MECHANISM OF DEATH 303 



death rate will decrease. This must happen, e.g., 

 with mixtures of old and young cells. 



Organisms with more than one such '^ reacting mole- 

 cule" show an increasing death rate, and a survivor 

 curve bulging above the straight line. 



Bacteria which have a tendency to clump or cluster, 

 and whose plate count does not reveal the actual number 

 of cells, but only the number of clumps or clusters, 

 must give survivor curves corresponding to more than 

 one ^^ reacting molecule" because the only method of 

 counting living bacteria is the plating method, and 

 here, each colony represents a clump of cells whose total 

 number of reacting molecules equals the number of 

 cells in the clump. 



IV. DEATH OF DRY BACTERIA 



The following subchapters will treat the various causes 

 of death. That of dry bacteria has been chosen as the 

 first cause to be discussed because conditions here are 

 simpler than with other causes. When bacteria are in 

 a moist condition, endogenous catabolism is decreasing, 

 the cell substance continuously, while in dry cells, lack of 

 moisture prevents this. Thus the uncertain factor of 

 catabolism does not enter into the death of dry bacteria. 



(a) ORDER OF DEATH OF DRY BACTERIA 



Dry bacteria will die in the course of time ; some species 

 within a few minutes, most of them within weeks or 

 months, and the spores of some bacilli eventually not 

 for a decade. The viability or longevity of dried bac- 

 teria depends not only upon the species, but also upon 

 the temperature and the stratum on which the cells are 

 dried. 



