Science Buddies: "Ask an Expert"

My project is asking if uv light kills bacteria and if sunscreen protects it. I made 2 groups of bacteria half with/without sunscreen(SPF60). I put it under UV light(UVA) for 45 minutes everyday for three days. To test if bacteria died, I transfered the bacteria to new Petri dishes and see if it grew. Turns out sunscreen does protect bacteria and UV light kills bacteria. I didnt use human cells becasue that is unethical. I have a few questions though: What is the link between bacteria and skin cells? What should i improve?

Hi,

In your first question you ask what is the 'link' between bacteria and skin cells [with regard to UV irradiation]. To answer that think about what UV radiation does to all living cells--it damages DNA. That is why UV is germicidal. Bacteria and human cells both have DNA and sunscreen absorbs the energy from the UV so that it cannot cause DNA damage and cell death.

Actually, cell death is not the problem with UV. The harm comes from DNA mutations in living cells that can transform them into cancer cells. That is why you should use sunscreen and wear a hat and sunglasses. Using cultured human cells to do medical experiments is a legitimate and necessary scientific method. Without such experiments advances in medical knowledge and development of new treatments and new drugs would be impossibly difficult.

Your second question asks how you could improve your experiments. You did not say what was your source of UV light, what sunscreen you used and how you 'applied' it, which bacteria you used, how you irradiated them, how many samples you tested, whether you did statistical tests, etc. Without knowing the details of how you did the experiments no one can tell you how to improve them. Often experimental data is questioned because of lack of proper controls or insufficient numbers of experimental samples. Those are the two major areas that usually need improvement.

The best thing you can do when planning a science project is to present your hypothesis and experiment plans to Scibuddies and let us help you set it up from the beginning. That way you can avoid problems later that can make your data useless. Also, when you choose a question for a project, try to pick something where the answer is not already known. For example, you could have asked whether there was a difference in three different brands of sunscreen all with the same SPF. Or you could have done a comparison of three sunscreens of the same brand but with increasing SPFs to see which offered the most protection.

If you have any specific questions about your results or how to present them, let us know.

Good luck!

Sybee

Thank you kindly, Sybee
I hypothesized that UV light will kill bacteria that is unprotected because UV light can cause mutations in DNA. I also hypothesized taht sunscreen will protect bacteria because sunscreen is made to absorb and reflect rays.
I used a UVA light bulb from a pet store.

Procedure:
1) make agar by mixing 6grams of nutrient agar powder and 1 cup of water in a pot on medium heat making sure to stir.
2) pour the agar into 14 Petri dishes until halfway. Let it sit to cool and harden into a jelly.
3) take a cotton swab and wipe it on a toilet seat the wipe it on all the agar dishes.
4) Keep the lid on the bacteria plates to let bacteria form. Make sure it is in a warm place
5) Once the bacteria has grown, take a cotton swab with some sunscreen on it( banana boat spf 60) and rub it on half(7) bacteria plates.
6) Take all 14 plates of bacteria and place them under UVA lightbulb for 45 minutes everyday for 3 days. (make sure to use UV light bulb carefully)
7) To see if bacteria has died, repeat steps 1 and 2 to make 3 new plates of agar.
Take a cotton swab and transfer bacteria from the previous plates onto the new ones. ( transfer bacteria from the plates with sunscreen first then do the same for the ones without sunscreen).
9) check everyday to see if the bacteria grew or not.

Results: We have found that when we tested if all the plates of bacteria has died or not, the plates with sunsreen grew and the plates without sunscreen didint grow. this means that sunscreen does protect from UV light and UV light does kill bacteria.


I have a few questions:
1) how should i expand my results
2) I did a graph showing the results of when i was testing the bacteria if it died or not. Should i make a graph of all 14 Petri dishes ( number of colonies)?
3) What should i include in my research? The judges said i should inclusde research about the connection between bacteria and skin (like why I tested bacteria instead of skin cells) I have this so far:
Uv light is natural light coming from the sun. It contains UV photons ( particles that carry energy in the form of light). THese photons fall outside the visile spectrum which means you cant see them. But wen a person is exposed to these rays, photons may hurt you. the short term effects of the photons hitting your skin may just mean that you get a small sunburn. But the long term effects may cause skin cancer. This is why people prefer to wear sunscreen outside. In this case, sunscreeen acts like a solid to protect your skin from harmful ray. Sunscreen contains organic praticles that soak up UV light and inorganic particles that reflect UV light. Usually, sunscreens have a label called SPF. THis stands for sun protection factor. The higher the SPF is viewed on your bottle the more protection you receive.
PS. I did this all at home becuase i dont have access to any lab

Thank you so much Sybee. I hope you know how much I appreiciate your help

Hi again. I’m happy to be able to help you. Thanks for the procedural details. Now I understand how you tested your hypothesis. You did a good job of setting up an experiment that was doable at home but still gave you enough data to answer your question.

