5 o2 S. E. LURIA AND M. DELBRUCK 



from each other or from the sensitive strain in their fermentation reactions on 

 common sugars and in the characteristics of their growth curves in nutrient 

 broth. In particular, the lag periods, the division times during the logarithmic 

 phase of growth, and the maximum titers attained are identical for the sensi- 

 tive strain and for the two variants. Both variants, therefore, fulfill the require- 

 ments for the applicability of the theory developed above. 



In the presentation of our experimental results we have lumped the counts 

 of the two types of colonies together, because: (i) theoretically, this is equiva- 

 lent to summing the corresponding mutation rates; (2) experimentally, we 

 are not certain whether each of these types does not actually comprise a diver- 

 sity of variants; (3) experimentally, no correlation appeared to exist between 

 the occurrence of these variants, which shows the independence of the causes 

 of their occurrence. 



Cultures of B were grown either in nutrient broth (containing .5 percent 

 NaCl) or in an asparagin-glucose synthetic medium. In the latter, the division 

 time during the logarithmic phase of growth was 35 minutes, as compared 

 with 19 minutes in broth. In synthetic medium, the acidity increased during 

 the time of incubation from pH 7 to pH 5. 



In cultures of strain B, between io~ 8 and io~ 6 of the bacteria are found 

 usually to give colonies resistant to the action of virus a when samples of such 

 cultures are plated with large amounts of virus. In order to be reasonably cer- 

 tain that the resistant bacteria found in the test had not been introduced into 

 the test culture with the initial inoculum, the test cultures were always started 

 with very small inocula, containing between 50 and 500 bacteria from a grow- 

 ing culture. Thus any resistant bacterium found at the moment of testing 

 (when the culture contains between io 8 and 5X10 9 bacteria/cc) must be an 

 offspring of one of the sensitive bacteria of the inoculum. 



All platings were made on nutrient agar plates. The plating experiments for 

 counting the number of resistant bacteria in a liquid culture of the sensitive 

 strain were done by plating either a portion or the entire culture with a large 

 amount of virus a. The virus was plated first, and spread over the entire sur- 

 face of the agar. A few minutes later the bacterial suspension to be tested was 

 spread over the central part of the plate, leaving a margin of at least one cen- 

 timeter. Thus all bacteria were surrounded by large numbers of virus par- 

 ticles. 



Microscopic examination of plates seeded in this manner showed that lysis 

 takes place very quickly; only bacteria which at the time of plating were in 

 the process of division may sometimes complete the division. The resistant 

 colonies which appear after incubation are therefore due to resistant bacterial 

 cells present at the time of plating. 



The total number of bacteria present in the culture to be tested was deter- 

 mined by colony counts in the usual manner. 



The resistant colonies of the large type appear after 12-16 hours of incuba- 

 tion, the colonies of the small type appear after 18-24 hours, and never reach 

 half the size of the former ones. Counts were usually made after 24 and 48 

 hours. 



