16 HUMAN BIOLOGY 



5. (Optional.) Make careful drawings at intervals of several days 

 to show the difference in the number of colonies in the dishes, 

 and the change in the size and appearance of the colonies. 



14. Are bacteria present in water, milk, and other foods ? 



Laboratory demonstration. 



Allow the water to run from the faucet for several minutes, 

 and then spread a drop on the surface of dish No. 4. Spread 

 a drop of milk on the surface of the agar in dish No. 5. On 

 the agar surface of dish No. 6 put a bit of raw meat, a bit 

 of apple peel, and bits of other kinds of food. Put the dishes 

 in a warm, dark place as directed above, and examine at the 

 end of several days. 



1. State the difference in the treatment of dishes No. 4, No. 5, 



and No. 6. 



2. In which of the three dishes do you find bacteria colonies ? 



Describe the colonies in each dish as to position, number, 

 and color. 



3. What do you infer as to the presence of bacteria in water, 



milk, and other foods that you have tested ? 



15. Are bacteria present on various parts of the human 

 body ? Laboratory demonstration. 



Touch the surface of the agar in dish No. 7 with the 

 finger tips ; lay a hair on another part of the surface, and 

 touch a third part with a toothpick that has been used to 

 scrape the teeth. Put the dish in a warm, dark place as 

 above, and examine at the end of several days. 



Describe fully this experiment, stating your observations 

 and conclusions. 



16. Distribution of bacteria. From our study of the 

 culture dishes we have learned that bacteria are very com- 

 mon organisms. In fact, they are doubtless the most 

 abundant of all living things; for they are found not only 

 in air, water, and milk; not only in countless numbers 

 wherever dead plant or animal material is allowed to accu- 

 mulate ; but al*o ; unfortunately, in living tissues. 



