96 



S. E. LURIA AND R. DULBECCO 



took place, dilutions were made to bring the total dilution of the irradiated 

 sample to a constant value, and an aliquot plated for plaque count. The plaque 

 counts represent infected bacteria that liberate active phage. Although the 

 total dilution of the irradiated phage on all plates is the same, it is seen that 

 the plaque counts are higher when bacteria have first been placed in contact 

 with a more concentrated phage lysate. 



This means that bacteria may produce active phage if they pick up the ir- 



Table 2 



Dependence of plaque counts on irradiated phage T6r on the concentration 

 of the phage sample that is mixed with bacteria 



A sample of phage T6r containing 1.5X101° particles/ml was irradiated for 20 seconds. The 

 bacterial suspension (B) contained 2X10^ cells/ml. Each plate received 0.05 ml of phage dilution 

 and 0.2 ml of suspension (B). 



0.1 ml T6r-^Q.9 ml (B); kept 10 min. 



at 37°C; diluted 1 : 10^, 0.05 cc plated 1 : 10 



1:10^ 



1318 



0.1 ml {T6r 1:10) -^0.9 ml (B); kept 

 10 min. at 37°C; diluted 1:10^, 0.05 

 ml plated 



0.1 ml {T6r l:103)-»0.9 ml (B); kept 

 10 min. at 37°C; diluted 1:10, 0.05 

 ml plated 



0.05 ml {T6r 1:10^) plated 



1:10^ 



1:10* 



Less than 



1:105 

 (on plate) 



1:10* 



1:10* 



1:10* 



474 



250 



57 



radiated particles from a concentrated phage suspension, but not from a dilute 

 one. The immediate explanation is that from a concentrated lysate the bac- 

 teria receive some other "factor," which, inside the bacterium, somehow 

 reactivates an "inactive" particle and which is not present in dilute lysates. 

 An "inactive" particle can be defined as one that has lost the ability to initiate 

 production of active phage unless adsorbed by a bacterium together with the 

 unknown "factor." 



Reactivation still occurs after storage of irradiated phage for weeks in an 

 ice-box. Reactivation gives rise to fully active phage particles. This can be 

 proved either by sampling phage from the plaques or by letting the bacteria, 

 in which reactivation occurs, lyse in liquid and then testing the lysate for 

 active particles. 



250 



