Science Buddies: "Ask an Expert"

Hello. I'm trying to find the products I will need to help my son complete his science project. I'm not sure if I need Agar and if I do I'm not sure which one I would need. I've seen many different types so, I was hoping if I give you details of the project you would be able to help me decide which products I would need to purchase and how he would go about conducting the experiment.
My son is 7 years old and in the second grade. The project he wants to do is.. "Who's mouth has more bacteria.. Dogs or Humans?" When he mentioned this to me my first thoughts of how we could test this would be to use 2 dogs and 2 humans. We could rub the insides of each mouth with a swab and place them in separate petri dishes. Now.. I've heard that heat helps the bacteria grow faster but I'm not sure if that is correct. Also, I'm not sure if this is something we will be able to see without a microscope and if we would need agar to conduct the product. If we do need agar, which type would we need for this project?

Thank you,
Melinda

Hi there,

I would recommend starting with some research – look up what kinds of bacteria are typically found in a human’s mouth or in a dog’s mouth and what types of media these bacteria typically grow on. I would recommend using a rich media to start with that will allow growth of many different bacteria. Rich medias include, for example, Tryptic Soy Agar, Blood Agar or LB (Luria-Bertani) Agar. See the Following link for details.

https://www.sciencebuddies.org/mentorin ... Agar.shtml

Also, I would advise that you read a little bit about sterile technique and include appropriate controls in your experiments. For further information see the following link:

https://www.sciencebuddies.org/mentorin ... ?from=Home

Other options include preparing agar plates using gelatin and beef/chicken broth – see the following link for more information:
http://www.freesciencefairproject.com/b ... teria.html

As for heat, bacteria (especially those from your mouth) typically grow best at approximately 37 degrees Celsius (body temperature).

Prior to your experiment you should consider scientific method:
https://www.sciencebuddies.org/mentorin ... thod.shtml

Remember to state your hypothesis. For example, your statement should be something like “a human’s mouth has more bacteria than a dog’s mouth�

Prepare/obtain the media and use sterile swabs to swab the mouth of a dog and of a human and streak the agar plates.

I think that using 2 people and 2 dogs is a good idea and you should do more than 1 trial for each.

Keep your plates in a warm place for at least 24 hours. You will then be able to count the number of colonies on each plate and can also describe the colonies that grow. If you want to determine that types of bacteria that grow you will need a microscope and further supplies.

I hope this helps. Please do not hesitate to post further questions.

Mary

For kicks, if you can get a hold of Mythbusters epidose 39, Original Air Date: October 19, 2005, Jamie and Adam investigated the "five second rule". While they were playing with Agar plates, they used a dog and Adam to investigate this very topic.

Didn't exactly use stellar scientific technique, but it was fun TV.


Here's a synopsis of the episode from the Mythbusters web site:

http://www.mythbustersfanclub.com/mb2/c ... iew/35/27/

"Dog's Mouth Vs. Human's Mouth"

The Myth:

A dog has a cleaner mouth than that of a human.

During the Five Second Rule testing, Adam decides to test the cleanliness of a dog's mouth vs. human's (Adam's) mouth myth. Adam licks one of the contact plates, while Lulu (one of the MB's assistants' dog) licks another. The samples are placed into an incubator to allow them to develop.

I'll leave out what conclusion they came to.

Since they obviously edited out most of the useful information about sterile technique, microbiological testing, etc.. you will need to research all of that yourself. And you might very well get exactly the opposite result that they did.