124 LABORATORY GUIDE IN BACTERIOLOGY 



3. Expose the flask to sunlight for several hours. 



4. Make another plate with i c.c. of the suspension, 

 and place in a thermostat. 



5. After 48 hours count both plates, and compare 

 the results. 



EXPERIMENT II 



1. Melt a tube of agar and cool to 43°. 



2. Inoculate with B. coli (or any other organism). 



3. Pour into a sterile Petri dish. 



4. After solidification, turn bottom side up, and paste 

 a strip of black paper on the glass, covering part of the 

 surface. 



5. Expose to direct sunlight for several hours, and 

 note the result. 



EXERCISE IV. INFLUENCE OF MOIST HEAT 



Read the methods of determining the thermal death- 

 point of bacteria in the textbook. 



1. Prepare six broth-cultures each of B. coli and B. 

 subtilis. 



2. Place four cultures of each organism in the water- 

 bath and heat. 



3. Remove one of each at 40°, one of each at 60°, 

 one of each at 80°, and keep one of each for 10 minutes at 

 100°. 



4. Place one tube of each organism in the autoclav, 

 and heat to 120° for 5 minutes. 



5. Now place all twelve tubes in the thermostat, 

 including one of each organism as a control. 



6. After 24 hours, make plates of each tube in agar, 

 and place them in the thermostat. 



7. After 24 hours, count the colonies and compare 

 the results. 



