growing bacteria in petri dishes

[cjonesmath]

I am helping my 4th grader with his science project, which is to see what place in his school is the germiest. We prepared his petri dishes with nutrient agar, gathered samples from the school with sterile cotton swabs, and swabbed the agar immediately after swabbing the test site each time. How long should it take for us to see something growing? We put the dishes upside down in the oven and covered them with a black pan, so the light would not get to the dishes through the oven window. The only thing I see so far is some clear spots, which appear to be at the bottom of the dishes. Through the top of the dishes I can see where the sample was applied because it looks 'textured' as opposed to where the sample was not applied is smooth. Does the oven need to be on at 100 degrees so that they have a constant temp, because my house is pretty cool (70 degrees)?

[Survivor Kid 909]

We did this in class and we quit after a week, also with the heat i dont know. We had a heater type hting that kept them just right.

[canalon]

Beside the fact that you have to be careful with those plates (you can grow pathogenic bacteria this way) most culture are maintained in an incubator with a temperature at 37ºC. Of course different species can grow at different temperatures, but they might take a while to grow enough to be seen (many days).

You should also incubate your dishes upside down (so condensation do not fall on the plate and blurs colonies).

And if something grows:
1-be careful with the colonies, you don't know what they are, and they can be nasty. Wash your hands frequently for at least 30s after you touch the plates.

2-dispose carefully (in a sealed bag, the best would be in spaecial waste availables in a lab, other wise drench in bleach for hours before throwing out)

[cjonesmath]

We did store them upside down, I had read that they should be because of the condensation. As for the incubator, if I turn the oven on warm, do you think that will work?

Thank you for your help!

[Survivor Kid 909]

I think that should work fine!