Bacterial Mutation and Conjugation 



309 



resistant mutants on drug plates arise spon- 

 taneously, prior to exposure to the drugs 

 and, there jore, are preadaptive in origin. 



Large numbers of bacteria can be easily 

 tested for mutations. For example, a billion 

 drug-sensitive individuals can be plated on 

 agar containing the drug, and the number 

 of resistant mutant clones detected by count- 

 ing the colonies formed; or, similarly, the 

 number of mutants to prototrophy can be 

 scored by plating auxotrophs on agar which 

 lacks the nutrient required for their growth 

 and counting the number of colonies formed. 

 To give information in terms of a rate of 

 mutation, however, it is necessary to state 

 the number of mutants occurring per unit 

 event. In multicellular organisms, mutation 

 rate is usually expressed in terms of muta- 

 tions per cell, per individual, or per genera- 

 tion. This definition can be applied to bac- 

 teria also. Thus, the mutation rate from 

 streptomycin-sensitivity to -resistance in one 

 particular strain of E. coli (a different strain 

 from that used previously) is one per billion 

 bacteria — one of the lowest mutation rates 

 so far measured in any organism. 



It is sometimes desirable to express muta- 

 tion rate in terms of mutations per unit time; 

 for example, in describing the increase in 

 mutations obtained by aging Drosophila 

 spermatids or sperm (Chapter 14). In bac- 

 teria, one can considerably vary the length 

 of time required to complete a generation. 

 For generation times between 37 minutes 

 and two hours, the shorter the generation 

 time, the larger the mutation rate per hour. 

 When generation time is lengthened from 

 two to twelve hours, the rate of mutations 

 per hour is constant — each hour of delay 

 increasing the number of mutants by the 

 same amount. (Thus, in the two- to twelve- 

 hour range, the number of mutations in- 

 creases linearly per generation. ) Even when 

 the generation time is extended from twelve 

 hours to infinity (the nondividing cells kept 

 alive in a medium which provides a source 



figure 23-3. Replica-plating a bacterial lawn 

 for the detection of mutants to streptomycin- 

 resistance. (After J. Lederberg and E. M. 

 Lederberg. ) 



of energy), some mutations are found to 

 take place. 



It becomes apparent, therefore, that mu- 

 tation rate is best defined 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), 

 mutation rate is usually measured with one 

 generation as the unit of time. 



Conjugation 



Genetic recombination in bacteria may occur 

 as a result of genetic transformation. The 

 transformation process has two features 

 hitherto unencountered in discussions of ge- 

 netic recombination in multicellular organ- 

 isms: 



1. The donor DNA enters the host bac- 

 terium without intervention of any other or- 

 ganism, as is shown by the infectivity of pure 

 DNA. (Although transformation involves 



