Death Rays: What Duration of Ultraviolet Exposure Kills Bacteria?

This science fair project involves the use of the bacteria E. coli. While E. coli is not considered a biohazardous or dangerous bacteria, it is important to always properly clean and dispose of bacteria and supplies that come in contact with it. See the Bacterial Safety guidelines for more details on how to handle bacterial cleanup and waste.

1. Follow the directions in the kit to reconstitute the dried E. coli. Let the reconstituted E. coli sit at room temperature for five minutes.
2. Prepare 15 nutrient agar plates with the E. coli bacteria: while wearing gloves, gently shake the reconstituted vial of E. coli, add two drops of the bacterial suspension to a plate, use a sterile cotton swab to spread the bacteria around the entire plate. Cover the plate and wait 5 minutes for it to dry. Repeat for all 15 plates, using a fresh cotton swab every third plate.
3. For the UV light exposure you will use the plates in five groups of three plates each. All plates should be at the same distance from the UV source. The table below shows the suggested UV exposure time for each group of plates. Remember to read and follow the UV light safety precautions (above) while performing this step.
4. Just before exposing the plates to UV light, remove the lid of each of the plates. Protect half of each plate from UV light by using aluminum foil to cover half of the open plate. Make sure that the aluminum foil does not touch the nutrient agar.

4. Immediately after the UV light exposure, put the lids back onto each of the plates. Use a permanent marker to indicate which half of each plate received UV light, and the duration of the exposure.
5. Next, incubate the plates, inverted (lid down and agar-side up), overnight at 37°C (or longer if at lower temperature).
6. Count colonies in both halves of each plate.
7. For each group of plates, calculate the average and standard deviation of the number of colonies in each half of the plate.
8. Make a graph showing the average number of colonies (y-axis) as a function of UV exposure time (x-axis).
9. On the same graph, you can also use a different symbol to plot the average number of colonies on the unexposed (control) side of each plate.
10. Is the average number of control colonies consistent across the five groups of plates? Why or why not?
11. What duration of UV exposure results in 50% bacterial mortality? Are there any plates with 100% bacterial mortality? If so, what duration of UV exposure results in 100% bacterial mortality?

Bacterial Safety

Bacteria are all around us in our daily lives and the vast majority of them are not harmful. However, for maximum safety, all bacterial cultures should always be treated as potential hazards. This means that proper handling, cleanup, and disposal are necessary. Below are a few important safety reminders.

• Keep your nose and mouth away from tubes, pipettes, or other tools that come in contact with bacterial cultures, in order to avoid ingesting or inhaling any bacteria.
• Make sure to wash your hands thoroughly after handling bacteria.
• Proper Disposal of Bacterial Cultures
  • Bacterial cultures, plates, and disposables that are used to manipulate the bacteria should be soaked in a 10% bleach solution (1 part bleach to 9 parts water) for 1–2 hours.
  • Use caution when handling the bleach, as it can ruin your clothes if spilled, and any disinfectant can be harmful if splashed in your eyes.
  • After bleach treatment is completed, these items can be placed in your normal household garbage.
• Cleaning Your Work Area
  • At the end of your experiment, use a disinfectant, such as 70% ethanol, a 10% bleach solution, or a commercial antibacterial kitchen/bath cleaning solution, to thoroughly clean any surfaces you have used.
  • Be aware of the possible hazards of disinfectants and use them carefully.