|MadSci Network: Microbiology|
There are a couple of ways to make the assay you were using more accurate. It is important to note that by soaking the paper discs in peppermint extract and then placing them on the plate that you are not making sure that each paper disc has an equivalent amount of peppermint extract. In order to ensure that each paper disc has the same amount you need to place the paper disc on the agar, after you have spread the E.coli, and add a single drop of peppermint extract. If you use a dropper that has a small hole, as opposed to a dropper with a large hole, it will be easier to get consistent sized drops. This will eliminate one variable.
It seems that you are measuring zones of inhibition. If you aren't this is what we do to determine the sensitivity of the bacteria to that compound. We spread the solution of E. coli on the plate, add the paper disc with the drug, and the next day we use a milimeter ruler to measure the distance that the bacteria is from the disc. If the bacteria grows closer to the disc then it is more resistant, if it requires a large distance from the disc then it is more sensitive.
Another way to approach this problem is to determine the percentage of bacteria that are sensitive to a particular concentration of peppermint extract. The way this is done is to add peppermint extract to the agar before pouring the plate. Different plates have different concentrations of peppermint extract. Then you would grow the bacteria in broth, and add exactly the same amount of broth to each plate. We typically add 100 microliters [0.1 milliliters] to a plate and spread the broth evenly across the plate. On plates that do not have peppermint extract you would plate dilutions of the broth so that you could determine how many bacteria were in 100 microliters. The next day you could count how many bacteria were reseistant to the different levels of peppermint extract in the agar. This would give you a different way to look at the effects of peppermint extract.
Try the links in the MadSci Library for more information on Microbiology.