Science Buddies: "Ask an Expert"

Hi this is my first time posting on this forum. My 7th grade daughter decided to conduct her science experiment to find out the effectiveness of antibacterial soap. The original experiment says to use blood agar, and chopsticks dipped in saliva to swab in each petri dish, after using antibacterial soap, regular soap, no soap and washing for specified lengths of time i.e., 20 seconds, 2 minutes etc.

However according to the rules, she cannot use blood or saliva. The teacher modified it to use agar and yeast. However, we're trying to figure out how to carry that out. We have already prepared the petri dishes with the agar. On the package directions it says to add hot boiling water (3tbls) and stir until fully dissolved. We did that, and poured the mixture into the petri dishes, however, you can still see some brown gelatin like pieces, after we put the petri dishes into the fridge overnight.

Is the agar supposed to be completely dissolved until it is totally clear?
Also we have small petri dishes.
How much of the dissolved agar should be placed in each petri dish?
She poured a tsp in each dish, which filled the petri dish up about halfway.

Finally, are we supposed to prepare the yeast according to pckg directions (adding water and sugar until it foams)? Next dip the chopsticks into the yeast, wash off using soap types (and length of time). It doesn't seem like it would work like the saliva. Seems like the yeast will just be washed off. One of the chopsticks will not be washed off at all with any soap, as per the experiment.

Or is there any other type of bacteria we can use? Rubbing the chopsticks on something else?
Below is what is NOT allowed:

4. No projects that deal with PHBA e.g. bacteria, virus, fungi, recombinant DNA, human or animal fresh/frozen tissues, fluids, or blood. (Would require a SRC approval and need to be carried out in a research facility/lab)

***Project is due 1/23/14***

Thanks in advance for your help.

Hi,

I'll try to answer your questions, but I have to preface this with the fact that I am not an expert on the science fair rules, so maybe another expert can chime in and help there.

First, your petri dishes:
It's a little unclear to me what kind of agar you are using. We use agar as a material for the bacteria (or yeast) to grow on, but agar alone doesn't actually have the right nutrient mix for most microbes to grow, so generally we make or buy agar with specific nutrients added. One common one is LB agar, so if you're using just straight up agar (from a specialty foods store, for example), I think you'll have some problems growing your microbes.
As for the pouring of the agar, I'm not sure why you are getting brown "gelatin like pieces." The agar should form a uniform, firm, jelly-like substance. It won't necessarily be 100% clear (especially if it is a nutrient agar designed for growing bacteria, in which case they are usually a light yellowish, brown color), but it should be uniform, without bits of agar strewn throughout. It's possible that you didn't get everything dissolved, or that putting it in the fridge cooled things down before it was evenly distributed. Agar should solidify at room temperature, so you could potentially make it by pouring in the hot agar liquid and just letting it set overnight on the counter.
Filling the petri dishes halfway up should be sufficient for your purposes. It's not an exact amount, so there's some leeway there.
P.S. It is possible to buy plates with pre-poured nutrient agar for you to use as well. You can find some at Carolina Biologicals.

I am actually very confused about why the teacher would have you use yeast for this experiment. Yeast is NOT bacteria, so using it to see how well antibacterial soap works versus regular soap versus no soap is very strange, because there is no reason to believe the antibacterial soap should be more effective than the regular soap when it is not being used against bacteria. If you're not allowed to use any bacteria, then I'm not sure I have a good answer for you, though. There are some very non-pathogenic bacteria that are commonly used in the lab (E. coli K12 for example), but it sounds like all bacteria are forbidden, so that won't work. Another thought would be to actually use the soap on human skin and then swab and plate that area, but I suppose that might violate the "no human tissues" rule. No other ideas come immediately to mind, but maybe another expert can chime in with something.

I will say that wood (i.e. chopsticks) are somewhat porous, so you may be able to soak up enough of the yeast mixture that it won't automatically slide off of the chopstick. If that doesn't work, you could try something similar with a cotton swab where the tip soaps up some of the yeast solution and then you wash (swish around?) the tip in the different soaps.

I hope this helps somewhat, and I'm sorry I wasn't able to give you any brilliant new ideas.

Please post again if you need more help!
JMP

In my high school biology classes, I ran this experiment quite successfully each year as a review of scientific method and controlling for all variables (factors) except for the one you are testing.
I found the easiest method was to do what the previous post suggested - use the pre-made LB agar plates that you can buy from the scientific company (check with the science teacher for the best one). This way you will know that the plates are coming to you sterile. It is sometimes a challenge for inexperienced students and even experienced lab scientists - to make sterile plates - one thing you want to do is to be certain that the bacteria that show up on your plates were not there because of non-sterile technique in making up the agar solution!

As the previous post mentioned - it is very important to use the LB agar (it has all the nutrients necessary for growing common bacteria).

At this point it is important to check with the teacher and make sure this is an acceptable experiment to be carried out at home. When I worked with science fair, we had special forms that needed to be signed when students worked with bacteria.

See safety guides about this.

hope this was helpful!