Agar

[danville44]

I am doing my science fair project is on what mouth contains the most bacteria? Cats, Dogs, Or mine. And to make it a good controlled experiment i am going to do it for one week at the same time eachday when i get home from school. But i only have one batch of agar to make. Can you store agar or will i have to buy 5 batches of it?

[ChrisG]

In my limited experience, yes, you can store agar. The only problem I have had is when (1) it is contaminated and starts to grow mold or bacteria (so be careful as possible not to introduce contaminants) or (2) when it starts to dry out (so keep it sealed). Hopefully the biologists can chime in on this too.
Good luck.
Chris

[donnahardy2]

Hi,

Chris is right. You can prepare sterile agar plates and store them in the original plastic bag, in the refrigerator, turned upside down, so the moisture doesn't condense on the surface for up to a week. However, if the agar is exposed to air for even a few seconds, it could get contaminated. Have you prepared agar before, or do you have someone at school to help you? Please let us know if you need more information on this.

It sounds like you are planning to test the same human, dog, and cat every day for a week. This would be a good experiment, but you could also consider testing multiple humans, dogs, and cats at once, and then incubate the plates at the same time and temperature. This would avoid the storage problem.

When using your plates, don't forget to keep one plate as a negative control (to show that the plates were sterile), and one plate as a positive control (to show that the plates will support the growth of bacteria).

Let us know if you have more questions. This sounds like a great project.

Donna Hardy

[danville44]

I made it and did the experiment the first time last night, it was kinda hard to swab a cats mouth with a cotton swab lol It is sitting in my kitchen now, it says to store it at 80 degrees but there isnt a place in my house that hot lol

[donnahardy2]

Hi,

You may not get a chance to test your cat again. If it were my cat, she would make herself unavailable for another experiment until after the project was due. You might want to pet your kitty just to let her know that there are no hard feelings. You should also plan to mention this difficulty in the discussion section of your project paper and board, as the difficulty in getting a good sample could definitely affect your results. If you set up the experiment again, you may want to do the cat sample first, and then try to use the same technique for the dog and human, so your samples will be comparable.

Actually, since the sample came from a warm-blooded animal, the optimum temperature for incubation would be about 98 degrees F. But generally bacteria grow more slowly at lower temperatures, so you will just need to incubate for a longer time.

You can keep your agar plates a little warmer by placing them near the water heater, in the oven with the light turned on, or in a cardboard box with a light bulb turned on. Be sure to measure the temperature of your incubator before you move the plates, and make sure the temperature does not go above 105, as this could inhibit growth. You should record the temperature of the location of your agar plates at least twice a day, and try to keep the temperature as constant as possible. You will get different results with plates incubated at different temperatures, so it's important to keep the temperature constant.

In science fair projects, it's best to use metric measurements, so you should convert the temperature to degrees Centigrade. Do you remember how to do this?

You're making great progress because you have actually started your project! Let us know if there are any other questions.

Donna Hardy