Subject: How can I measure the percentage of crude oil eaten by oil eating microbes?

I was interested in doing a (science fair) project on the effects of oil by 
OEMs over a period of time, but was unsuccesful in finding a way to measure 
the percant of oil eaten by oil eating microbes, as what i am looking for is 
something very specific. I wanted to have a starting amount of oil, and see 
how much the oil eating microbes degrate over a period of time, and see how 
the percentage changes in between each time frame. I am not sure how i would 
observethe oil/microbes (through a petrie dish?), nor am i entirely sure how 
to measure the percentage of oil eaten (i wouldnt know how to go about 
measuring the amount of bacteria). Could you help?
thank you
Jessica

Re: How can I measure the percentage of crude oil eaten by oil eating microbes?