Subject: How do I measure bacterial growth in a jar?

I collected samples of public bathroom toilets by rubbing potato pieces 
around them and placed them in jars.  My control was just a regular potato 
inside a jar.  My control and one bathroom don't seem to have any 
bacterial growth because the potatoes are dried out. All the other 
potatoes seem to be decomposing and liquifying. I am not sure how to 
measure this bacterial growth or graph it.  My orginal science experiment 
question was in short "compare the bacterial growth in different 
bathrooms". Can you help me?

Re: How do I measure bacterial growth in a jar?