Caffeine experiment

Some possible things to test is:

1) Their ability to grow (is it an inhibitor or simulant)
2) Speed of growth
3) Changes in colony formation
4) Changes in shape of bacteria (given you have a good microscope)

You need to choose one or two of these factors to test. To test growth, you'd plate the bacteria on agar that contains caffeine. You may want to plate bacteria on various concentrations of caffeine too, because you could discuss how much of a concentration is needed if you notice if caffeine does have an effect. You could collect bacteria from the colonies that growth and discuss their density using a spectrometer or via cell count. You may have to keep environmental factors constant, such as temperature and exposure to oxygen.