Science Buddies: "Ask an Expert"

Hello, we are trying to test diffrenet household disinfectants against E.coli bacteria. We ran into a problem of how to prepare the dilution for testing. Can we just test the effectness of the products by plating the bacteria and placing a filter paper disk saturated with disinfectant and then measure the zone of inhibition. Would this be a right way to go about it? Thank you.

Hi!
There is actually a similar experiment on the ScienceBuddies website that you might want to check out the procedure of:

https://www.sciencebuddies.org/science- ... p013.shtml

Remember to have supervision because E.coli can be very dangerous, and best of luck!

Hello there,
Yes definitely check out that link...it has some good information!
But you are on the right track...just make sure that the filter paper disks are the same size and that you do a good job equally distributing the bacteria across the plate...Also remember to leave enough space around each paper disk to see the zone of inhibition and try to get the same amount of disinfectant on each disk...like dip each disk completely, etc...
Other than that you're on the right track and all the best!
Post if you have any other questions... :]

When I was in middle school I performed a project very similar to the one that you are interested in. My partner and I examined the antimicrobial properties of various natural herb extracts compared to commercial disinfectants against a strain of E. coli. We used the filter paper method that you described above, measuring the zones of inhibition after incubation overnight.
This method is known as the Kirby-Bauer disk diffusion method, and it is generally used to determine the effectiveness of a particular antibiotic against a strain of bacteria. Here is a link describing the process, http://aminj.myweb.uga.edu/KIRBY-BAUER.html. I would recommend consulting a microbiologist to determine the proper dilutions for your project. When we did our experiment, we asked the microbiologists that worked in our local hospital for input and they were very helpful. You want to make sure that the dilutions of the various disinfectants are equal, so you are not introducing other variables into the experiment.

Hope this helps,
Nithin