96 LABORATORY GUIDE IN BACTERIOLOGY 



days, and on the last day prove the relative growth 

 by making agar plates with i c.c. of each culture, 

 and count the colonies after 24 hours. 



7. Tabulate the results, and state conclusions. 



EXERCISE 3. INFLUENCE OF SUNLIGHT 



Experiment i — 



1. Inoculate a flask containing 100 c.c. of sterile 

 water with B. coll. 



2. After thoroughly shaking, take i c.c. by means of 

 a sterile pipette, and plate in agar. Place the plate in 

 the thermostat. 



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 2— 



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



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 4. INFLUENCE OF MOIST HEAT 



Read the methods of determining the thermal death- 

 point of bacteria in the textbook. 



I. Prepare six broth cultures each of B. coli and B. 

 subtilis. 



