402 PHYSIOLOGY OF BACTERIA 



An example of the changes of the generation time is 

 given in Table 133 representing the growth of Streptococcus 

 lactis in sterile milk at a constant temperature of about 

 20°C. In the first three hour period, seventy-five to 

 eighty minutes are needed for the cells to double, but 

 soon the minimum time of forty-five to fifty minutes is 

 reached, and the cells divide in less than one hour. 

 This period of most rapid growth lasts only six to twelve 

 hours, and then growth slackens, the generation time 

 increases progressively, and soon reaches infinite 

 which means that the culture has ceased growing 

 altogether. 



Amount of Inoculum : It is not unusual to find state- 

 ments in literature where it is anticipated that by dou- 

 bling the amount of inoculum, the time for completion 

 of the fermentation can be reduced to one-half. In 

 thinking of the geometrical progression of bacterial 

 multiplication, it is evident that this is an error; the time 

 saved by doubling the inoculum is just the time which 

 is required for the inoculum to double, i.e. the time of 

 one generation which is usually not much more then 

 one hour. By using one thousand times the inoculum, 

 the amount of time saved is the time of ten generations, 

 since 2^^ = 1,024. 



We can test this in Table 133 where the two parallel cultures differ 

 only inasmuch as the first was inoculated with one thousand times as 

 many cells as the other. The average generation time for the first 

 few hours is about sixty minutes. It should therefore require ten 

 times sixty minutes or ten hours for the first culture to come to the 

 point where the second culture started, and from then on, there 

 should be no difference between the two. This is actually the case. 

 After nine hours, culture I has 23,000 cells per c.c. which, after ten 

 hours, would have increased to about 50,000. The time saved in the 

 first experiment by using 1,000 times as many bacteria for seeding 

 the culture was about nine and a half hours. 



