154 PHYSIOLOGY OF BACTEKIA 



cooling to — 1°C. does not seem to injure them, and 

 they produce lactic acid at a slow rate, as would be 

 expected, but still at a rate reasonable for such a low 

 temperature. But the rate of fermentation soon begins 

 to decrease; something seems to interfere. It can- 

 not be the fermentation products because only 0.3% 

 have been formed when the fermentation ceases alto- 

 gether. Nor are other products interfering for fermen- 

 tation begins again at higher temperatures and reaches 

 a normal endpoint. It must be that some change in 

 the cell prevents fermentation. This may be the 

 deterioration of the fermenting enzjnne which cannot 

 be regenerated at temperatures below the minimum of 

 growth. Above this temperature, the enzyme is grad- 

 ually reconstructed, and normal fermentation starts again. 



Some doubt in this explanation is justified. The endpoints of 

 growing cultures, or of heavily seeded cultures just above the growth- 

 minimum, seem to have a lower endpoint than at higher temperatures 

 while according to the above assumption, it should be at least as 

 high or higher. 



This is shown by an experiment with Strept. lactis. Four milk 

 cultures were kept at 30°C. until they were just beginning to coagu- 

 late. Then, they were cooled rapidly in water, one to — 1°C., one 

 to 5°C., one to 10°C., while the last one remained at 30°C. They 

 required 5.5 c.c, 5.8 c.c, 5.7 c.c. and 6.1 c.c. of n/10 normal KOH 

 for neutralization, immediately after cooling. Fourteen days 

 later, the neutralization required 6.1 c.c, 6.8 c.c, 8.0 c.c, and 8.8 

 c.c. respectively. Later titrations showed no increase in acidity. 

 This strain of Strept. lactis grew distinctly at 5°C. in cultures with 

 small seeding. The low minimum of 6.8 c.c n/10 KOH at 5°C. 

 against 8 cc at 10°C. and 8.8 c.c at 30°C. cannot be explained 

 simply by lack of reconstruction power of the bacteria. 



SUMMARY OF FACTS 



At temperatures well above the growth-minimum, the 

 final amount of fermentation products decreases with 



