WHERE BACTERIA ARE FOUND 



185 



Growth of bacteria in an impure drop of water 

 placed on a sterilized culture medium. 



air of a near-by street ; the air of the woods or high mountains 

 contains fewer still. Why ? 



Fluids the favorite home of bacteria. Tap water, standing 

 water, milk, vinegar, wine, cider, all can be proved to contain 

 bacteria by experiments similar 

 to those already suggested. 

 Spring or artesian well water 

 would have very few, if any, 

 bacteria, while the same quan- 

 tity of river water, if it held 

 any sewage, might contain un- 

 told millions of these little 

 organisms. 



Individual Project. Deter- 

 mine by experiment whether bac- 

 teria will grow without fluids being 

 present. Try dry and moist beans. 



Demonstration 3. To deter- 

 mine the foods most favorable 

 for the growth of bacteria. 



Materials. Raw meat, cooked meat, white of egg, beans, Indian 

 meal flour, cake, sugar, butter, test tubes, and absorbent cotton. 



Method. Moisten all of the above food substances. Place small 

 particles of them in test tubes with a little distilled water. Expose all to 

 the air for half an hour. (This can be done during a class period.) Plug 

 the tubes with absorbent cotton and allow to stand for several days. 



Note the appearance and odor of the various substances after five 

 days. 



In which substances was there rapid growth of bacteria ? 



Food of bacteria. Bacteria, since they contain no chlorophyll, 

 are unable to make carbohydrate food, but must absorb their 

 foods, ready formed, from decaying organic matter. Some bacteria, 

 however, seem able to build up the protein, which they need for 

 growth, out of absorbed carbohydrates and simple inorganic nitrog- 

 enous substances. 



What bacteria do to foods. When bacteria feed, they digest 

 the food substances by means of enzymes which they secrete. 

 The food is decomposed and eventually rots. The material left 

 behind after the bacteria have finished their meal is quite different 

 from its original form. It is broken down by the action of the 