As I suggested in my previous email you could have made your project more interesting by testing different brands of sunscreen or different SPFs. Keep that in mind for your next science project.

I hope you took photos of your bacterial colonies because people [including judges] like to look at pictures. You should make a poster display with 2 or 3 photos of the dishes without sunscreen and compare them with photos of the dishes that were treated with sunscreen. You can make a table that shows your results using minuses ( - ) and pluses ( + ) to indicate the degree of growth. Give each plate a score based on how many or how few colonies you saw.

Showing a photo of your UV lamp with the Petrie dishes under it would be a great idea too so people will see instantly how you did the experiment.

Another thing you might do is use Powerpoint to make a slide showing the visible light spectrum with UVA at the far left. Get the peak wavelength numbers from the information on the UV bulb package or go to the company’s website. Then you can mark these on the UV part of your spectrum.

Note that the energy of ultraviolet light is different for different wavelengths (http://www.physlink.com/Education/AskExperts/ae300.cfm). On your poster you need to explain the differences between the three types of UV—A, B and C. UVC is the most powerful and is the light that comes from germicidal lamps, but recent studies have shown that UVA and UVB are also harmful (http://www.skincancer.org/prevention/uv ... va-and-uvb).

In case you haven’t seen it, Scibuddies has a project similar to the one you did except it uses a culture of the bacterium Escherichia (E) coli as the test organism (https://www.sciencebuddies.org/science- ... ml#summary).

Let us know if you have any more questions about preparing your presentation.

Good luck!

Sybee

Hi again sybee. I have one last question I promise! so I know my independent variable which is half of the Petri dishes have sunscreen and the other half don't. My only question is, what would my dependant variable be? Would it be how long the bacteria took to grow??

Thanks again

Hi Dot,

I would say that your dependent variable would be how much of the bacteria grew back, not how long it took. Did you let all of the plates grow for the same length of time? Photos of the bacteria with and without sunscreen would let people see exactly what happened. Then they can make up their own minds as to the effectiveness of sunscreen in blocking UV. Do you understand the difference between UVA, -B and -C? If you have more questions, keep asking. We're your virtual teachers and we want to help you get the ideas of science in your own mind.

Happy weekend!

Sybee

Hello again Sybee,

I am doing my research and I only have one more thing to answer. Its how does UV cause damage to cells. I've been looking for a good website for hours but did'nt find one. Have any recomendations?

Hi,
I'm having a lot of trouble on how to write a conclusion for my lab. Any advice?
?

Hi,

I worked in a radiation biology lab when I was in college and we studied the effects of UV on mutation rate in bacteria.

The main target of UV light in a cell whether bacterial or human is DNA. The photons of UV light have more energy than those of visible light and when they strike a cell they easily pass though the membranes and then may transfer their energy to the bases that make up the DNA chain. The UV causes a chemical change in the DNA that can result in a mutation, an improper reading of the DNA code.

The cell tries to repair the damage but sometimes it cannot and it simply dies. In other cases, the change in the DNA becomes part of the genome and can be passed on when the cell divides. This is how radiation can cause cancer. UV light has lower energy than X-rays, gamma rays or cosmic rays and thus is less able to cause DNA damage, but years of exposure to UV from sunlight can cause an accumulation of DNA damage can that result in skin cancer.

Here are some references that have pretty good information about the effects of UV on cells and DNA:

1. http://en.wikipedia.org/wiki/Direct_DNA_damage
2. http://genetics.thetech.org/ask/ask402
3. http://www.scientificamerican.com/artic ... olet-ligh/

You asked about your conclusions. What did your experiment show? That more bacteria grew on the plates where they had been treated with sunscreen before UV exposure? Your conclusions could be that exposure to too much UV light from sunlight can kill certain kinds of bacteria and that blocking UV with sunscreen can protect the bacteria and other cells such as human skin cells from damage to DNA.

I hope this helps. If you have specific questions just make a list and post it and I will try to help you understand this better.

Sybee

Thank you so much Sybee. With your help we have won second place in both school and the state wide science fair!! I appreciate all your help and one again, thank you.

Excellent! I am happy to help you succeed as a scientist. Let us know when you are planning your next project.

Good luck!

Sybee