GROWTH 199 



(c) THE DECREASING GROWTH RATE 



Several explanations have been given for the decreasing 

 growth rate, and probably, all of them are correct. 

 It has been stated that increasing accumulation of 

 fermentation products decreases the rate of multiplica- 

 tion. In Table 133, p. 400, the generation times for 

 Strept. ladis in milk are given for each three-hour period, 

 and also the amounts of acid formed. The generation 

 time is slowest, i.e., growth is most rapid just before the 

 acidity begins to become noticeable. But it does not 

 seem probable that an increase of only 0.005% of lactic 

 acid could decrease the growthrate 26% (culture I) 

 or that 0.007% increase of acid could cause a drop of 

 40% (culture II). Nor is this beginning retardation 

 likely to be caused by lack of food because addition 

 of 1% peptone did not change the numbers of bacteria 

 of this very strain appreciably (p. 210). 



With other streptococci, however, cessation or retarda- 

 tion of growth may be due to lack of food. It has been 

 shown on p. 149, that many streptococci do not find 

 enough available nitrogenous food in milk to produce 

 maximal growth. Addition of peptone increases the 

 final number per c.c. 



Attention has been called already (p. 80) to the very 

 rapid disappearance of oxygen in cultures of aerobic 

 bacteria, and to the fact that only on the very surface 

 layer, cells can multiply rapidly after this stage has been 

 reached. These facts will be discussed in more detail 

 later, in the chapter on the concentration of food. 

 Lack of food, then, may be the cause of retardation of 

 growth in some cases, but cannot be considered so 

 generally. The growth rate frequently decreases before 

 any appreciable amounts of food have been used up, 



