144 PHYSIOLOGY OF BACTERIA 



Rahn (1911) neutralized a milk culture of Strept. lactis repeatedly 

 until the acid formation became very weak. The results given in 

 Table 29 show a gradual decrease of the fermenting capacity. 

 The plate count showed a decrease with Strain II from about 2,200 

 million cells to 1,700 million after the seventh neutralization, and to 

 820 million after the eighth neutralization. With Strain IV, the 

 bacteria even dropped to 40 million after the fourth neutralization. 

 The fermenting capacities are not altogether comparable because 

 some of them had to be computed from a period of forty-eight hours, 

 while most of them are given for a twenty-four hour period. The last 

 datum of each series is high because it is based on the rapidly declining 

 plate count, and it is quite probable that many cells which have lost 

 the power to make colonies will still continue to ferment. This has 

 been observed by Rubner with yeast (p. 13). 



By both strains, fermentation is resumed rapidly after each 

 neutralization (see also Table 62, p. 232 for Strain II), but upon 

 repeated neutrahzation, they lose gradually the power to ferment. 

 Strain IV much faster than Strain II. The total acid formed shows, 

 by a simple calculation, that not all of the lactose can have been 

 fermented. At least 1 % must have remained in the medium. 



The limiting factor in fresh cultures is the hydrogen 

 ion concentration. In a strongly buffered medium, 

 such as milk, much more lactic acid is formed before the 

 limiting pH is reached than in a medium with less 

 buffer, such as whey or lactose broth. The titratable 

 acidity of these latter cultures is, therefore, much lower 

 than that of milk cultures, though the pH is the same. 



Van Dam (1922) and Holwerda (1921) proved, later, 

 that the undissociated lactic acid molecules also have an 

 inhibiting effect. If lactates are added to milk cultures, 

 the fermentation stops at a definite concentration of 

 undissociated lactic acid, and not at a constant pH. 

 This is the explanation of the decreasing rate of fermen- 

 tation after repeated neutralization. 



Rogers and Whittier (1928) have confirmed these 

 explanations. They grew pure cultures of Strept, lactis 



