134 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



temperature influence to the effect of temperature on a single enzymatic 

 reaction, postulating the falling oft' at higher temperatures as being due to 

 thermal denaturation. It may be that in some cases the temjierature de- 

 pendence of growth rate does correlate with a single reaction but this is 

 prol)ably not generally the case. The effect of sudden temperature changes 

 on the metabolism or growth has been investigated hardly at all and in 

 considering the physiological effects one should bear in mind the magni- 

 tude of the temperature changes with respect to optimal temperature and 

 normal temperature range. Although there is outwardly considerable dif- 

 ference between the synchronization systems, it does seem that there may 

 be a ciualitatively similar effect of temporary exposure to a non-optimal 

 temperature on a particular stage of the division cycle. 



Hotchkiss (6) in interpreting the synchronization of Pneumococcus ex- 

 jiressed the idea that the sudden cooling, to a temperature well below the 

 optimum, of an exponentially growing culture might selectively slow 

 down, or even stop, the metabolism of that fraction of the cells which had 

 not passed a definite stage of the division cycle thus giving an opportunity 

 for different cells to come into phase with each other. The later demonstra- 

 tion by Szybalski and Hunter-Szybalska (19) that an almost similar 

 method will work with B. inegaterium shows that the mechanism is not 

 specific to a single organism. Furthermore, similar though smaller effects 

 occur if S. typhiniurium or Tetrahymena are subjected to a similar type 

 of temperature treatment. Nevertheless, it should not be concluded that 

 this represents a universal response of microorganisms to this type of tem- 

 perature treatment. Experiments of this type by the author, using various 

 strains of E. coli, failed to disclose any synchronization. Different types of 

 temperature treatment can be effective in accomplishing the same net end 

 result, e.g. selectively blocking a specific step in the division cycle. This 

 is nicely illustrated by the experiments on Tetrahymena which involved 

 a relatively complex temperature pretreatment in which the temperature 

 was raised from the optimal level. Zeuthen (22) expressed the opinion that 

 certain fractions of the cell jiopulation were differently affected by raising 

 the temperature to the sublethal level. Dividing cells continue division and 

 growth, cells hit between divisions continue synthesis; however cells hit in 

 some stage just prior to division continue growth but do not for a time 

 enter division. 



Zeuthen's suggestion that the step which is blocked is one just before 

 the cells enter into division is consistent with the results obtained with 

 Pneumococcus inasmuch as the first synchronized burst of divisions in 

 the Pneumococcus system occurs a few minutes after returning the cul- 

 ture to 37° from the cold. With B. inegaterium, on the other hand, there 

 is a lag following return of the culture to optimal temperature from the 



