Expert: John Locke - 3/9/2009

Question
My son would like to grow bacteria in a petri dish and then use a variety of substances that are suppose to destroy bacteria (soap, dog saliva, bleach, & hand sanitizer).  WE did buy a kit to grow bacteria.  What is the safest approach in trying to destroy the bacteria because I read that it is not good to open the petri dish after growing the bacteria.  So how is one suppose to do this experiment?
If you are unable to answer this question, please let me know the best way to find out this answer.  Thanks

Answer
Thanks for using AllExperts. The general reason that Petri dishes are not supposed to be opened right after swabbing is to avoid contamination; it is not really dangerous to open the lid right after the plate has been swabbed. However, it is a good idea not to open the plate after heavy bacterial growth has taken place--that's when you could potentially be exposed. Keep in mind that it takes 24-72 hours for that kind of heavy growth to occur.

This kind of experiment can be conducted safely without exposing yourself to bacteria that have grown on the plate; you only open the plate at the beginning of the experiment, then seal it permanently. Usually, the protocol is something like this:

1. Use a Petri dish with sterile agar. Keep the lid on until step 4.
2. Use a marker to divide up the bottom of the dish into sections, each of which will be tested with a different substance.
3. Collect the source of bacteria that you'll be using with a sterile swab.
4. Open the lid slightly and swab the entire surface evenly. Remove the swab and close the lid.
5. Use small, clean paper discs for this step. Saturate each disc with a separate test agent, and keep them apart from each other. Use one disc with nothing on it as the control.
6. Open the lid slightly. Use sterilized forceps to place the discs in the separate areas of the dish (note what you're putting in each area), then close the lid. Seal the dish with tape--you do not want to open it from this point on.
7. Incubate the dish in whatever manner you are able, with the lid on the bottom and agar uppermost. Inspect the results.
8. Dispose of the dish in a safe manner.

This method avoids exposing yourself to significant bacterial growth. Keep in mind that right after the plate has been swabbed, the bacteria are firmly attached to the surface and are not going to come off into the air. Thus, it is not really dangerous to open the plate briefly a second time to add the test discs.

Here's the basic protocol:
http://www.stevespanglerscience.com/experiment/00000165

And here's another one:
http://www.sciencecompany.com/sci-exper/petridishes.htm
Experiment 3 is closest to what you're trying to do.

Good luck!