V 

 THE THERMAL DEATH POINT OF BACTERIA 109 



C EXERCISE LVI ' 

 DETERMINING THE THERMAL DEATH POINT OF BACTERIA 



164. It is important to know the minimum temperature 

 which will kill bacteria, especially the pathogenic forms. The 

 uses to which such knowledge can be put are numerous in 

 practical sanitary medicine, disinfection, and pasteurization. 

 For the various methods employed in making these determi- 

 nations, see text-books and special articles on this subject. 

 The method here given, and which can be followed by a full 

 section of students, will give only approximate results. It 

 should not vary, however, more than one degree from the 

 actual thermal death point in moist heat of the organisms 

 tested. 



165. Work for this Exercise. — Inoculate 5 tubes of bouillon 

 from each of 2 cultures (^B. subtilis, old culture, and B. typhosus) 

 furnished. 



In inoculating be sure not to touch the sides of the tube 

 with the inoculating loop. The tubes should have stood in the 

 water bath at 60° C. for at least 15 minutes before they are 

 inoculated. 



Place one of these tubes in the incubator for a control. 

 Stand the others in a wire basket and set them in the thermo- 

 regulated water bath adjusted at 60° C. The water should 

 come just above the liquid in the tubes. Remove the tubes, 

 one of each species, as follows : one in 5 minutes, one in 10 

 minutes, one in 15 minutes, and one in 20 minutes. Label 

 and place them in the incubator. 



At the next exercise examine the heated tubes and note 

 which are clear and which contain a growth. If the tubes 

 heated for 10 minutes or longer have a growth, repeat the 

 experiment at 70° C. If this fails to destroy them, repeat at 

 80° C, and if necessary apply a still higher temperature. 



