OLE MAAL0E AND KARL G. LARK 



by the lower curve in Figure 3. It should be noted that the lag and the subsequent 

 rise may easily be overlooked. As illustrated in Figure 3, the rise causes the linear 

 portions of the growth curve to intersect near the zero-time axis, which means that 

 the lag period will be registered only if a sufficiently large number of counts are made 

 during the first 30 minutes after the temperature increase. The observations just 

 described are, as we shall see later, of great significance for the experiments with 

 bacteriophage and for the cytological studies to which we shall turn next. 



The phage-sensitive strain of Salmonella typhimurium used for this investigation was 

 obtained from Dr. Lwoff together with the two closely related bacteriophage strains 



(/) 



I- 

 z 



D 

 O 



u 



> 

 z 

 o 

 _J 

 o 

 u 



o 

 o 



100 120 140 160 180 



TIME IN MINUTES 



200 



ib. Experiment carried out with a vigorously aerated broth 

 culture with about 3 X io 7 bacteria per ml. For explanation 

 see above. 



Figures la and lb. Growth curves showing step-wise cell divisions. 



A and Ac. The ways in which these phage strains interact with the host cells must be 

 briefly reviewed. Infection of a sensitive bacterium with the yl-phage leads to one of 

 the two following results : either, the J-phage multiplies in the cell and this eventually 

 lyses, or the infected cell survives and multiplies, and in that case it and its descend- 

 ants have become immune both to the A- and to the ylc-phage (Lwoff, Kaplan and 

 Ritz, 1954). The surviving cells give rise to pure cultures in which occasionally a cell 

 lyses and liberates a large number of new yl-particles; a culture of this kind is there- 

 fore called lysogenic. To account for the remarkable stability of such lysogenic 

 cultures it must be assumed that the phage particle infecting the parental cell is 

 completely integrated in the cell structure and that, from then on, it multiplies as 

 part of the cell. In the integrated state, the phage particle is referred to as prophage, 



162 



