Discussion 63 



However, when these investigators varied the growth rate by controlling 

 the supply of a required amino acid, they found that the mutation rate 

 remained constant per unit time rather than per generation. Thus, when 

 an organism was growing at one-twelfth its normal rate it produced 

 twelve times as many phage-resistant mutants per generation as when 

 growing at the full rate. I don't know that anyone has carried out 

 parallel experiments on the effect of slowing growth inhibitors, but this 

 work raises an interesting possibility ; when we are dealing with border- 

 line drug concentrations that slow growth but do not kill the organisms, 

 mutation rates per generation may be very much higher than what we 

 are accustomed to. 



