OLE MAAL0E AND KARL G. LARK 



would not at the same time affect the rate of cell division. To investigate this possi- 

 bility, we returned to the simple system described earlier (p. 160). A culture was 

 grown overnight at 25 C. from a very small inoculum; when a density of about 

 10 million organisms per ml. had been reached, the culture was aerated for 90 

 minutes before the temperature was raised to 37 C. During these 90 minutes and 

 the following 60 minutes samples were withdrawn for determination of total colony 

 counts and of lysogenization frequency as described above. 



Figure 3 shows the results obtained just before and during the period after the 

 temperature shift. For clearness of presentation, the points corresponding to the 



Z 'o 



o - 



— X 



N Z 



5§ 



ID u 



140 



160 160 



TIME IN MINUTES 



Figure 2. Frequency of lysogenization at various times during 

 synchronous cell division. Points show individual frequency deter- 

 minations based on total cell counts and counts of lysogenic colonies. 

 The broken curve shows on an arithmetical scale the increase in total 

 colony count, and thus corresponds to the growth curves in Figures 

 la and lb. 



counts of lysogenic colonies have been plotted at a convenient distance above the 

 points marking the total cell counts. The lower curve is an ordinary growth curve, 

 representing viable counts, and it exhibits the characteristic lag, followed by an 

 abrupt rise which already has been described (page 161). The upper curve, represent- 

 ing the lysogenic counts, and its relation to the growth curve may be interpreted as 

 follows: before and during the first four to five minutes after raising the temperature 

 to 37 C. the two curves are parallel, which indicates that the lysogenization fre- 

 quency is constant; within the next three to four minutes the lysogenic counts double, 

 and, since the division rate remains very low, this corresponds to approximately a 

 doubling of the lysogenization frequency; then follows a period of 10 to 15 minutes 

 during which the lysogenic counts are almost constant, and during which the lyso- 

 genization frequency gradually decreases as a consequence of cell division; finally, 

 when normal growth has been established at 37 C, the lysogenic counts increase 



164 



