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!

[Dinu14]

ya keep ur oven at 100 or 99 at the best.....
coz bacteria need lukewarm climate to grow fast.....
if it is the proper temp. bacteria can multiply within 20 mins

[canalon]

ya keep ur oven at 100 or 99 at the best.....
coz bacteria need lukewarm climate to grow fast.....
if it is the proper temp. bacteria can multiply within 20 mins

Well it depends on the bacteria. The factors influencing bacterial growth are: water, energy, pH, temperature, oxygen. For each of those the optimum varies for the different species. Some bacteria will grow best at high temperature (Thermus aquaticus) and other will prefer much lower temperature (Aeromonas hydrophila) and the same could be said for all the other parameters.
Asfor the doubling time, it is also very dependant on the conditions, but even the optimum can be very different between species. E.coli have an optimum close to 20 minutes, but Clostridium perfringens is only 10 minutes, and Mycobacterium tuberculosis has one that is counted in hours (10 to 90h).

Beware the generalizations when talking about bacteria.

[jdl]

What I did was store my petri dishes in a cardboard box, taped it shut, left it in a room for 3-7 days. I've got great outcomes, and I can tell the difference between bacteria.

[greatmicrobiologist]

I am absolutely supporting of what Mr Moderator has said.

Working with germs require very much aseptic conditions, sometimes we Microbiologists have to wear gloves for handling cultures.

As a part of your project concerned I don't know the motive of your project and from where you have isolated the organisms.

Normally as Mr Moderator said you should try to maintain the temperature of 37C for the growth of pathogenic bacteria. Some microorganisms does grow at high temperatures and some even at cold temperatures. So ideally try to maintain it at 37C.

How you are going to maintain that is the biggest question now.

Take a woonden box, make hole in it so that a small electric bulb can fit into it. Wrap it with black paper. Keep the plates inverted under wooden box and on the light... and off that after half an hour .... try to maintain it for long as 5hours with a intermediate half an hour gap.

I tried it in my lab as we had some Incubator problem during my research and it worked, even though some plates got contaminated. Try it, hope it works...