42 



J. H. NORTHROP 



VOL. 12 (1953) 



cation of the phage cannot be followed continuously, since it occurs inside the cells 

 and an infected cell gives rise to one plaque, no matter how many phage particles it 

 contains. The original and final number of phage particles is therefore all that can be 

 determined. All the cells liberate phage at about the same time. The variation of the 

 phage yield with the rate of growth of the cells must, therefore, be due to changes in the 

 number of particles formed per cell, and not to changes in either the fraction of the total 

 number of cells which liberate phage or to variation in the time at which the phage is 

 liberated. In this case the rate of increase is not proportional to the free phage concen- 

 tration and adding different amounts of phage has no effect on the yield, provided all 

 the cells are infected at the beginning of the experiment. 



In the present experiments it has been found that the same relation between phage 

 growth and cell division holds for lysogenic megatherium cultures (Megatherium 899a) 

 (De JONG=*). 



During the lag period, the growth rate of the phage is much higher than that of the 

 cells, as in the case with the phage-sensitive cell system. As soon as the lysogenic culture 

 enters the range of log growth, however, the phage growth rate suddenly drops to 

 exactly that of the cells, so that, after this, the ratio of PfB remains constant as long as 

 the system remains in log growth. 



EXPERIMENTAL RESULTS 



Resting cells of lysogenic B. megatherium 899a were added to 5% peptone and the 

 suspensions shaken at 34° C. The free phage, cells, 

 cellular protein and cellular RNA per ml of the sus- 

 pension were determined at various times. 



The results of such an experiment are shown 

 in Fig. I in which the various values have been 

 plotted (on a log scale) against the time. 



The curves for protein per ml and cell count 

 per ml are nearly parallel throughout, so that the pro- 

 tein per cell remains nearly constant [cf. Caldwell, 

 Mackor and Hinshelwood^ PRiCE^).The RNA/ml 

 increases more rapidly at first than the cell count 

 and the P/ml very much more rapidly. All the curves 

 become parallel during the log-growth phase and 

 then flatten out. The first part of the phage curve 

 is autocatalytic (logarithmic) * and has a much stee- 

 per slope than do the other curves. The increase in 

 phage, in this part of the curve, may, therefore, be 

 written dP/d/ = KpP, where P is the external phage. 

 If the value of P is varied by adding more from an 

 outside source, however, the equation fails, since 

 the added phage has no effect on the rate of forma- 

 tion of new phage (Fig. 2). Since the value for P/ml 



This fact does not contradict the assumption that 

 individual cells liberate phage particles in bursts {cf. Bur- 

 net25, Northrop^). 



References p. 50. 



Fig. I. Concentration of free phage 

 (P), RNA, protein, and cells in a 

 suspension of megatherium. 899a, 

 growing in 5% peptone. Cells from 

 an 18 h 5% peptone-agar slant 

 suspended in 10 ml 5% peptone, 

 centrifuged, the cells added to 100 ml 

 5% peptone. 10 ml suspension put 

 in each of 10 2 x 20 cm test tubes 

 and the tubes shaken at 34°. i ml 

 samples taken from each tube at the 

 time indicated and combined. The 

 combined sample was then analyzed 

 in duplicate sets as described under 

 experimental prodecure. 



