216 



LIFE ON THE EARTH 



Heat Petri dishes in a hot oven for two hours or keep them 

 in boiling water for thirty minutes, to kill all bacteria. 



Nutrient agar is an excellent food on which to grow 

 bacteria. It can be prepared in the following manner. Peel 

 and wash several small potatoes. Cut them into slices and 

 boil in a quart of water for twelve minutes. Drain off about 

 a pint of water from the potatoes and strain it through a 

 clean white cloth. Cut a half ounce of agar (sea-weed jelly) 

 into small pieces and stir it in the liquid. After a few min- 

 utes heat the solution again until the agar is all dissolved. 

 Strain the liquid again and pour it into test tubes, making 

 them about half full. Plug each tube with a wad of cotton. 

 Stand the tubes in a dish of water. Boil the water for thirty 

 minutes. The agar is now ready for use in bacteria gardens. 



Experiment 135. Where are bacteria found? 



Prepare some culture plates by putting some of the steril- 

 ized nutrient agar in several of the sterilized Petri dishes. 

 If the agar in the test tubes has jellied from cooling, it will 

 be necessary to melt by heating the test tubes in a beaker 

 of water. Allow them to cool. Expose one dish to the air. 

 Touch another here and there with your fingers. Hold an- 

 other in front of some person's face when he sneezes. Steril- 

 ize the end of a thin wire by heating it in a flafne and touch 

 it to the cavity of a decayed tooth; then touch the agar in 

 several places in one of the dishes with the wire. Cover 

 the dishes and set them aside in a warm, dark place for 

 several days. On the floor under a radiator is a good place. 

 Examine them each day and record your results in your 

 notebook. 



Experiment 136. How may bacteria be killed? 



Pour some melted nutrient agar into four test tubes. 

 Label the tubes 1, 2, 3, 4. Find a Petri dish with colonies 

 of bacteria growing in it. Touch the colonies with the tip 

 of a needle which has been sterilized by holding in a flame 

 and transfer some of the bacteria to the agar in each of 

 the test tubes. Add a teaspoonful of iodine solution to the 

 agar in test tube number 2. Add a teaspoonful of hydrogen 

 peroxide to test tube number 3. Add a teaspoonful of some 

 common disinfectant such as Zonite or Lysol to tube num- 

 ber 4. 



Pour the contents of each test tube into a separate Petri 

 dish. Label each dish. Put the dishes in a warm, dark place. 

 After four days examine the Petri dishes again. 



Are colonies of bacteria growing in dish No. 1 ? In dish 

 No. 2? In dish No. 3? In dish No. 4? 



Why did bacteria grow in one Petri dish and not in the 

 others ? 



Why do we put iodine or hydrogen peroxide on a wound ? 



Did you ever gargle your throat? Why? 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



What are bacteria and how large are they? Bac- 

 teria are the smallest and simplest living things. 

 They are tiny one-celled plants. They are not green 

 in color because they do not possess chlorophyll. 

 Hence they are not able to manufacture their own 

 food, but are dependent upon other living organisms 

 for their nourishment. 



There are many kinds of bacteria, and they are 

 found almost anywhere. There is no difficulty in get- 



ting material for study. Bacteria are in the air, in the 

 soil, and in water. They may live on either dead or 

 living material. A poorly ventilated schoolroom con- 

 tains many bacteria in the air, and even a well ven- 

 tilated room has some. Soil that is rich in decaying 

 plant and animal life teems with bacteria. We eat, 

 drink, and breathe bacteria in countless millions ; yet 

 most of them do us no harm. We shall learn later 

 about bacteria that cause human diseases. 



S V- 



BACILLI , 

 (Rod-Shaped) 



FIG. 344. KINDS OF BACTERIA 



Single bacteria are so small that they are visible 

 only when greatly magnified by a microscope. They 

 range in size from about 1/25,000 to 1/1,000 of an 

 inch in length. Only colonies containing millions of 

 bacteria can be seen with the naked eye. Many per- 

 sons who have never seen bacteria imagine them to 

 be horrible looking creatures. They are, however, 

 very simple in structure. They are generally color- 

 less, transparent cells. 



There are many kinds of bacteria. Most of them 

 may be grouped as one of three forms. The three 

 forms are shown in Figure 344, but keep in mind that 

 they are greatly magnified in these diagrams. The 

 straight, rod-like ones are called bacilli; the spheri- 

 cal ones are called cocci ; and the curved and spiral 

 ones are called spirilla. If a microscope is available in 

 your school, scrape the inside of your cheek with 

 your finger nail. Place the material on a slide, add 

 a drop of water, and cover with a cover glass. Ex- 

 amine it with the high power of the microscope and 

 you may see all three of these forms. .These bacteria 

 from your mouth are not particularly harmful. Among 

 the many that may be moving, observe the character- 

 istic snaky motion of the curved ones (spirilla). 



How do bacteria grow? Like higher forms of life, 

 bacteria have certain definite requirements for growth. 

 Conditions of temperature, light, water, and food 

 supply must be suited to their needs. Bacteria differ 

 greatly in their requirements, and some of them are 

 able to survive almost anywhere. 



Water. Water is a first essential in the life of bac- 

 teria. Their bodies contain about eighty-five per cent 

 water, and all of their food must be dissolved in water 

 before it can be absorbed. Therefore bacteria live in 

 liquids, and if the medium on which they live dries 



