MECHANISM OF DEATH 273 



the time differs with the species used. The tuberculosis 

 organism is extremely resistant to alkali; molds can 

 tolerate more formaldehyde than most bacteria; spores 

 are much more resistant than vegetative cells. 



The experiment is usually carried out by taking 

 samples after certain intervals, transferring them to a 

 good nutrient medium, and waiting for a certain length 

 of time to see whether growth develops. The results 

 obtained are of the nature shown in Table 75. 



The accuracy of this method depends upon the frequency of the 

 time intervals. In the above example, the interval is relatively 

 large; the span between four and six minutes is 33% of the longest 

 time. The last one of the four experiments of the above table is 

 more accurate. The correct death time hes between twenty-five 

 and thirty minutes. The span within which we know the exact 

 time to be, is only five minutes out of thirty, or 18%. This relative 

 accuracy will prove important where the endpoint method is used 

 for computing death rates. 



It has been known for many years that more time is needed to 

 kill a larger number of bacteria. This is important for the technique 

 of the endpoint method. In order to obtain comparable data, it is 

 essential that the same number of cells is used for all data to be 

 compared. If that proves to be impossible, the initial number of 

 cells acted upon should be determined. It has become quite cus- 

 tomary to count the initial number of cells. 



(b) The Plate Count Method.— The plate count 

 for determining the efficiency of a disinfectant has been 

 in use for a very long time. It has the advantage of 

 showing more of the kinetics of the disinfection process 

 than other methods, and requires a smaller number of 

 samples, but more time, and more media; therefore it is 

 used mostly for studies on the laws of death, and less 

 frequently in applied bacteriology. 



(c) Staining Methods. — Most cells cannot be stained 

 easily as long as they are alive, but take the dye readily 



