44 



J. H. NORTHROP 



VOL. 12 (1953) 



The results show that the cells reached log growth in about an hour and a half. 

 The phage growth rate and cell growth rate are already equal and remain equal through 



6 Hours 



Fig. 4. Changes in P/ml, i?/ml, RNA/ml, and protein/ml 

 in suspensions of lysogenic megatheyiuin Sgqa in 5% 

 peptone kept in log growth by repeated dilution. Cells 

 from 18 h 5% peptone slant washed in 5% peptone, 

 diluted to I • lo^B/ml. Stand 25°, 18 h and then shake at 

 34*^. Suspension diluted 1/5 with fresh 5% peptone as 

 soon as B/ml = 500-10*. 



Fig. 5. Changes in free phage 

 and Bjml in suspensions of 

 megathermin sensitive + 

 phage T. Cells from 18 h 5% 

 peptone agar slant washed 

 off in 5% peptone and dilu- 

 ted to 4- 10' cells/ml. 80 "T" 

 phage particles/ml added 

 and then tubes shaken at 

 34°. Suspension diluted 1/5 

 when S/ml = 2-10*. 



all the successive growth steps. The RNA and protein growth rates are also equal to 

 the cell and phage growth rates. In other words, the composition of the cells remained 

 exactly the same with respect to phage, protein and RNA throughout the experiment 

 and would continue to do so indefinitely until some change in the media occurred, or a 

 new type of cell appeared. The fact that such a constant ratio of P/B would result in a 

 lysogenic culture was pointed out by Delbruck". 



Rate o\ phage production and cell groiuth with sensitive cells 



The results of an experiment in which T phage was added to a culture oimegatheriiim 

 sensitive, growing in 5% peptone, are shown in I'ig. 5. In tins case, the phage growth 

 rate remains at its original higli value throughout, and as a result the ratio of PjB 

 continues to increase very rapidly. When this ratio reaches about 50/1, lysis occurs. 

 Lysis caused by such high PjB ratios is not accompanied by an increase in the number 

 of phage particles (Northrop and Krueger^), (Northrop'^), (Delbruck*'). This explains 

 the fact that there is no change in slope of the P curve when lysis starts. 



References p. 50. 



