232 



PHYSIOLOGY OF BACTERIA 



concentration of food, the total crop shows distinct 

 relation to the quantity of food (see next chapter). 



Probably the most common assumption is that the 

 accumulation of the products of fermentation prevents 

 further growth. This can be most easily proved by 

 removing the fermentation products from a full-grown 

 culture. This has been done by Rahn (1911) by 

 neutralizing the acid in milk cultures of two streptococci. 

 The result was increased growth with Strain II, but no 

 increase with Strain IV. 



Table 62. — Increase in Numbers of Cells of Acid Forming 

 Streptococci after Neutralization of Culture 

 (Numbers indicate millions per c.c.) 



Strain II is not affected by peptone (see p. 210). 

 It is capable of utilizing the protein matter of the milk, 

 and its growth limit is primarily fixed by the hydrogen 

 ions. If these are removed, growth continues until 

 another inhibiting factor intervenes. 



Strain IV does not find sufficient building material 

 in milk; the final number of cells is increased if peptone 

 is added. Neutralizing a culture which contains no 

 peptone does not further the growth because there is no 

 building material. Lack of food is the limiting factor 

 of Strain IV, fermentation products are the limiting 

 factor of Strain II. 



Rogers and Whittier (1928) give the following final 

 crops for their strains of Strept. lactis: 



