Science Buddies: "Ask an Expert"

For my biology experiment:
I grew E.coli in the petri dishes, counted how many colonies there were, and sprayed the petri dishes with different types of toothpaste that was mixed with water. I left the petri dishes to incubate for 24 hours. After 24 hours, in some petri dishes I grew lawns but in some I did not grow anything-- I didn't really grow any more colonies.
My question is: how would I compare the before and after growth through numbers? Because before, I had colonies but after I grew lawns instead. I want to be able to determine the average growth.

Hi,

I was not sure what your hypothesis was. Are you saying that some kinds of toothpaste kill E coli?

The way to test compounds like toothpaste, mouthwash, or antibacterial soap is to spread E coli on a Petri dish first and then apply the test product on a filter paper disk. If the compound inhibits bacterial growth then there will be a clear area around the paper disk, while the bacteria form a lawn on the agar elsewhere on the plate.

This project (https://www.sciencebuddies.org/science- ... inhibition) uses zones of inhibition to test products and explains how to do it. There are also many sources of information about this technique online.

Sybee