GROWTH PHASES 75 



parasitic existence in man. It does not follow that this is 

 the optimum temperature for bacterial metabolism. In fact, 

 many soil organisms cannot grow successfully at temperatures 

 as high as 37° C. A few studies have been made of the effect 

 of growth temperatures on enzyme constitution, and it has 

 been shown, for example, that the amino-acid decarboxylases 

 of some strains of Escli. coli are formed to a greater extent 

 when growth occurs at 20° C. than when at 37° C. Several 

 workers have shown that the efficiency of protein synthesis 

 increases as the temperature falls. 



THE AGE OF THE CULTURE 



When an organism is inoculated into a suitably nutrient 

 medium, it begins to increase in size until, in due course, the 

 enlarged organism divides into two daughter- cells apparently 

 similar to the mother-cell. This process will go on until 

 some nutrient in the medium is exhausted. The growth 

 process can be followed experimentally in two main ways: 

 by counting the number of cells per ml. of medium, or by 

 determining the mass of cell-material (measured as cell- 

 nitrogen, cell-carbon, dry weight of cell-material, etc.). If we 

 record the amount of growth against time, we find that we 

 get curves of different shape if we measure growth by cell- 

 numbers or by cell-mass, as shown in Fig. 5. 



If we follow the increase in cell-numbers with time we find 

 that the curve can be divided into a number of phases. 

 Starting from the time of inoculation, we get 



1. an initial stationary phase during which no increase in 

 cell-numbers takes place; 



2. a lag phase during which the rate of multiplication 

 increases with time; 



3. a phase of logarithmic growth when the rate of multiplica- 

 tion is constant; 



4. a phase of negative growth acceleration during which 

 the rate of growth decreases with time; 



5. a maximum stationary phase. 



