Il6 LABORATORY BACTERIOLOGY 



EXERCISE LII 



DETERMINING THE THERMAL DEATH POINT OF 

 BACTERIA 



169. 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. In this exercise students may work in groups with 

 satisfactory results and with the saving of much media. 



170. Work for this exercise. Take 10 tubes of bouillon, 

 place 8 of them in a wire basket, and stand it in the thermo- 

 regulated water bath at 60 C. After 15 minutes remove the 

 tubes and inoculate 4 of them from a culture of B. subtilis 

 and 4 of them from a culture of B. typhosus or B. cholera suis 

 (the cultures will be furnished). In inoculating the tubes be 

 sure not to touch the sides above the surface of the bouillon 

 with the wire. 



After the tubes are inoculated return them to the 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. 



Inoculate the other two tubes of bouillon, one from each of 

 the cultures used, and place them in the incubator for controls. 



