Science Buddies: "Ask an Expert"

I am testing the effects of UV radiation on Serratia Marcescens. I tried the tests and even after exposure for four hours the bacteria grew a lot. I am using a 254nm light. Is it possible that after I isolated the bacteria from UV that it repaired the damage or that the covers on the petri dishes are blocking out the UV?

Hello there,

There are several possible reasons why you are seeing what you are seeing. First of all, it is possible that four hours of exposure may be enough to induce mutations in the DNA, but not enough to cause death, and that may be why you are still seeing a lot of bugs growing. Have you tried exposing the bugs to UV light for different amounts of time? It may be useful to look up some studies where people have exposed Serratia or other bacterial species with UV and see what experimental procedures they have used and compare them to your own methods.

Also, have you actually quantified the amount of bacteria that would grow without UV exposure and compared that to the amount of bacteria that grew after UV exposure? It is possible that there are less bugs growing after you expose them to UV light and that this difference is difficult to tell just by looking without counting. Also, the UV light may have induced mutations that don't affect the viability of the bugs at all, and that could be why a majority of them are still growing. Just out of curiosity, how did you actually expose the bugs to UV light? Did you grow up a lawn on petri dishes and then shined UV light on them? Or did you grow them up in liquid culture, exposed them to UV light, and then plated them for counting viable colonies? If you let us know a little more about your experimental procedures, we can perhaps identify what steps you can try changing to troubleshoot the problems you are having.

Hope that helped! Please feel free to post back with more questions!

Best,
Connie

I have tried exposing the bacteria to UV from all times from 0 minutes to 4 hours. My procedure was to put 30ul of the bacteria on petri dishes and place them under a UV light for a given amount of time. After this I placed them in a box and checked on them every few days. I found previous records of this experiment online and some of them said that this is an incredibly UV sensitive bacteria that will die after two minutes. This leads me to believe that something is blocking the radiation from the bacteria. To solve this I tried removing the clear screen on the UV light but my results remain the same. It is possible that there is less bacteria in the dishes that were exposed to more radiation, however, the difference should be greater between 0 minutes and 4 hours based on my research. I hope this information can help bring out the error in my procedure.

Hi there,

Are you doing the "Death Rays: What Duration of Ultraviolet Exposure Kills Bacteria?" project on the Science Buddies website with Serratia instead of E. coli? Even if you are not, the procedures described for this project sound very similar to what you are doing, so you can definitely compare the methods you used versus what is described here: https://www.sciencebuddies.org/science- ... ml#summary.

As you had mentioned before, having the lid on could have made a difference, but it is very strange that even after you removed the lid, there still isn't a noticeable difference. One other thing I could think of is there could have been a problem with the equipment you're using. Did you check to make sure that you are exposing the bugs to the correct wavelength? Did you make sure that the equipment you used was actually working, or if you pressed the correct buttons? If you have a teacher or mentor who is helping you with your project, discussing your problems with them could help. Other experts, if you have any other ideas of what could have gone wrong here, please chime in!

Please let us know if you have anymore questions!

Best,
Connie

Hi Phillipio,

You have already gotten some great feedback from Connie. Based on the information you provided, it sounds like you are leaving the covers on your petri dishes. The lids of the petri dishes are very effective at blocking and refracting light. You need to expose the bacteria to the UV light with the lid off the petri dish.

I hope this helps.

Heather