290 PHYSIOLOGY OF BACTERIA 



(/) DECREASING DEATH RATES 



Decreasing death rates can be accounted for by varied 

 resistance of the cells. If we have a mixture of sensitive 

 and resistant individuals, the death rate at the beginning 

 of the experiment will be high because all sensitive cells 

 will die rapidly. After most of these are killed, the 

 rate will be largely that of the resistant cells, and this 

 is lower. 



It is most probable that in a culture which is not 

 especially cared for, there will be organisms of different 

 physiological conditions, and therefore of different 

 resistance to the cause of death. Rahn (1930) computed 

 a theoretical case for bacteria with three different 

 grades of resistance, and the curve of survivors is shown 

 in the dotted line of Fig. 32. 



This effect of variability of resistance had already 

 been forseen by Harriett Chick (1910) who tried to avoid 

 this decrease of death rate by using very young cultures, 

 three hours old, and by transplanting them repeatedly 

 in order to get rid of old and more resistant cells. The 

 results are seen in Table 82. 



The degree of constancy or variability of K can be best compared 

 by the variation in the relative death rates based on the average K 

 of each experiment = 100. The lowest line of Table 82 shows that 

 the relative spread of variation in the two experiments with twenty- 

 four hours old cultures is quite large, 137 — 41 = 96% in the one and 

 140 ~ 56 = 84% in the other. With the three hours old culture, 

 the spread of variation was 108%, i.e., larger than in the twenty- 

 four hour cultures; perhaps there were many old cells left among the 

 new growth. But upon transferring this culture repeatedly every 

 three hours, the uniformity improved, and the spread of variation 

 dropped to 26 and 44%. 



Reichenbach (1911) tried a similar experiment with 

 the same organism. Bad. paratyphosum, exposed to heat. 



