|MadSci Network: Microbiology|
A couple thoughts on your experiments and then some suggestions for your experiments. Household products such as 409 and Lysol are classified as disinfectants. Disinfectants are chemicals used on non-living surfaces and objects to decrease the number of microbes on that surface. The effectiveness of a disinfectant depends on a couple of things: the length of exposure of the surface to the disinfectant, and the concentration of the disinfectant (some household products recommend that you actually dilute the product before using it). Another important thing to realize is that some disinfectants actually kill microbes. These are called bactericidal (or microbicidal) agents. Other disinfectants inhibit the growth of bacteria. These are called bacteriostatic agents. Now that that has been said, here are a couple suggestions for your experiment: 1. If you really want to test the effectiveness of Lysol, 409, etc,... be sure to read the label to discover how they are supposed to be used. Should you leave the disinfectant on the surface of the object for a short while or for 10 minutes? Should you use the disinfectant at full strength or should you make the disinfectant less strong (by diluting it in something like water)? Any household product should have these directions on it. As for your actual experiment method, in a microbiology lab that I helped instruct we were interested in some of the same things that you were and this is how we did them. 1. For testing the effectiveness of lysol we took a tube full of lysol and combined that with a culture of already growing bacteria. We then compared how the bacteria grew on an agar plate depending on how long the bacteria were exposed to the lysol. We did an agar plate where the bacteria were not exposed to the lysol at all, an agar plate where the bacteria were exposed to the lysol for 5 minutes and one for 10 minute lysol exposure. Unfortunately, this might be difficult for you to do since we used a bacterial culture that was already growing and therefore we used a lot of bacteria in the experiment. This is something you might not have access to. 2. A person can also take a small piece of paper in a circular shape and spray it with the household product of choice. After swabbing the blood agar plate with the swab of bacteria you can then carefully put the piece of paper directly in the center of the blood agar plate. Try not to handle the paper with your hand if possible. You might use a tweezers to pick up the paper and put it on the plate. If the chemical either kills the bacteria or inhibits their growth you should actually see a larger circle surrounding the paper disk where no bacteria grew. The bacteria should have grown on the rest of the plate. IT IS VERY IMPORTANT that you make sure you swab the whole plate with your swab of bacteria! If you do not this technique will not work very well, since this technique depends on the fact that there is bacteria spread across the entire blood agar plate. 3. In terms of your suggested experiment, where you wanted to spray the surface and then swab it again, there might be a couple of problems. If the disinfectant is bacteriostatic (it inhibits but does NOT kill bacteria) then once you swab the "disinfected" surface and put the bacteria on the plate, the bacteria are no longer exposed to the chemical that is inhibiting their growth. Because of this, your technique will only work if the disinfectant actually KILLS the bacteria. Give it a try, but don't be surprised if your disinfectant doesn't actually kill bacteria but just inhibits their growth. As for your second suggestion about how to do your experiment, I think that my suggestion number 2 is a variation of this. I would swab out the bacteria on the plate and then put the piece of paper with the disinfectant on it on the plate. One last thing. Be careful not to touch things like the blood agar plate and the paper disc with your bare hands. There are lots of bacteria on your hands! Good luck and I hoped that helps. Some experiments were taken from Johnson, T.R. & C.L. Case. "Laboratory Experiments in Microbiology". Brief Edition. 3rd Edition. Benjamin/Cummings Publishing Company, Inc. 1992.
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