Science Buddies: "Ask an Expert"

Dear Science Buddies,
I am doing the project Death Rays: What Duration of Ultraviolet Exposure Kills Bacteria?
1. Do I need to keep the petri dish lid on during the UV exposure? In my research I read that the petri dish lid may not let the ultraviolet light get through. My bacteria will be Escherichia coli.
2. What strength of ultraviolet light do I need? The project says to use a short wavelength lamp. My school has a UV light that is shortwave and longwave at the same time.
3. I am serially diluting the bacteria. My research showed me that if I am going to count the bacteria colonies, serially diluting the bacteria will help because the original sample has a very high number of cells. A high number of cells means there will be too many colonies to count (TMTC). I am going to test both 1/100,000 and 1/1,000,000. Do you think that will be diluted enough?
Thank you,
Reagan Jones
6th Grade

Hi Regan,

This is a really great project from the Science Buddies website:

https://www.sciencebuddies.org/science- ... #materials

Here are answers to your questions:

1. The directions are not clear and do not mention removing the cover from the Petri dish. However, since plastics can block or scatter some UV rays, it would be better to remove it for the experiment.

However, then it would not be clear how to cover half of the plate with aluminum foil. It’s probably best to mark the bottom of the Petri dish first and then gently place the aluminum foil over the protected half of the plate. Recover the Petri dish with the lid as soon as possible after UV light exposure to minimize the possibility of contamination.

2. This is a good question. Here is a website that shows the UV emission spectrum of various UV lamps. For this experiment, you need a UV lamp that emits light in the short wavelength range, about 253 nm.
http://www.topbulb.com/find/uv.asp
You should be able to find out the manufacturer and catalog number of the UV lamp that is available at school. Do an internet search for the manufacturer’s website and check the specifications for the lamp you will be using to verify the wavelength.


3. If you are growing a broth culture overnight before starting the experiment, then a 1:100,000 to 1:1,000,000 dilution of the original culture should work very well. It is always a good idea to start with a freshly grown culture as this should be one of the controlled parameters of your experiment.

Please post again in this thread if you have more questions.

Donna Hardy