VICTOR G. BRUCE 133 



Scherbaiim and Zeuthcn (18, 23) induced synchronous division in the 

 protozoon Tetrahijmena pyriformis by alternately subjecting an expo- 

 nentially growing culture to 30-minute periods at the optimal temperature 

 of 29° and 30 minutes at the sub-lethal temperature of 33°. Treatment for 

 6 to 10 hours with this cycling causes the cells to grow bigger without 

 dividing, and if they are subsequently returned to a lower temperature 

 (24° j, there follow successive peaks of division activity, about lYz hours 

 apart, separated by periods in which no divisions occur. In some of their 

 early experiments these workers investigated the effect of temporarily 

 lowering the temperature of a culture growing exponentially at optimal 

 temperature. A synchronous burst of divisions (25-30% of the population) 

 occurred after the temperature was raised. However, they were unable 

 to improve the synchrony by repeating the temperature changes between 

 the low (7°) and the optimal temperature. 



Szybalski and Hunter-Szybalska (19), working witli Bacillus mega- 

 ferium, used a method very similar to that used by Hotchkiss in order to 

 obtain synchronous divisions of this bacteria. The temperature of a 

 culture growing exponentially at 34° was lowered to 15° for 30 minutes 

 after which it was again raised to 34°. Two or three cycles of synchronous 

 divisions occurred after the second temperature transfer, with a jirogres- 

 sive loss of synchrony. 



The last example to be cited here in which temperature changes have 

 been used to synchronize divisions is the system described by James (9) for 

 Amoeba proteus. In this system a number of dividing amoebae are manu- 

 ally selected and subjected to a 24-hour temperature cycle of 12 hours at 

 26°, and 12 hours at 18°. The dividing amoebae are started off at the 

 beginning of the warm period and divide again at the beginning of the 

 next warm period 24 hours later. The amoebae divide over a spread of 

 time equal to about 5% of the total generation time and after five or six 

 divisions begin to get out of synchrony again. This method of course 

 combines physical techniques with temperature changes and thus is some- 

 what different from the previous systems. 



DISCUSSION 



The several systems which have been briefly described showing how 

 temperature changes can effect synchronization of cell division raise the 

 question of whether a common mechanism is operating and if so what 

 is the nature of the temperature effect resulting in the synchronization. The 

 main generalization that can be made about the effect of temperature 

 on microbial growth is the accelerating of the division rate, with a Qio 

 of approximately 2 up to an optimum temperature, and with some falling 

 off of the rate at higher temperatures. Speculations have linked this 



