Germ Invasion - Microbiology

[jamiesims70]

We left the agar plates out and uncovered for 48 hours in different locations like the project procedures suggested. But, when we went to cover them and incubate them together, we noticed that all of the agar had evaporated out of most of the plates. The only ones that still had some were the control one we didn't open, the one in the refrigerator, back porch, and garage (and these last three had very little and it was very dried out). All of the others had none, or a dried almost nonexistent crust on the bottom. We're worried that this will not work and culture the bacteria. Do we need to start over and not leave the plates out? We don't have a lot of time left, and would need to order more plates I'm guessing.

[scibuddyAK]

Hi,

In my experiences with agar and bacteria culturing so far, I have never left agar plates out for that long, but I believe this is necessary for the purposes of the project you're undertaking. I'm not fully sure how to answer this - perhaps another Expert can chime in please? - but my first concern would be - had the agar solidified (like a gelatin-looking structure) when you left it out? Or was it still in liquid form? With my experience, the agar had to solidify before culturing any bacteria.

And yes, you would need to order more plates.

I'm sorry this may not have exactly answered your question. Hope this helps, though.

[connief]

Hi there,

When you ordered the agar plates, did you order them pre-poured, or did you have to pour them yourself? Regardless, if the agar plates were poured too thinly, they can easily dry up if you don't keep them covered and/or in a humid environment. The environments where you left your plates opened could also be too dry, and 48 hours is a long time to leave agar plates open, so those could also be reasons why your plates have dried up. Which project procedures are you following? According to what you've posted, it seems like leaving the plates open for 2 days is necessary for the project, but in my experience, I have never left agar plates open for more than a day because they can dry up. Do the project procedures suggest anything for keeping the plates moist enough despite being open to the environment so that they won't dry up?

Out of curiosity, why do you have to leave the agar plates open to the environment for 48 hours? Is it to give the microbes enough time to stick to the plates? To get around your problem of plates drying up, you can either pour your plates thicker, or leave them out for a shorter amount of time before covering them up and incubating them to look for bacterial growth. Microbes are found everywhere, and I think two days is more time than necessary to get enough microbes on your plate, but I could be wrong. It would be useful if you provide more details about your experimental procedures, or if you have a word document or link to them, you can also post it here so I can take a quick look at it to see if there are any steps that you can troubleshoot.

I hope that helped. Please let us know if you have anymore questions or concerns, and good luck with your experiment!

Best,
Connie

[jamiesims70]

I got the project procedures from the sciencebuddies site, under the project Germ Invasion. The procedures said to leave the plates open for 48 hours, and even suggested sites such as beside a vent (that one really dried out). We actually did see growth on the plates that were left out in a more humid environment (back porch, garage, and beside the toilet), but nothing on the other plates that were more dried out. The procedures didn't have any info on keeping the plates from drying out, and since we haven't done this before, we didn't realize they would dry out until it was too late. We ordered more agar, documented the growth we did see (including that it was in the more moist environments), cleaned the plates with bleach, and are starting over tomorrow. This time we won't leave them out at all, just swab the area and then close the plate so we don't have the evaporation/drying out problem. We did have pre-poured plates, and I think they may have needed a little more agar in them. We ordered bottled agar this time and will pour it ourselves. Thanks for your information.

[jamiesims70]

The agar was solidified when we left them out by the way, it wasn't still liquid. We had stored the pre-filled plates in the fridge until we were ready to use them.

[connief]

Hello there,

I took a look at the Germ Invasion procedures, and I am quite surprised that they do ask you to leave the plates open to the environment for 48 hours because usually when I work with agar plates, they can start drying up very fast within several hours if they are poured very thinly. It is great that you're ordering agar to pour plates for yourselves because you can control the thickness of your agar. You probably want to pour the agar until it fills the height of the petri dish halfway. Too thin is not good because the plates will dry easily, but if you pour it too thickly, the agar may end up touching the cover.

Out of curiosity, what do you mean by cleaning the plates with bleach? Are you planning to reuse these plates by pouring fresh agar on them? It's always better to start off with fresh plates that haven't been used before to ensure sterility. However, I have never really worked with reused petri dishes before, so if that is something that is commonly done, then I could be wrong. I do agree with you though that instead of leaving the plates out for 48 hours, a good idea would be just to take a loop or cotton swab, swab a little of your environment of interest, and just streak it over the agar--that is a great alternative and will ensure that your plates won't dry out at all!

Let us know if you have anymore questions!

Best,
Connie