Assume you infected an E. coli culture with a virulent bacteriophage. Most of the cells lysed, but a few survived – 1×10^-4. You wonder where the resistant bacteria came from. Were they caused by the bacteriophage infection or did they already exist in the bacterial culture? Earlier, for a different experiment, you ha spread a dilute suspension of E. coli onto solid medium in a large Petri dish, and, after seeing that about 10^5 colonies were growing up, you made an imprint of the colonies on that plate and transferred it to three other plates – a process known as replica plating – which creates identical patterns of the colonies on 3 plates. You realize that you can use these plates to distinguish between the two possibilities. You pipette a suspension of the bacteriophage onto each of the 3 replica plates so that the bacteria can be infected. What result do you expect if the bacteriophage cause resistance? What result do you expect if the resistant bacteria pre-exist?
