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Posted: Wed Jan 23, 2019 8:14 pm
by nicholasnee
Hi, I'm in the 7th grade and doing a science project on the effects of different dilutions of disinfectants on bacterial resistance. I know exactly what I'm doing and the procedure of the experiment, but all my problems are occurring during the experiment. It seems that when after I incubate by plates, I see no zones of inhibition around my disinfectant-impregnated filter disks though I see my bacterial colonies, M. luteus, growing close enough to the disks that it seems there's no disinfectants on the disks (I'm using the Kirby-Bauer disk diffusiong method). Help!


Posted: Wed Jan 23, 2019 9:56 pm
by 17eugenekim
Hello and welcome to Science Buddies!

That is an interesting problem. My first suspicion would be whether your dilutions are too weak. Do you have any controls in your experiment? I performed a similar disk diffusion method on antifungals several years ago, and I had a negative control with just water or saline solution; if you have one (which I recommend), are you seeing anything strange with that? Additionally, a good "positive control" might be an undiluted disinfectant that you know works. Also make sure you have a good "lawn" of bacteria - that's just because irregular colony density can be harder to analyze later.

I hope some of that helps. Please come back with more questions as you proceed!