Help me help my kid use her Agar plates.

My daughter is doing her science fair project for 5th grade. She is trying to see how much bacteria there is growing on three types of flooring, wood tile and carpet. She determined that she needs to use Agar plates to count the bacteria. So I ordered her some plates and am trying to figure out the right way for her to apply samples to these things. We’ve read a few sites that try to explain but they aren’t totally clear. Does she take a dry cotton swab and wipe it on the three floor types and then smear that onto the agar? Does she wet the swab then take the sample and wipe it on the agar? It sounds like the isn’t supposed to break the skin of the agar but then several sites show zig zagging through the stuff.

Can someone tell me how to properly take the samples and get them onto the agar without hosing everything up?


Normally you use a swab that has been moistened. Using a dry swab won’t work well because all you will be collecting are those spores that are affected by static cling or that have barbs that physically stick to the fibres. Anything even vaguely moist or mucilaginous will be left behind.

Ideally you should use sterile, distilled water. Since you are unlikely to have that, then boil some tap water and let it cool. Most tap water contains enough chlorine to give some really screwy results if it isn’t boiled first.

Wipe the swab on the surface of the agar in a zig zag pattern. There are techniques where you have to break the surface, but that is primarily for culturing organisms that can’t tolerate oxygen, so they have to be buried under the agar to survive. It’s not required for what you are doing.

One thing to bear in mind is that, using this technique, you are highly likely to get a solid white smear from all floor types. There are so many microbes on most surfaces that doing a straight transfer like this is just going to return a “Too Numerous to Count” result from all surfaces. Not really an ideal result. If that happens you will need to do some sort of basic dilution series, which is pretty simple.

My mom used to do this with us when we were kids, and while the agar will flex under the Qtip pressure, you shouldn’t rupture it.

Make sure you culture the plates agar side up at the appropriate temp (about 98 degrees). We would put the plates in our gas oven and the heat from the pilot light was enough to incubate the bacteria.

Hmm. Would keeping them under a blanket under a lightbulb be good for incubating?

Also how long do you figure the things should cook? Will it take a week to reach capacity?

One thing to remember is to be consistent: if your daughter swabs the hardwood floor over and over again but only does one swipe of the carpet, that’s going to affect the results (I hope for obvious reasons, but it’s something she might want to show as part of her project).

You daughter could come up with a swabbing protocol: for example, she could cut a 1" x 1" square hole out of paper, and use that as a guide for the area size she will swab. Then swab the entire area, making sure to pass over each bit only once in a zig-zag pattern.

And don’t forget your control plate!

A couple days at around 100 F should do. Put the plates in the incubator upside down (agar up).

Whatever grows can be dangerous. Don’t open the plates to count the colonies. We always used an autoclave (fancy pressure cooker) in microbiology class to sterilize glass and metal equipment while plastic plates got incinerated.

In order to be a “fair test” (or controlled experiment), lay out a 1" square on each surface and fully swab the entire 1" to make sure that all have the same coverage.

The first things can start to show up at 16 hours, but usually a couple of days is more likely. You can keep growing until the plates dry up and shrivel (2 weeks is possible, but I don’t know the quality of plates you got). And I am not sure that I would agree that you’ll see a “lawn” of growth. Floors aren’t particularly infested (there isn’t much food, protection from light, or moisture).
A little bleach will kill anything and then the plates can just go in the regular garbage (but obviously this is just done by adults after the experiment). Otherwise, I agree with california jobcase, don’t open the plates during the growth phase.

The 5 second rule has had some investigation done and I thought the mythbusters had done an episode on it, but I can only find their study of the bathroom germs. If you can get a rough idea of the number of germs, you may be able to guide the experiment for how large an area to sample to get a countable number.

This is a good idea I hadn’t thought of. I’ll recommend this to her.

an incandescent bulb in a small enclosure made of flammable material is a fire hazard.

Yeah I know. I meant put the plates under the blanket and shine the light on the blanket.