Chapter 23 



BACTERIAL MUTATION 

 AND CONJUGATION 



Mutation 



Practically all of the mutants produced after 

 treatment with a mutagenic agent have non- 

 adaptive or detrimental phenotypic effects 

 (Chapter 16). The detriment produced by 

 these mutants clearly is not dependent upon 

 the mutagen's continued presence in the en- 

 vironment. Similarly many of the rare mu- 

 tants that increase adaptability continue to 

 be beneficial in the absence of the mutagen 

 which induced them. On rare occasions, 

 however, a mutagen (like X rays) produces 

 a mutant with an adaptive advantage in the 

 presence of the mutagen (for example, re- 

 sistance to the genetic or nongenetic detri- 

 mental effects of X rays). Is such an adap- 

 tive mutant produced by chance, or is it a 

 special genetic response elicited by the mu- 

 tagen? The same question can be raised 

 about adaptive mutants that occur "spon- 

 taneously." Are these mutants produced as 

 an adaptive genetic response to unidentified 

 factors in the environment? 



This general problem can be illustrated 

 with a particular strain of E. coli which ap- 

 parently has never been exposed to the drug 

 streptomycin. If such a strain is plated 

 onto an agar medium containing this drug, 

 almost all the individuals will not grow and, 

 there lore, will not form colonies. These in- 

 dividuals are streptomyt insensitive. How- 

 ever, about one bacterium in ten million 

 does grow on this medium and forms a 

 colony composed o\' streptomycin-resistant 

 individuals, the basis for this resistance 

 306 



clearly being transmissible. Is the adaptive, 

 resistant mutant produced in response to the 

 streptomycin exposure, with the streptomy- 

 cin acting as a directive mutagenic agent? 

 Or, do streptomycin-resistant mutants occur 

 in the absence of streptomycin, sponta- 

 neously, with the streptomycin acting only 

 as a selective agent to reveal the prior oc- 

 currence (or nonoccurrence) of resistant 

 mutants? Or, are both explanations true? 

 Restating the problem more generally, we 

 ask whether mutants adapted to a treatment 

 are postadapted (having arisen after treat- 

 ment), pre adapted (having already been 

 present before treatment), or of both types. 



Clearly, an ambiguous decision results so 

 long as it is necessary to treat the individuals 

 scored with what is being tested — strepto- 

 mycin, in this example — for, under these 

 conditions, one cannot decide whether the 

 resistant mutant had a post- or preadaptive 

 origin. This difficulty can be resolved. If 

 streptomycin-resistant mutants are preadap- 

 tive, they should occur in the absence of the 

 drug and give rise to clones all of whose 

 members are resistant. It should be noted 

 again that the mutation to streptomycin- 

 resistance is a very rare event however it 

 originates. Consequently, one must grow 

 about ten million clones on streptomycin- 

 free agar medium and test each clone for 

 streptomycin resistance by placing a sample 

 of each on a streptomycin-containing me- 

 dium. After this transfer, part or all of one 

 sample is expected to be resistant to the 

 drug. If resistance is due to a preadapted 

 mutant, one can return to the appropriate 

 original clone — which has never been ex- 

 posed to streptomycin — and readily obtain 

 other samples which prove to be resistant. 

 If, on the other hand, the mutant is post- 

 adaptive, additional samples of the original 

 clone will have no greater chance o\' furnish- 

 ing resistants than additional samples taken 

 from different clones. 



Three clone-sampling procedures are 



