DELAYED EXPRESSION OF MUTATIONS 473 



that the growth rate is higher in the mutant clones than in the parent strain — 

 is ruled out by the results obtained using method 3. The only plausible in- 

 terpretation, therefore, seems to be that a mutation to resistance is not pheno- 

 typically expressed until after a number of divisions have taken place. 



Two possibilities exist with regard to such a delay: (1) that after a fixed 

 period of one or more generations all of the offspring of the original mutant be- 

 come phenotypically resistant at the same time, and (2) that the delay is vari- 

 able within the mutant clone, some lines of descent becoming resistant earlier 

 than others. As pointed out by Luria (1946), the presence of cultures having 

 only one resistant bacterium eliminates the first of these. Thus the delay must 

 be variable within the mutant clones, and a single individual may develop 

 phenotypic resistance in the first instance, being followed later by other mem- 

 bers of the clone. 



As pointed out, the presence of phenotypically susceptible individuals in the 

 younger mutant clones, and the occurrence of changes in these to phenotypic 

 resistance, results in a high apparent mutation rate when the members of the 

 clone are dispersed and the changes are thus rendered detectable in each in- 

 dividual, and in a low apparent mutation rate when the members of a clone 

 are grouped together and only the first change to phenotypic resistance is 

 detectable. In the early growth on agar a situation exists in which the members 

 of mutant clones in the inoculum are dispersed over the plate but the products 

 of subsequent divisions of individual bacteria are grouped together. In such 

 circumstances the apparent mutation rate would be expected to be high during 

 the first division, as in liquid cultures, and to decline during subsequent genera- 

 tions to the apparent rate obtained when only the numbers of resistant clones 

 are considered. This, in fact, has been observed and lends support to the con- 

 clusions reached. 



In this connection, however, it should be noted that the rate of appearance 

 of resistant bacteria during the first division on agar following a resting stage 

 is higher than expected, by a factor of approximately three or four. This does 

 not affect the main line of reasoning, although it is of interest in itself and ap- 

 pears to be well substantiated by the data. 



Attempts to interpret this as due to the bacteria's failing to adsorb phage 

 during the early stages of growth after a resting stage have not been successful, 

 since one would have to suppose complete failure of adsorption over a period of 

 at. least two generations. 



Two possibilities remain: (1) that of a genuine high rate of mutation during 

 this division, together with physiological conditions favoring immediate pheno- 

 typic expression, since otherwise the products of these mutations would not be 

 detectable; and (2) that of physiological conditions favoring phenotypic ex- 

 pression of mutants which had not hitherto become resistant. So far, no 

 methods have been devised to distinguish between these two possibilities. 



Thus the rate of appearance of resistant clones during early growth on agar 

 is in agreement with the concept of a delayed phenotypic expression, although 

 an additional phenomenon appears to be involved. 



The size of a mutant clone at the time of appearance of the first resistant in- 



75 



