Expert: MGR - 11/6/2009

Question
we are trying to devise a 3rd grade science fair project using petri plates with agar to test activity of had sanitizers against bactera. We are buying plates already made from lab store. We need to know what we should use as source of bacteria. Can we just use swabs from mouth or hand or grow mold on bread. We have less then a month. We need to make sure our source can be measurable so we can get the same amount of bacteria in each petri dish. Then we need advise on how to test the sanitizer. If we put the sanitizer on at the same time as the bacteria, I do not no if sanitizer is strong enough to prevent growth of bacteria. On the other hand if we wait for the bacteria to grow and then add sanitizer observe howmuch bacteria is killed then we have to open containers containing bacteria. May not be too safe for 3rd grader.

Answer
Hi Jawan

The easiest thing to do would be to swab surfaces that you touch a lot - like a door knobs or sink, etc.  If you put the plats in a somewhat warm dark place, the bacteria will grow quickly.  The only problem is, you will likely wind up with different numbers of bacterial colonies per plate.

You can try putting some hand sanitizer on the plates before you put the swab down, and see how many colonies grow, and compare that to how many colonies grow on the unsanitized plates.

You can swab after it grows and it shouldn't be a problem.  However, now these bacteria are in much larger amounts, so the sanitizer would likely have less of an affect.