CHAPTER II 



THE RATE OF GROWTH OF BACTERIA 



". . . .It is perfectly true that all changes in form, inasmuch 



as they necessarily involve changes of actual or relative magnitude, 



may in a sense be properly looked upon as phenomena of growth; and 



it is also true, since the movement of matter must always involve an 



element of time, that in all cases the rate of growth is a phenomenon 



to be considered." _,, _ 



D Arcy Thompson 



The outstanding fact established by my studies is that bacteria 

 are continually varying in morphology with increasing age of the cul- 

 ture in which they are growing, and that the morphologic variations 

 are correlated with the rate of growth, the transition from one type 

 to another occurring at the points of inflection between phases of 

 the growth curve. Before discussing these morphologic variations, 

 therefore, it is desirable to devote some space to a review of what 

 is known concerning the growth curves of bacteria and the factors 

 which determine them. 



When we place viable bacteria in a suitable medium they gen- 

 erally do not commence to grow at once, but after a longer or shorter 

 period of quiescence they begin to divide, at first rather slowly, then 

 with greater and greater rapidity until a maximum growth rate is 

 attained, after which they continue to divide more and more slowly 

 until growth ceases altogether. After a longer or shorter period 

 during which the number of cells remains practically constant, they 

 begin to die, the rate of death becoming greater and greater until 

 a maximum death rate is attained, then becoming slower and slower, 

 some cells remaining viable in most cultures for long periods of 

 time. Thus if we plot a curve of the number of living cells in the 

 culture, against time, there is obtained a characteristic S-shaped 

 curve of growth, which is repeated in reverse during the period of 

 death. 



Such growth curves for populations of microorganisms in cul- 

 tures are essentially similar to the curves obtained for the growth 



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