336 



CHAPTER 36 



of Streptomycin, that all mutants resistant to 

 the drug are preadaptive — that is, this drug 

 does not induce resistant mutants. The same 

 results were obtained with the drug chloram- 

 phenicol, so that we may extrapolate from 

 such resuhs and conclude that generally all 

 resistant mutants on drug plates arise spon- 

 taneously prior to exposure to the drug. 



Large numbers of bacteria can be tested 

 for mutations with ease. For example, a 

 billion drug-sensitive individuals can be 

 plated on agar containing the drug, and the 

 number of resistant mutant clones readily 

 detected by counting the colonies formed. 

 Or, similarly, the number of mutants to pro- 

 totrophy can be scored, by plating auxotrophs 

 on agar lacking the nutrient required for their 

 growth, and counting the number of colonies 

 formed. In order to give information in 

 terms of a rate of mutation, however, it is 

 necessary to state the number of mutants that 

 occur per unit event. In multicellular organ- 

 isms, mutation rate is usually expressed in 

 terms of mutations per cell, individual, or 

 generation. This definition can be applied 

 to bacteria, also. Thus, the mutation rate, 

 from streptomycin-sensitivity to resistance in 

 one particular strain of E. coli (other than 

 the one used beginning on p. 333) is one per 

 billion bacteria, the lowest mutation rate so 

 far measured in any organism. 



It is sometimes desirable to express muta- 

 tion rates in terms of mutations per unit time. 

 This could have been done, for example, in 

 describing the increase in mutations obtained 

 by means of aging Drosophila spermatids or 

 sperm (Chapter 23, p. 201). In the case 

 of bacteria, it is possible to vary consider- 

 ably the length of time required to com- 

 plete a generation. For generation times 

 between 37 minutes and 2 hours, the shorter 

 the generation time, the larger the mutation 

 rate per hour. When generation time is 

 lengthened to occupy from 2 to 12 hours, the 

 rate of mutations per hour is constant — each 

 hour of delay increasing the number of mu- 



tants by the same amount. (Thus, in this 

 2-12 hour range, there is an increase in the 

 number of mutations that occur per genera- 

 tion.) Finally, even when the generation 

 time is extended to infinity (when nondividing 

 cells, provided with a source of energy, are 

 studied), some mutations are found to take 

 place. 



It becomes apparent, therefore, that it is 

 best to define mutation rate as the chance of 

 a mutation per cell (or individual) per unit 

 time. When, however, each of the division 

 cycles or generations requires the same length 

 of time (as would be true for bacteria under 

 optimal environmental conditions), time is 

 usually measured in terms of generations. 



What causes the spontaneous mutation 

 rate in bacteria? We have already discussed 

 how the spontaneous mutation rate is influ- 

 enced by naturally occurring physical muta- 

 gens, hke ultraviolet light or ionizing radia- 

 tions (Chapter 21), and possibly by naturally 

 occurring chemical mutagens (Chapter 23). 

 We also know that there is some genetic 

 control of mutability (Chapter 25). We may 

 be able to learn more about these or other 

 factors influencing the spontaneous mutation 

 rate after considering the results of certain 

 experiments with E. coli. 



In the studies of the effect of generation 

 length on spontaneous mutation rate, muta- 

 tions from sensitivity to resistance to infection 

 by virus were investigated.'- In some experi- 

 ments virus T5 and in others virus T6 were 

 used (a mutant resistant to one of these viruses 

 is still sensitive to the other). Generation 

 time was lengthened by limiting the amount 

 of one of the nutrients required by the bac- 

 teria for growth. The nutrient medium was 

 simple and contained lactate, ammonium, 

 chloride, and magnesium sulfate. Since the 

 E. coli employed was a mutant requiring sup- 

 plementary tryptophan in order to grow, this 

 substance was also added to the medium. 



2 Based upon work of A. Novick and L. Szilard 

 (1951). 



