210 



PHYSIOLOGY OF BACTERIA 



that peptone is an excellent food for both energy formation and 

 building material. However, we really know very little about 

 either. For the non-liquefying organisms commonly used for labora- 

 tory experiments we scarcely know which compounds of peptone 

 they can use, and which are indigestible for them. Some of these 

 bacteria may be able to use only a very small portion of the peptone 

 given, and though 1% peptone may seem ample food, it is very 

 little, if 95 % of it indigestible. 



Most streptococci do not find sufficient food for optimal growth 

 in milk, and grow to higher final numbers if peptone is added to the 

 milk (see p. 149). That the rate of growth is also influenced may be 

 seen from the following data of Rahn (1911) : 



Table 54. — Generation Times of Streptococci in Milk 



Strain II must have been able to use the nitrogenous material 

 of milk readily for construction while Strain IV was plainly starving 

 for available nitrogen in the midst of all the milk protein. 



With liquefiers, we are more certain about their being able to 

 utilize most of the peptone and meat extract. 



Attention has already been called to the fact that a 

 change in the type of substrate for fermentation may 

 cause a more pronounced change in growth rate than an 

 increase in concentration. Such a change of the growth 

 rate by a change of source of energy is very typically 

 shown in Fig. 20 representing the yeast growth and sugar 

 consumption in aerated wort as found by Balls and 



