Origin of Bacterial Variants 



Numerous bacterial variants are known which will 

 grow in environments unfavourable to the parent 

 strain, and to explain their occurrence two conflicting 

 hypotheses have been advanced. The first assumes 

 that the particular environment produces the observed 

 change in some of the bacteria exposed to it, whereas 

 the second assumes that the variants arise spontan- 

 eously during growth under normal conditions, tho 

 part played by the adverse environment being purely 

 selective. These are known respectively as the 

 'adaptation' and the 'spontaneous mutation' hypo- 

 theses. 



In order to discriminate between them an experi- 

 mental approach (the 'fluctuation test') was developed 

 by Luria and Delbriick in 1943 l . This test has been 

 applied to a number of variants, some of them in 

 widely separated strains of bacteria (see Table ] ) . 

 and in each case the conclusion reached has been 

 that the variant arose by spontaneous mutation. The 

 validity of the fluctuation test has not been challenged, 

 at least so far as I am aware ; but on the other hand 



Bacterial variations shown by means of the fluctuation 

 test to arise through spontaneous mutation 



it has gained only limited recognition (see ref. 2). 

 In part this may be due to the statistical and essen- 

 tially indirect nature of the argument on which it 

 is based, and if so the more direct experimental 

 evidence described below would seem to be of value. 

 Bacteria of Escherichia coli strain Bjr susceptible 

 to phage Tl were plated on agar and incubated until 

 a limited population increase had taken place. On 

 alternate plates the bacteria were redistributed over 

 the surface of the agar by spreading with 0-1 c.c. 

 of sterile saline. All were then sprayed with phage T\ , 

 and counts made of the colonies of resistant survivors 



[Reprinted by permission of MacMillan & Co. Ltd. from Nature 164 : 150, July 23, 1949] 



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