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If you want to be as objective as possible, there is something you should probably also consider - controls. Often times, food have bacteria on them, so this may skew your results. What I would recommend is doing two conditions - one where you do no drop the food on the floor, and one where you do for a certain period of time. That way, you can establish a base line, and you can be sure if the control has no growth, all of the bacteria did indeed come from the floor.

Also, cotton swabs would not be a good tool to use, as unless these are sterile, they are a perfect source of contamination. I would recommend applying the food to the agar physically, as the second poster suggests, and then further, using a tool that can be easily sterilized to spread the sample out on the agar. It is important to spread the sample out, because there is likely to be the highest concentration of bacteria on the point of contact, and if there are too many, you will not be able to count the colonies easily. Use a disposable plastic spoon, which you have wiped with rubbing alcohol, and let thoroughly dry (this is very important, or else it will kill the bacteria), and use it to spread the sample all over the plate.

Then, just invert, incubate and wait for your results to grow. Hope this helps. =]

I would seriously suggest doing a control sample first. Prep your agar gel and quickly cover it and store it in your freezer if you can (just until you can apply the swab to it)--it's the least contaminated area in your house, unless you're doing this in a well-filtered lab. Collect surface samples from the target (food) and apply to agar (which you've removed from the freezer). Cover and set aside in an air-tight box if you can.

Now you have a means to more accurately judge growths independent from the control sample. To further control from cross-contamination (other sources), wear new latex/lab gloves, and wear a face mask--don't want to sample rhinitus bacteria, do you? Before you even start the collection, determine where the least amount of air- or surface-borne cross-contamination can occur. Quite often it's a "guest bathroom" without air sprayer smellums, etc. It's a good place to store your samples...just tell little brother not to eat the gel in the flat disks. Try to keep the temperature as stable as possible and determine if you want to control for light or unlighted growing conditions.

Edit: I should have pointed out that there are sterile swabs. I agree with the next poster about direct application of the sample source.

place the food that was dropped (contaminated side to the agar) on top of the agar and then remove the food without spreading it around. replace the top back on the agar dish and incubate the agar dish upside down for 24 hours at 37 degrees C. then count the bacterial colonies. Be sure to label each dish so you know which is which.

First, get some cotton buds and wipe them on the area in which you think the germs are. Then just wipe it on the agar plate in zigzags for 10 seconds. This should give you excellent results. Good luck.