76 



THE FORMATION OF ENZYMES IN BACTERIA 



If we count the number of viable cells {i.e. the cells capable 

 of further division) rather than the total number of cells 

 present, we find a similar curve (Fig. 5), though the number 

 of viable cells is always less than the total number of cells 

 present. Following the maximum stationary phase, during 

 which the number of new cells is balanced by the number of 

 dying cells, we get a falling off in numbers as the cells die 

 at an increasing rate. 



If, however, we estimate growth by cell-mass rather than 

 by cell-numbers, we get a different curve, as shown in Fig. 5, 



I 



PHASE I PHASE 2 PHASE 3 PHASE 4 PHASES 

 AGE OF CULTURE »- 



Fig. 5. 



which shows no initial stationary or lag phases but a steady 

 increase in mass until growth ceases. The difference between 

 the two curves lies in the fact that the size of the cells is not 

 the same throughout the growth period. When the inoculum- 

 cells enter the new medium they begin to grow in size, i.e. in 

 cell-mass, but do not divide and so give rise to the stationary 

 phase of cell-numbers. The cells do eventually divide, but 

 whereas they may divide at a limiting size x in the logarithmic 

 phase of growth, they will grow to a size considerably larger 

 than X before division occurs in the lag phase of growth. 

 Consequently we have a steady increase in cell-mass, but 



