Science Buddies: "Ask an Expert"

Dear Science Buddies:
Attached is my son Brian’s plan for his science fair project. Could you review and make suggestions.
One of the main questions he has is what to contaminate his tiles with. Do you have a suggestion? In the Mythbusters test, they used beef broth. Would that be a better contaminant than milk?
Thanks so much for your kind assistance.
Sincerely,

Suzanne and Brian Williams

Brian’s Science Fair Project – January 2007

Name: Brian Williams
Grade: 5th
Age: 10
School: Gault Elementary School

Question: Is the 5 – second rule true or false?

Definition of 5 – second rule (from Wikipedia)
The rule states that any germs that can be contracted by grounded food in under five seconds are of such a small amount that they will easily be destroyed by gastric acid, thus causing no harm to the body.

Brian’s question:
Does it make any difference if you leave a piece of bread on a contaminated tile surface for 1 second, 5 seconds, 10 seconds, 15 seconds, 30 seconds, 1 minute, 5 minutes?

Controls:
same type and size of bread – Orowheat oatnut bread – 1 slice per time test
same surface – tiles soaked in milk for 5 minutes and then dried for 10 minutes in the sun

Procedure:
Place slice of bread on tile for designated time.
Place bread with on petri dish for 3 minutes. Remove. Place the Petri dishes on a card table in the dining room along the wall with no windows. (to prevent different amounts of light from being a variable). Observe results after 3 days and again after 1 week.
Leave bread on a clean cookie sheet for 3 days and observe. Observe again after 1 week.

Record results:
Count cultures on the Petri dishes under a microscope.
Make observations of the bread.

Repeat

Make conclusions
Write up report

Materials list:
tiles – check at tile store near Costco for donation
bread – purchase from Safeway
milk - raw milk purchased from a health food store
pans to soak tiles – check with Elizabeth at school for large trays
Petri dishes – research science supply stores on the internet located in
Santa Clara/San Jose. Ask Dad to purchase
Agar – same as above

For fun:
Take a poll of how many boys and girls believe in the 5 second rule.
25 - 5th grade boys/25 - 5th grade girls
25 – 3rd grade boys/25 - 3rd grade girls

Survey:
Have you ever heard of the 5 – second rule?
If you have heard about it, do you believe it is true or false?
Have you ever applied the 5 - second rule to food you have dropped on the floor?
Are you a boy or a girl?
What grade are you in?
Do you speak Spanish or English in your home?
Thank you for answering these questions.

Questions:
Dad suggests we use the kitchen floor or the back wooden deck as our surface. Would this be appropriate? He contends that our tile soaked in milk will not produce results and that the tile soaked in milk does not portray a kitchen floor which is walked on, spilled on, had the dog walk on it, etc. He also contends the tile is not porous and will not be contaminated by the pasteurized milk therefore we will not get any results.
Do you have a suggestion? What would be more likely to get results? Should we contaminate the tile with additional substances or use the kitchen floor or back wooden deck?

We also need help in how to prepare or get already prepared petri dishes for cultures. I think they need agar, dextrose, water and have heard perhaps potatoe water to provide a medium for material to grow.

In response to your questions:
Dad suggests we use the kitchen floor or the back wooden deck as our surface. Would this be appropriate? He contends that our tile soaked in milk will not produce results and that the tile soaked in milk does not portray a kitchen floor which is walked on, spilled on, had the dog walk on it, etc. He also contends the tile is not porous and will not be contaminated by the pasteurized milk therefore we will not get any results. Do you have a suggestion? What would be more likely to get results? Should we contaminate the tile with additional substances or use the kitchen floor or back wooden deck?

I think the kitchen floor or back deck would add too many variables you couldn't control. Therefore, I think it is preferable to use tiles as you've described. Other studies have described using tiles and still finding bacterial growth. The key seems to be moisture. Very dry surfaces remain "cleaner" because they don't support bacterial growth well. You intend to dry the plates for 10 minutes. If you don't see growth using this method, perhaps you could consider wiping lightly with a paper towel to dry and applying the bread shortly thereafter. (If you do this, it will be important to come up with a consistent and reproducible method.) As for milk vs beef broth, I would think either would work.
I'm including here a link describing a "5-second rule" test done a couple years ago. You might find it interesting.
http://www.cbsnews.com/stories/2006/07/ ... 4287.shtml


We also need help in how to prepare or get already prepared petri dishes for cultures. I think they need agar, dextrose, water and have heard perhaps potatoe water to provide a medium for material to grow.
You can buy prepared plates. This would certainly be easiest, but a bit more expensive than buying your own. I'm listing a couple biological supply stores where you can buy agar and petri plates, or already prepared agar plates. You will want to get "nutrient" agar or "Luria" or something similar known to be "tasty" to bacteria. Search for "agar" at these places and see if there's something that might work.
http://www.wardsi.com
http://www.sciencekit.com
http://www.ipmscientific.com/Microbiology%20media.htm


I also came across a place called The ScienceShop, which I believe is in your area. For info see this webpage: http://www.scienceshopusa.com/

Brian and Suzanne,
I just posted to you, and realized I pasted in a different link than I meant to. I would encourage you to check out this website for media and prepared plates. The tabs for both "prepared agar plates" and "microbiology kits" have good information. You will see some are the potato dextrose agar plates, like you mentioned, and these are particularly good for growing fungi. I like this page because besides telling you what the media is good for, it gives you instructions, so you can get a feel for whether it's something you want to do. (I don't however, have experience with this company.)
http://www.ipmscientific.com/education.htm

Your project sounds cool. Have fun with it!