400 



BACTERIA 



A method of preserving foods is by sun-drying The picture shows plums exposed to sunlight 

 until dry. This changes them to prunes and safeguards them from the attacks of bacteria. 



growth of most bacteria. In the refrigeration process, the flavor 

 is not Hkely to be spoiled. It is an expensive method, however. 

 Canning or preserving makes use of extreme heat to sterilize, then 

 by placing the food in air-tight containers no new bacteria can 

 enter. Smoke contains creosote, which is poisonous to bacteria, 

 but, in small amounts, is not poisonous to people. Hence, smok- 

 ing is a method of preserving food. Bacteria in an alkaline or 

 acid solution are more easilv killed than those in a neutral solu- 

 tion. Housekeepers have found that canned fruits seldom spoil, 

 but canned vegetables frequently cause trouble. Sometimes 

 sugar, salt, and vinegar in quantity act as poisons to bacteria. 

 They are not injurious to people. Producers frequently add pre- 

 servatives such as benzoate of soda or mild acids to help kill the 

 bacteria. Many of these may be injurious. 



Problem. What is the number of bacteria found in the air in vari- 

 ous places in the environment f 



Prepare seven or eight Petri dishes of sterile agar media. 



