GROWTH 223 



A possible explanation is the assumption that the available energy- 

 is no longer sufficient for synthesis. Each synthesis requires a 

 minimum amount of available energy, not only in quantity but 

 probably also potentially. This is furnished readily in suitable 

 media at normal growth temperatures. Even then, a considerable 

 amount of the available energy is not utilized for synthesis (see 

 pp. 180-188). This apparent waste might be due to radiation or 

 conduction of heat, or to the general effort of keeping up a certain 

 energy potential. In the first case, the absolute loss would be 

 greater at lower temperatures; the relative loss would be very much 

 greater, because the amount of available energy is greatly reduced 

 by low temperatures. If the maintenance of a certain energy poten- 

 tial is necessary to bring about growth, there may be a temperature 

 well above the freezing point where certain organisms lose so much 

 of the small amount of energy furnished them by their fermentation 

 that the potential in the cell cannot be maintained, and the cell 

 ceases to grow. It has been shown on p. 134 that streptococci in 

 milk at — 1°C., gradually lose the power to ferment. Their endo- 

 enzyme deteriorates and cannot be replenished at such low tempera- 

 tures. If the cell ceases to be able to make repairs, it must gradually 

 die from the slow, but certain deterioration of its essential, ther- 

 molabile components. (See also influence of kind of food, p. 157.) 



The data obtained by the author with streptococci 

 indicate that bacteria die at temperatures too low for. 

 growth. The death rate is very slow, but distinct. 

 Table 59 shows the number of cells in milk cultures 

 of three streptococci. Strept. cremoris No. 18 is the 

 only one which does not grow at H-5°C. 



The generation times at 5°C. for the first time interval 

 were 40.7 hours for Strept. cremoris 23 and 55.5 hours 

 for Strept. lactis. It takes about 2 days for one cell to 

 double. At 10°C., the generation times are 24.2 hours 

 (Strept. cremoris 23) and 12.5 hours (Strept. lactis). 

 It seems remarkable that a growth process can be so 

 slow and yet not come to an end. At — 1°C., all three 

 cultures showed a decrease in numbers. A gradual 

 decrease of the number of cells of Lactobacillus acid- 



