272 PHYSIOLOGY OF BACTERIA 



Attention may also be called to the observation of Hofmann (p. 256) 

 that from the same dilution of a pure culture, a larger plate count 

 might be obtained by adding small amounts of poisons like AgNOs 

 or CuCU to the medium. According to our definition of death, the 

 chemical stimulus brought some of the "dead" cells back to life. 



As long as a cell can recover from injury, the death 

 process is reversible. It seems that the point of irre- 

 versibility is not reached very soon after the process 

 of dying begins. This becomes evident in chemical 

 disinfection. Bacteria treated with a poison may 

 remain incapable of reproduction even when all poison 

 has been washed away from the cells. An antidote that 

 combines with the poison which has already reacted 

 with the protoplasm, may be able to make the cell grow 

 again (p. 358). 



This discussion leads to the conclusion that our defini- 

 tion of death means the fixation of one point on the 

 continuous line of a gradual chemical process. Dying 

 is a gradual time process, of measurable velocity, and 

 may be reversible, after removal of the cause, during 

 the first stage by the mechanism of the cell itself. 

 Gradually, it becomes irreversible for the cell, but may 

 still be made reversible by some outside influences such 

 as antidotes. Finally it reaches a state where it becomes 

 irreversible under any condition. This may be due to 

 secondary chemical changes which could take place 

 because some part of the cell was not functioning suffi- 

 ciently to ward it off as in normal cells. 



II. METHODS OF MEASURING DEATH 



(a) The Endpoint Method. — The oldest method of 

 measuring the power of a disinfectant was to determine 

 the shortest time in which all bacteria added to a certain 

 concentration of the poison would be dead. Naturally, 



