Science Buddies: "Ask an Expert"

Hello there,

I am conducting an experiment on growing skin bacteria. For about 3 days, I incubated my streaked plates (informally streaked, I just swabbed it in a predictable pattern with a sterile applicator) at 20C for 72 hours. I saw no growth so I took it and incubated it since 35 hours ago (it is still being incubated) at about 35 - 39C. however, I am seeing a LOAD of condensation on my petri lids. Quite a bit of moisture is collecting on the lids (of course I incubated them upside down). I am using standard nutrient agar and incubated my plates right after the 20C incubation period. Is this normal, or preventable? I plan to incubate my plates for about 24 more hours. Will my agar dry out?

Thanks!

Saternet,
Yes, it is very common to get condensation on the lids of your plates. You were right to incubate the plates upsidedown; if the condensation falls onto the agar, it can streak and mess up the bacteria. Plates can become too dry, especially in an incubator. I would recommend that you put your plates into sandwhich baggies. They'll still get condensation, but should dry out less overall. (p.s. Before flipping a plate right side up if there is a lot of condensation, I'd recommend giving the lid a quick shake over a sink.)
Good luck!