MICROORGANISMS AND HUMAN WELFARE 19 



minutes. Now take the tray out of the water, cool the 

 bottles with cold water and ice as quickly as possible, 

 and keep them at this low temperature till used." 

 Place one bottle of Pasteurized milk (No. 3) beside the bottle 

 in the room temperature, and the other (No. 4) in the 

 refrigerator beside bottle No. 2. 



1. At the end of three days shake the two bottles kept at 



the room temperature and open them. Smell or taste 

 of the milk in each. State your observations and con- 

 clusions. 



2. In a similar manner, test the two bottles that have been 



kept on ice for a week. State your observations and 

 conclusions. 



3. Why are milk, meat, and other foods of the kind put into 



the refrigerator, especially in summer time ? Does this 

 kill the bacteria ? How do you know ? 



4. Why are meats cooked, milk Pasteurized, and fruits boiled 



before they can be kept for any length of time ? 



C. The effect of lack of moisture. Expose for ten minutes 

 two Petri dishes of nutrient agar in a dusty room or 

 corridor (as in A above). Place the two dishes (No. 

 11 and No. 12 side by side in a warm room (over 

 90). Cover dish No. 11 and leave dish No. 12 un- 

 covered. 



1. Describe the similarity and the difference in the 



treatment of dishes 11 and 12. 



2. How is the agar mixture affected by removing the 



cover ? 



3. In which dish do colonies of bacteria develop? 



4. What do you conclude, therefore, as to the necessity 



of moisture for the growth of bacteria? 



5. Why is hay dried before it is put into the barn? 



Name some foods used by man that are kept for 

 a long time after being dried. 



6. As a conclusion from these experiments (in A, B and 



C) state what conditions you have found favorable 

 for the growth of bacteria. 



