CHAPTER V 



Reproduction i^j* ^ ^^ ^ 



y! 



A: THE GROWTH CYCLE 



(8, 11,20,21, 24,25) 



THE growth cycle of bacteria, whether in culture or under natural 

 conditions, follows a regular pattern of which the main outlines have 

 long been known, although the underlying chemical and cytological 

 changes have more recently been discovered. 



When bacteria are transplanted upon a new medium, suitable for their 

 growth, from an older culture, which has passed its period of active re- 

 production, there is at first an interval of time, known as the lag phase, in 

 which no numerical increase occurs. The bacteria may increase in size but 

 do not divide. The lag phase lasts from one or two hours to six or seven 

 or longer, after which the logarithmic phase commences. The bacteria 

 reproduce by fission, sometimes at very short intervals, and increase in 

 numbers at an approximately logarithmic rate. Much later, aher a period 

 which is a function of food-supply, temperature and the physical conditions 

 in the culture, the rate of growth falls steadily and eventually almost ceases. 

 In the dcchne phase the numbers may actually decrease, although they usually 

 remain static for a long period. 



It has been discovered that the chemical constitution of the cells, and 

 especially their nucleotide content varies according to a similar cycle. Bacteria 

 in aged cultures have a low nucleotide content, which, when they are trans- 

 planted to a new medium, rises to a high level during the course of the lag 

 phase and falls off gradually as the culture ages. The nucleotide content thus 

 corresponds closely to the state of activity o£ the nucleus, and presumably 

 ' indicates the actual level of nuclear material in the cell. 



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