GROWTH 



191 



If growth is determined by any method other than counting cells 

 per c.c, the growth rate can be computed by the same formula, 

 just as the formula for the generation time would apply. As an 

 example, Slator (1916) measured the growth rate of Lact. Delbrucki 

 of which plate counts would give no accurate measure, by comparing 

 the turbidity of the culture with a standard asbestos suspension, and 

 obtained the following results: 



The average for 0.434K = 0.84 corresponds to a generation time of 

 21.5 minutes (see also the computations for yeast growth p. 211). 



Slator went even further than this and claimed that, since the 

 fermentation is proportional to the number of cells, the growth rate 

 can be computed from the increase of the fermentation products. 

 This will be permissible only if all conditions are entirely under control 

 for we have seen (p. Ill) that the rate of fermentation is easily 

 influenced by the products already formed. This same principle 

 has been used as early as 1877 by Nageli and Schwendener to deter- 

 mine the number of cells. Meyerhof (1917) used it to compare the 

 numbers of active cells in Nitrosomonas cultures. 



The growth rate of bacteria under optimal conditions 

 is very large. With common saprophytic bacteria 

 in a suitable medium at room temperature, the genera- 

 tion time is about one hour. At high temperatures and 

 in excellent media, especially with a good source of 

 energy, the generation time may drop to fifteen minutes. 

 This means that one cell can multiply to 16 cells in one 

 hour. This rapid rate can be maintained only for a 

 few hours. With yeast, the fastest rate is about 1 hour. 



However, some microorganisms grow very slowly. 

 Among the slowest is My cob. tuberculosis requiring about 



