Mutations: induced or selected

We have determined that the T4-resistance in E. coli is stable and specific.

Some kind of genetic mutation has occurred in the resistant strains.

Do such mutations arise spontaneously, as a kind of genetic noise, or are "induced " by the presence of the phage itself, like a kind of immunity?

Luria & Delbruck set out to answer this question.

Before doing the experiment, write down which view you think is most likely to be correct and why.

The logic of their experiment was quite simple, although statistical.

If mutations that convey phage resistance on the cells that carry them are present within the population at low frequency, then the number of resistant bacteria found after the addition of phage, will vary widely.  

Some cultures will, by chance, not contain any resistant bacteria. Others will have many (something very much like a founder effect). There will be wide variance (also known as fluctuation) in the number of resistant bacteria between cultures.

If, on the other hand, phage resistance is "induced" by the presence of phage then the variance between cultures should be significantly lower - all cultures should respond in a similar way. 

Do you understand why this is to be expected?

The general method Luria and Delbruck used should be familiar to you by now. 

In this experiment, we will use T4 phage to eliminate all of the sensitive bacteria from the culture. 

We can then count the number of resistant bacteria that remain, and from that number estimate the mutation rate.

• Record the variance and the mean of your fluctuation analysis (above); is it high or low.

Answer these questions:

• Why would induced mutation produce low levels of variation between cultures? 
• Do the bacteria somehow "decide" to produce mutations that lead to phage resistance?