82 



inoculate a tube of bouillon and make a series of 2 agar plate 

 cultures. 



At the next exercise describe these cultures and examine 

 the colonies microscopically to determine the genera of bac- 

 teria. If a micrococcus which grows in clumps is found, in- 

 oculate a tube of agar with it and at the following exercise 

 examine and describe its appearance. Indicate in the notes 

 the number of colonies of bacteria which developed in the 

 plate cultures and the genera which appear in the bouillon 

 culture. 



EXERCISE 



DETERMINE THE THERMAL DEATH POINT OF BACTERIA. 



145. Explanatory note. It is important to know the 

 minimum temperature which will kill bacteria, especially the 

 pathogenic forms. The uses for such knowledge are numer- 

 ous in practical disinfection and pasteurization. For the 

 various methods employed in making these determinations, 

 see text books and special articles on this subject. The 

 method here given and which can be followed by a full sec- 

 tion 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. 



146. General directions. Inoculates tubes of bouillon 

 from each of two cultures (It. 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 10 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 which is at 60 C. The water should 



