CAN YOU HELP ME, PLEASE !

[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!

[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!