Project on Science Buddies

[JJSamsel]

Why does the material's list for the project, "Death Rays: What Duration of Ultraviolet Exposure Kills Bacteria?" list a bunsen burner and ethanol as supplies for this project? There are stages, but an outline/rough draft of the research paper is due next week, and I am supposed to explain the procedure. I cannot figure out from reading this experiment on this Science Buddies site why a bunsen burner is needed unless simply to bend a glass stirrer. Can I do this without a bunsen burner? If I need a bunsen burner where can I buy the right kind of ethanol? Also, does any UV shield that covers the face and neck work? Do they sell ones specifically for UVC rays, since I have not found one that says it is specifically for UVC rays.

[donnahardy2]

Hi,

Welcome to Science Buddies! I think you are doing this very excellent project from this website:

http://www.sciencebuddies.org/science-f ... p017.shtml

The Bunsen burner and ethanol are used to sterilize inoculating loops and glass rods used in the sterile transfer of the bacteria. A Bunsen burner is used to sterilize an inoculating loop directly in the flame until the metal loop glows. For a glass rod, the rod is dipped in ethanol briefly and then exposed to the flame to sterilize it by burning off the ethanol.

In this project you will be transferring the bacterial culture from an agar slant to a Petri dish to streak it out so you can select a single colony to transfer to the broth culture that will be used to grow the broth culture.

The microbiological techniques section of the Science Buddies website is excellent, and I recommend reading through all of the sections so you can become familiar with the basic techniques.

http://www.sciencebuddies.org/science-f ... ques.shtml

You can use sterile cotton swabs instead of an inoculating loop and glass rods, and eliminate the need for the Bunsen burner and ethanol. You can use 70% isopropanol available from the Dollar or drugstore instead of ethanol. Let me know if you need any other suggestions for substitutions.

One important consideration in doing a science project is to keep all parameters controlled, except the one independent variable that you are changing in the experiment, which in this experiment, is the time of UV exposure. So you will want to make sure that the bacteria you use are at the same growth stage at the time of inoculation. Probably the best way to do this is to transfer a single colony to a broth culture and incubate it overnight (or for a certain number of hours) until it is transferred to the Petri dish immediately prior to the UV step.

Here is information of one of many examples of a suitable UV face shield. The specifications state that it filters out the full UV spectrum, 200-405 nm. This is an expensive item, so hopefully you can borrow something suitable. If you do borrow something, check to make sure that the screen filters out all of the harmful UV rays.

http://www.fishersci.com/ecomm/servlet/ ... &langId=-1

I hope this helps. Good luck! Let us know if you have any other questions.


Donna Hardy

[JJSamsel]

Is there a good way to analyze the data if individual colonies did not emerge easy to be counted, but streaks instead? The UV treated sides clearly did not grow bacteria, or barely grew any. Should I count the number of grids intersected by the line of bacteria, and then I could say the percentage of grids that had some bacteria on the non-treated side compared to the percentage of grids that had bacteria on the treated side? It is not a lawn effect, so area won't work, and it is not individual colones, so I cannot count colonies. I used the same streaking pattern and same amount of streaks on each side though. It might not be the most accurate way, but the difference is so great between the sides, and it is the only way I can think to do it. I have heard I could dilute it ten fold, and do a test run then, but I cannot afford more supplies.

[donnahardy2]

Hi,

It’s difficult to tell what happened from your description. However, you have an excellent idea. The science fair judges will be looking for some sort of measurement, so if you could count the number of grids showing bacterial growth, that would provide some semiquantitative results. It would be helpful also, if you could take a photograph or draw a picture showing the results.

Since you used a consistent quantity and technique in streaking the plates, you can compare the results between the UV treated and control sides of the plates. You can explain in your discussion section what you would have done if you could have repeated the results again.

Donna Hardy