CHAPTER V 



GROWTH CURVES OF BACTERIA 



R. E. BUCHANAN 



Iowa State College 



The following discussion of growth rates of bacteria and their graphical representa- 

 tion in growth curves will be concerned solely with rates of increase. 



CHARACTERISTICS OF GROWTH CURVES 



The general characteristics of growth curves may be developed through considera- 

 tion of the changes in numbers of bacteria which follow inoculation into a medium 



suitable for growth. For the purpose 

 of preliminary discussion it is advan- 

 tageous to assume that the inoculum 

 consists of bacterial spores, for such 

 material will permit of the maximum 

 opportunity for differentiation of 

 stages or phases of growth. 



Examination of such a culture at 

 suitable intervals will show that an 

 appreciable time elapses before any 

 increase in numbers occurs, i.e., some 

 time is required before any of the 

 spores germinate and vegetative cells 

 develop and divide. This may be 

 termed the "initial stationary phase." 

 It is scarcely to be anticipated that 

 all the spores will germinate at the 

 same instant. However, after cell 

 division has been initiated it will pro- 

 ceed with a considerable degree of 

 regularity. Finally, all the viable 

 spores will have germinated, and the 

 culture will have completed its second or "lag phase." For a time thereafter the 

 numbers of cells will increase more and more rapidly, with the rate of growth per cell 

 remaining nearly uniform. This is the third or "logarithmic phase," during which 

 there is a geometrical increase in cells with time. Conditions eventually become less 

 favorable, and the rate of growth decreases. This is the phase of "negative growth 

 acceleration." Finally the bacteria cease to multiply, and the "maximum stationary 

 phase" is instituted. It is thus possible under favorable conditions to differentiate 

 some five different growth phases. 



46 



a 10 jz J4 

 Time in Hours 



20 22 2^ 



Fig. I. — Growth curve with various phases 



a-h. Initial stationary phase 



h-c. Lag phase 



c-d. Logarithmic phase 



d-e. Phase of negative acceleration 



e-/. Maximum stationary phase 



