So I left the Petrifilm plates smeared in poo in the incubator for two days. When I took them out this afternoon they all had all turned a purplish red colour and had lots of little air bubbles in them. This is gas produced by the E. coli colonies which can be found in proximity to each bubble. However, the fact that the whole film changed colour makes me suspect I have grown a lot more than a few colonies. Below is a picture of one of the films.