A study of bacterial populations with induced nuclear and cellular divisions 



out on sufficiently dense cultures, and Figure \b shows the results obtained with a 

 vigorously aerated culture containing about 30 million bacteria per ml. ; under these 

 conditions cycles of 28 minutes at 25 C. and 8 minutes at 37° C. gave the best 

 results. 



The type of experiment just presented was built up on the basis of preliminary 

 investigations which will be described in more detail elsewhere (Lark and Maaloe, 

 1954). In this connexion we shall mention only two observations of particular 

 significance. Firstly, it was found that the condition of the culture before temperature 



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120 140 



TIME IN 



160 180 



MINUTES 



200 



1 a. Experiment carried out with a non-aerated broth culture with 



about 5000 bacteria per ml. Points represent individual colony 



counts; the intervals between horizontal lines areo-q log units each, 



corresponding to a doubling of the colony counts. The continuous 



curve is theoretical; see text on page 160. 



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



cycling is begun is important. The best results are obtained when a culture, grown to 

 saturation at 37 C, is first diluted one hundred-fold and then aerated lightly for 

 about 75 minutes before the first lowering of temperature takes place. After this 

 preliminary period one ml. of the aerated culture contains 20 to 40 million organisms 

 which have just entered the logarithmic phase of growth. Secondly, a peculiar 

 observation was made concerning the behaviour of a culture when the temperature is 

 raised to 37 C. after the first 25 C. period. If this period is extended to two or 

 more hours, the rise in temperature is followed by a lag of some 20 minutes during 

 which the rate of division is less than or equal to that of the preceding 25 C. period. 

 After the first 20 minutes at 37 C. a sudden rise in colony count is registered before 

 the normal 37 C. growth rate is established. This set of observations is illustrated 



161 



