Bacteria : Clones and Mutation 



333 



in order to grow, are said to be auxotrophic. 



It is known that the vast majority of the 

 mutants produced following treatment with 

 a mutagenic agent have nonadaptive or detri- 

 mental phenotypic effects (Chapter 24). 

 Moreover, the detriment of these mutants is 

 clearly not dependent upon the continued 

 presence in the environment of the mutagen 

 which induced them. It is also true that 

 many of the rare mutants that increase adapt- 

 abihty are beneficial in the absence of the 

 mutagen which induced them. However, on 

 rare occasions a mutagen (like X rays) pro- 

 duces a mutant which has an adaptive advan- 

 tage in the presence of the mutagen (resist- 

 ance to the genetic or nongenetic detrimental 

 effects of X rays). Was this adaptive mutant 

 produced fortuitously, or was it a genetic 

 response specifically elicited by the mutagen? 

 The same question can be raised about adapt- 

 ive mutants occurring spontaneously. Are 

 these, by some mechanism, an adaptive ge- 

 netic response to some unidentified factor 

 present in the environment (so that we say 

 the mutant arose for no known reason, and 

 is, therefore, "spontaneous")? What we 

 are asking is whether there is or is not a type 

 of mutant which occurs specifically because 

 it provides adaptiveness to some factor in the 

 environment. 



This general problem can be illustrated 

 with a particular example. There is a strain 

 of E. coli that has, apparently, never been 

 exposed to streptomycin. If such a strain is 

 plated onto agar containing this drug almost 

 all the individuals cannot grow and do not 

 form colonies. These individuals are called 

 streptomycin-sensitive. However, in one par- 

 ticular strain, about one bacterium in 10 

 million does grow to form a colony composed 

 oi streptomycin-resistant individuals, the basis 

 for this resistance clearly being transmissible 

 and adaptive. Was the adaptive resistant 

 mutant produced in response to the strepto- 

 mycin exposure, so that the streptomycin 

 acted as a directive mutagenic agent? Or, 



do streptomycin-resistant mutants occur in 

 the absence of streptomycin, spontaneously, 

 so that the streptomycin acts only as a se- 

 lective agent to reveal the prior occurrence 

 (or nonoccurrence) of resistant mutants? Or, 

 are both explanations true? We can restate 

 the problem more generally and ask whether 

 mutants, adapted to a treatment, arose only 

 after treatment (being postadapted), or had 

 already been present before treatment (being 

 preadapted), or both. 



It becomes clear that an unambiguous de- 

 cision cannot be made so long as it is neces- 

 sary to treat the individuals scored, with the 

 factor being tested — streptomycin, in the par- 

 ticular example under discussion. For, under 

 these conditions, one cannot decide whether 

 the resistant mutant had a post- or preadap- 

 tive origin. This diflRculty can be circum- 

 vented in the following way. If streptomycin- 

 resistant mutants are preadaptive, they should 

 occur in the absence of the drug, and give rise 

 to clones, all of whose members should be 

 resistant. It should be noted again that the 

 mutation to streptomycin-resistance is a very 

 rare event, whatever its mechanism of origin. 

 Accordingly, what we need to do is to grow 

 about 10 million clones on streptomycin-free 

 agar medium and sample each one for strep- 

 tomycin-resistance, by placing each sample 

 on streptomycin. If this is done, at least 

 part of one sample will be resistant to the 

 drug. If the resistance is due to a preadapted 

 mutant, one will be able to return to the par- 

 ticular original clone (itself never exposed to 

 streptomycin) and repeatedly obtain other 

 samples which test as resistant. If, on the 

 other hand, the mutant is postadaptive, return 

 to the original clone will provide additional 

 samples which show no greater chance of 

 furnishing resistants than have additional 

 samples taken from different clones. 



There are three clone-sampling procedures 

 one may use for testing the pre- or post- 

 adaptiveness of mutants. The first method 

 starts with a single (presumably) streptomy- 



