Science Buddies: "Ask an Expert"

I am comparing regular hand washing (not antibacteria soap) with 2 forms of hand santizer (ROH and -ROH) to try to prove which is most effective at killing bacteria. This is my first time doing cell culture. I would like to test my hands after touching faucets and door handles at school but since it was closed when I started I thought I would try with something at home. First I washed my hands with the soap and used both hand santitizers to try to have my hands start at "zero". Then I squeezed our kitchen sponge onto my hands and took a culture of my hands with a sterile q-tip. (I think this is a control). Then I squeezed the sponge again and washed my hands with the soap for a set time and took another culture into another petri dish. For dish 3 I squeezed again and used HS#1 and took a culture. Then I squeezed the sponge again and used HS#2 followed by a culture. So for this first test I have 5 petri dishes that I am keeping in a closed box in my house. I look at them every day. The 5th dish just has the agar I made, nothing else so I could make sure it didn't grow stuff on its own. So far it hasn't but I think it may also be a control. After 5 days my results were suprising and I am not sure what to consider when I start to write up the discussion and I am also not sure what I should think about or change before I try this again. I was thinking of repeating the same steps above but instead of squeezing the sponge touching the door handles and faucets at school but I am wondering if there is some flaw or place I am getting contamination that I haven't thought about yet. Also I have looked at a lot of bacteria and mold pictures online but I am not even completely sure if what I am looking at is really bacteria (and I think 1 fuzzy mold bit in the side of dish #1) I will attach a picture so you can see. In the picture 1A is the top left (sponge squeeze culture) and I think it has small bacteria colonies & 1 mold on the right edge. The other 3 all have lots of bactera (and I don't think mold, the funny spot on #4 is from the flash - IRL it looks like all the other dots in the dish.). In this picture #1 looks like the other dishes looked on like day 2-3 but they were all started at the same time and kept in the same place. So I am wondering what I might think about discussing in my project as to why it appears I have more growth in the 3 different "cleaning" treatments than in the direct sample. I didn't include a picture of the other petri dish that I made and didn't culture anything in - it didn't fit in the same picture but it is clear. Thank you for giving me any ideas to think about. hmmm..can't figure out how to attach the picture so that will have to come later

Hi Lunar,

Welcome to Science Buddies! You are doing a great project and you have designed a very well controlled experiment. Thanks for posting the photograph; it is very helpful to see what happened.

You are correct about your controls. The agar plate with no bacteria added is the negative control that verifies that you did not have any unexpected contamination. The sample prepared with the kitchen sponge and no sanitizer is the positive control that you are using to compare with your samples.

All of the dots are probably bacteria colonies. The fuzzy colony is a mold.

I can understand your question about your results. It looks like you have more bacteria with the hand sanitizers compared to the control plate. You were expected to either see the same number of a fewer bacteria with the sanitizers.

I would say that your results are inconclusive at this point because the sanitizer samples contain more bacteria compared to the positive control. Here are possible explanations of the results.

1. There may have been something wrong with the plate used for the positive control; perhaps it had dried out.
2. There may be a lot of variation in the number of bacteria obtained when using the kitchen sponge as a source of bacteria. Perhaps the normal range is 2 or 3 colonies to several hundred colonies using the technique you described.
3. Perhaps something went wrong with the experiment that you were not aware of; maybe there was something on the sterile cotton used for the positive control that inhibited the bacteria; maybe you accidentally did not spread the bacteria around on the surface of the plate.
4. Maybe the sanitizers added bacteria to the samples. (This is unlikely, but a possibility considering your results).

Can you think of any other possibilities?

The resolution to the problem is to repeat the experiment exactly the same way again. Rather than using bacteria from other sources like the faucets and door knobs at school (which would add another variable), repeat the results with the kitchen sponge again. To ensure that you are starting with the same number of microorganisms for each plate, start with a very wet sponge, and squeeze the water out into a container and use exactly the same sample for each agar plate. If possible, set up two plates for the control and each sample.

This should help resolve the apparent discrepancy in your results. Since this is a science project, repeating the results and explaining what actually happens are important.

While you are waiting for the bacteria to grow again, you can start writing the sections for your display board. Here is the information from the Science Buddies website that includes the various sections. Your conclusion section for your display board is going to be very interesting with this project. You should double check your teacher’s written assignment to make sure you have included all of the required details.

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

Here is a similar project from the Science Buddies website that should give you additional background information for your topic.

https://www.sciencebuddies.org/science- ... #procedure

One more note. How are you planning to discard your plates? Since you are working with unknown bacteria, you should tape the plates and not open then again after they are incubated. And, you should soak the plates in a bucket of diluted bleach for at least an hour before discarding them.

Good luck! Please post again in this topic if you have any other questions.

Donna Hardy