192 PHYSIOLOGY OF BACTERIA 



six hours for one generation, and slower yet is the 

 organism of Johne's disease. No food has as yet been 

 found to speed up the growth of these organisms to 

 approximately the same rate as saprophytes. In the 

 organism of the host, these bacteria also multiply 

 extremely slowly. 



The data obtained by the plate count method are 

 averages. The growth rate of individual cells varies 

 greatly even if the cell material, as well as the environ- 

 ment is very homogeneous as will be shown on p. 266. 



(h) THE LAG PERIOD 



It has become customary to speak of ^^ growth curves'' 

 of bacteria or yeasts which means curves representing 

 the changes in the numbers of living organisms in a 

 given time. Often, instead of the numbers of the indi- 

 viduals, the logarithms of these numbers are plotted 

 against time. Frequently, these '^growth curves'' 

 include the stage where the individuals die from old 

 age, and sometimes this period is spoken of, and even 

 calculated, as '^negative growth." The conception of 

 death as negative growth cannot be considered a bio- 

 chemical definition; it is used only as a simile, for death 

 is not a reversed growth process. Since death in old 

 cultures will be treated together with other causes of 

 death, this phase will not be considered here. 



In Fig. 42 of Appendix (p. 404) a growth curve of Ps, 

 fluorescens is given. A set of logarithmic growth curves 

 is shown in Fig. 17 representing the multiplication of a 

 Bacterium A at different temperatures, as measured by 

 Max Miiller (1903). The data for these curves are to 

 be found in Table 48. 



In the logarithmic growth curve, the generation time 

 is constant as long as the curve is a straight line. This 



