli, and you should use care not to spill any of
the sample while conducting your experiments. Contact your instructor should you
spill anything.

Three important concepts that you must understand for this experiment:

Control:
A scientific control is that component of the experiment that permits comparison
of results. Covering half of the Petri dish with aluminum foil will block the UV
light and not allow it to reach the growing cells. This will permit maximum cell
growth and represents your control.

Experimental: Covering the other half of the Petri dish with sunscreen may only
reduce the passage of UV light reaching the cells causing some cell death and
reduced growth. This is your experimental group. In this experiment you will
have two experimental groups where you compare one typeof sunscreen with
another. Because in all cases half of each dish will be covered with foil, we
are allowing each Petri dish to serve as its own control.

Sample Size: One Petri dish represents a sample size of only one (N=1). This
gives you no reliable basis to judge your experiment. You need to use at least
three Petri dishes (N=3) for each test group to obtain reliable results. Your
experiment will require a total of six Petri dishes: three for one sunscreen and
three for the other. In each case each dish will serve as its own control.

PROCEDURE

-Each group will have six fresh Petri dishes at their bench. Three will be used
to evaluate the growth effects of one sunscreen and three for the other
sunscreen. These plates contain the growth media tryptic soy agar (TSA). This
agar contains all the nutrients necessary for bacterial growth.

- Do not remove the lid of the Petri dishes. Label the bottom of your six Petri
dishes with your group name and the date, and then divide the plate in half with
a line. Label one side of the line ‘C’ for control, the other side with ‘E’ for
experiment. Label three of the dishes with the first type of sunscreen you are
testing and then the remaining three dishes with the other sunscreen you are
testing. These will represent your two experimental groups. Remember, half of
each plate will be covered with foil and serve as its own control.

- Glove up. While holding the lid to avoid contamination, pipette 0.1ml of
bacteria from the sample into each Petri dish. Spread the sample evenly with a
hockey stick throughout the entire dish. This is called a bacterial lawn.
Quickly put the lid back in place.

- Cut pieces of Saran wrap large enough to cover the top of each Petri dish.
Remove the lid and cover the top of the Petri dish with the Saran wrap. Snuggly
fold the edges beneath the bottom of the petri dish. Secure the edges of the
Saran wrap with masking tape. Keep the Petri dish lids for later use.

- Weigh out (unless of course you are testing the efficacy of different amounts
of the same unscreen) 0.5 grams of sunscreen for each petri dish. Apply the
sunscreen evenly to the Saran wrap using the same technique for each.

- Cut and fold in place pieces of aluminum foil large enough to cover the
control half of each Petri dish.

- Locate the UV lamp you will be using for the experiment. Note the
wavelength in nanometers, and note the time of exposure necessary to kill
the bacteria.

- Expose your Petri dishes to the UV light using the same technique (duration,
position, and distance from light source) for each.

- Once your samples have been exposed to the UV light, remove the aluminum foil
and the Saran wrap. Put the Petri dish lids back on.

-Stack your plates into groups of three and tape them together with the masking
tape. Make sure your group name is clearly written on the masking tape.
Place them, inverted(explain the problem with condensation), into the carriers
provided.
 
RESULTS:

After 24 hours of bacterial growth at (37º C/ 98.6º F), the Petri
dishes will be removed from the incubator by the TMCC staff and stored in
the refrigerator at 4ºC. This will arrest the growth until you can complete your
experiment. You will be able to answer your experimental question as to which
sunscreen offers better protection by counting the number of bacterial colonies
(Colony Count) present on the control and experimental sides of each Petri dish.
The number of colonies represents the bacteria that survived the UV light and
reflects the amount of protection the sunscreen offered.

Accurately do a colony count on the control sides of the three Petri dishes for
your first sunscreen. These numbers represent your raw data. Add these numbers
together and divide by three. This number represents your averaged data for that
control. Repeat the control colony counts on the three plates containing the
second sunscreen tested. Add these numbers together and divide by three to
obtain an average. Now perform colony counts on the experimental sides of the
three plates from each of your two experimental groups. Record the raw data and
average for each group. 

Calculate the % survival for each experimental group:
- Average colony count with sunscreen (experimental)
- Average colony count with aluminum foil (control)

1) Prepare a table showing your raw data and averaged data for both the
experimental groups and their respective controls.

2) Make one bar graph comparing the percent survival of your two experimental
groups.