Expert: Florence M Rollwagen - 8/29/2008

Question
Hi, i recently finished an experiment in which i tested the effectiveness of disinfectants. I mixed in the different disinfectants into the agar plates and then smeared them with old smelly chicken meat. I found that Quaternary Ammonium Comounds were the best almost fully stopping the grwht of bacteria while benzalkonim chlorides came a close second and sodium hypochlorite (bleach) came last. I am having difficulty though actually finding reasons to why this happened and justifying it. I can find previous experiments of this on the internet however none that i have found have shown the results. If you could help it would be greatly appreciated.
Kassie :)

Answer
Hi Kassie:  Thanks for your question.

The problem with interpreting your experiment is that you have too many variables.   In science, we test one unknown at a time.  Let me explain:

The chicken meat had many different kinds of bacteria on it.  So you couldn’t really test whether the disinfectants worked.  You need to test one bacteria type at a time.

You mixed the disinfectants with the agar, so you didn’t have any quantitation.  The disinfectants were present all at a single concentration.  Usually when we test disinfectants we test at different concentrations.

The reason that your experiment isn’t giving you as much information as you wished is because each organism in the mixture of bacteria has a different susceptibility to your disinfectants.  So, let’s say you have more of bacteria A, less of bacteria B and still less of bacteria C.  If bacteria A is susceptible to your quats, then bacteria B and C will overgrow the plate, making it look as if the disinfectant didn’t work.

The test you ran is called a Kirby-Bauer test.  Usually we run it on antibiotics, but it can be used for disinfectants as well.  Here are sites:
http://www.life.umd.edu/classroom/bsci424/LabMaterialsMethods/AntibioticDisk.htm

http://www.sciencebuddies.org/science-fair-projects/project_ideas/MicroBio_p013....

So, if you don’t want to run the experiment again, you’ll have to generalize your answers.

First, look up the mechanism of action for each disinfectant that you used.  For example, hypochlorite is a strong oxidizing agent (chloride), which destroys the proteins of a cell.  Quats destroy the outer lipid membrane of bacteria.

Next, look up Gram positive and Gram negative bacteria, since they have different resistances to each disinfectant.  Because you used a mixture of bacteria, you must have had some of each kind in your sample.

Third, the optimal concentration for each disinfectant will be different with each species in your sample.  Since you used a mixture of bacteria and a single concentration of disinfectant, you can only generalize about the effects on specific bacteria.

If you ran this experiment for a science fair project, you will need a section on “future work”  Be sure to include all the above for more experiments.

I hope this answer has helped you.  Please write back if you have more questions.

FM Rollwagen, PhD