90 LABORATORY GUIDE IN BACTERIOLOGY 



utes; but, if milk containing tubercle bacilli has its 

 surface exposed to the air when heated to 60 C., the 

 pellicle which forms on its surface may contain living 

 tubercle bacilli after an exposure of 60 minutes. 



Study of the effect of the above two methods of 

 sterilization as compared with each other and with 

 unsterilized milk: 



1. From the fresh milk provided make three agar 

 plates, using i, 2, and 3 loopfuls, respectively. 



2. Fill about 10 c.c. into each of ten sterile culture 

 tubes, and keep one at room temperature and one in 

 the thermostat. 



3. Treat four of these tubes in the following manner: 

 Place water in a saucepan sufficient to cover completely 

 the milk when the tubes are immersed in it. Raise the 

 temperature to 65 C., and keep it there by regulating 

 the flame. The tubes of milk are then immersed in the 

 water, and kept there for 30 minutes, as it requires 

 about 10 minutes for the milk in the tubes to reach the 

 temperature of the water. The tubes are then taken 

 out and cooled quickly by standing them in cold water. 

 Place one of the tubes at incubator and the other at 

 room temperature. Aerate the other two by shaking 

 vigorously for i| minutes. Keep one of these at room 

 temperature, the other in the thermostat. 



4. Place two more milk tubes in the arnold at 

 100 C. for 30 minutes. Keep one at room tempera- 

 ture and one in the thermostat. 



5. The remaining two tubes autoclave at 120 C. 

 for 5 minutes, and place one in the thermostat and keep 

 the other at room temperature. 



