376 PHYSIOLOGY OF BACTERIA 



cubic centimeter (compared with about one billion in 

 broth) makes starvation the most probable cause. 



A case where fermentation products are the most likely- 

 cause of the decrease in living cells has been mentioned 

 by Rahn (1911) for a culture of a lactic streptococcus 

 in milk. The death rates in this experiment (Table 

 119) increase, but this is probably due to an 

 increase of lactic acid. Even after the maximum num- 

 ber has been passed and the cells already have begun to 

 decrease, the acid still increases, and this increase must 

 necessarily increase the death rate. 



Another set of data we owe to Chesney (1916) who studied the 

 death of pneumococci in old broth cultures. The death rate in this 

 case is quite constant. Probably, death is brought about by chemical 

 action of the products of metabolism. 



Buchanan and Fulmer (1928) distinguished between a period of 

 accelerated death rate, and a period of constant death rate. The 

 first-named phase must necessarily exist. Bacteria, after having 

 grown to a maximum number, will not die suddenly at a constant 

 rate. There is a gradual increase of the products beyond the 

 tolerable concentration, and the increase in death rate under these 

 conditions has been shown in Table 119. The period of 

 accelerated death is just an intermediate stage between maintenance 

 and death, and involves no new biologic principle. 



SUMMARY 



Death of bacteria in old cultures may be due to one 

 of the following three causes: starvation, fermentation 

 products, or specific thermolabile metabolic products. 

 The existence of the third cause has been questioned for 

 most species, but is not disproved. 



The order of death gives no clue as to which of the 

 possible causes is acting. 



The cause of death by old age does not seem to lie 

 in the cell, but in the unfavorable environment. 



