Science Buddies: "Ask an Expert"

Hi i am doing an experiment on mouthwash and i have tested the different concentrations of mouthwash on bacterial growth, and all of the concentrations decreases the bacterial growth by the same amount. I was just wondering can this be justified with any chemistry and is there any chemistry that supports dilutions decrease the effectiveness of the solute.

Hey Meredith,

I believe you can really take this experiment far.

I believe most of the mouthwashes on the market has alcohol as the active ingredient. When you dilute it to a very low level, the alcohol
would not be effective in killing bacteria because the concentration is too low to disrupt the membranes of the bacteria or to
denature its proteins. Gram-negative bacteria are more susceptible to killing due to its thin peptidoglycan layer.

I suggest that you

1) remember to do controls, both positive (full strength mouthwash) and negative (no mouthwash) controls.
2) do background research on the ingredients in the mouthwash you chose and the bacteria you chose. Think about
it on the molecular level too

It looks like you will need to accurately perform serial dilutions of your mouthwash. It all has to be done systematically.

Here are some sites that may be helpful:

http://abacus.bates.edu/~ganderso/biolo ... tions.html

http://www.austincc.edu/mlt/ser/Lab1Ser ... ll2007.pdf

http://www.waksmanfoundation.org/labs/r ... lution.htm

You will then need to count bacteria colonies, here are some more resources:

http://www.disknet.com/indiana_biolab/b038.htm

http://www.synbiosis.com/Putman_2005_JIM.pdf

I hope that helps!

Hi yes that is helpful thank you
the mouthwash i am testing contiains no alcohol and its active ingredient is cetylpyridinium chloride, would this make a difference to the expected results.
Thank you for your help

Hi,

Phonghuynh has given you some really excellent advice and background information for this project.

Cetylpyridium chloride is a quaternary ammonium compound, and like ethanol, disrupts the cell membrane of microorganisms. This site describes the possible mechanism of a quaternary ammonium compound on microorganisms:

http://en.wikipedia.org/wiki/Benzalkonium_chloride

You could include a mouthwash that contains only alcohol to the one that you are interested in. It would be interesting to include mouthwashes that contain different combinations and concentrations of the active ingredients in your project. And since you are working with commercial products, you could try contacting the manufacturers and ask if they would share the data they obtained in evaluating their products.

Donna Hardy

Thank you very much to you both for you excellent help