VICTOR G. BRUCE 



129 



The description of the division cycle seems appropriate also for bacteria, 

 although here of course the evidence is necessarily more inferential since 

 single cell measurements cannot be made. Suggestive evidence that the 

 principal growth takes place only during a part of the division cycle 

 derives from the fact that turbidity changes in a synchronized culture of 

 Sahnonella typhimurium increase in a step-wise fashion (2) as shown in 

 figure 1. Thus, if little or no growth takes place during a considerable 

 part of the division cycle, then there will be no turbidity increase during 



-20 



1.8 UJ 



1.4 



12 



9 



a. 



3 



O 

 O 



0.8 



20 40 60 80 100 120 140 

 TIME IN MINUTES 



160 180 



Fig. 1. Repeated tomperature cycling experiment. An overnight 37° culture of »S. 

 typhimurium was diluted 1:100 in broth/5, grown with aeration from t = —90 min. 

 to t = min. at 37°, and tlien subjected to alternate cycles of 25° and 37°. The hot 

 or cold broth, added to raise or lower the temperature of the culture, maintained the 



titer of the culture between 2 and 4 X 10" cells/ml A, , compound exponential 



growth curve constructed from the normal generation times of S. typhimurium in 



broth/5 at 25° and 37°; B, , turbidimetric reading in Klett colorimeter; C, 



, viable titer mea.sured as colony count. Turbidity and colony counts have been 



corrected for culture dilutions. 



the comparable part of the synchronized cycle. If this interpretation is 

 valid, then one inference would be that in an exponentially growing culture 

 only a part, perhaps only 30 to 50% of the cells, will be actively synthe- 

 sizing protein. The addition to such an exponentially growing culture of 

 the tryptophane analogue 5-methyl-tryptophane (51MT) might be expected 

 to stop the growth of all cells which are still actively synthesizing protein 

 and to allow those cells which are in the later phases of the division cycle 

 to proceed toward division and perhaps to divide once in the presence of 

 the 5MT. If, after roughly one half of the generation time has elapsed 

 in the presence of the 5MT, an excess of tryptophane (T) is added, then 

 one could expect a resumption of protein synthesis approaching asymp- 

 totically the exponential rate. One would also expect the colony count 



