278 APPLIED BIOLOGY 



trate it ; but yeasts and bacteria may develop later. This 

 opposition of different kinds of organisms is often called 

 antagonism. It is important, for harmless micro-organisms 

 often prevent the multiplication of other kinds. 



255. Study of Bacteria. (L or D) Clean by washing with soap 

 or soap-powder and rinsing some small bottles or test-tubes and plug 

 with cotton as described in 245. Place the plugged tubes in a 

 sterilizer ( 245) and keep the water boiling for a half hour. Pro- 

 tect the tubes from dust until needed. 



Any clear soup or bouillon will serve for the following experiment. 

 The clear concentrated soups commonly sold at ten cents per can 

 may be used after diluting with water. Test with litmus-paper, and 

 if acid add some baking soda (or KOH solution) , little by little, until 

 the soup is slightly alkaline in reaction. When thus prepared, the 

 bouillon may be kept in a bottle or flask by simply plugging the 

 mouth with cotton and sterilizing after each opening of the bottle. 



Fill sterile test-tubes half full with the bouillon; and replace the 

 cotton plugs, taking care that they do not become wet. Place the 

 tubes in the sterilizer for a half hour, and repeat on the following 

 day. The sterile tubes may now be kept indefinitely. It is best to 

 keep them covered so that dust from the air may not fall on the 

 cotton and thus increase the chance of bacteria getting into the tube 

 when the plug is removed. 



Examine tubes which have remained sterile for many days. If 

 bacteria develop, the bouillon will become turbid or cloudy. 



Take the plugs from some tubes with bouillon, and leave the tubes 

 open for several days. Do bacteria develop? What is the ex- 

 planation of the fact that similar tubes with cotton plugs remained 

 sterile ? The cotton certainly cannot keep air out of tubes ; what 

 then can be its effect on the air which enters the tubes ? 



Mount a drop of bouillon from a tube which has become turbid. 

 Examine with a high power. It is well to have some cotton threads 

 under the cover-glass to assist in finding the focus. Examine with 

 a high power. Look for exceedingly minute, transparent bodies, 

 much smaller than yeast cells and mold spores ; some rod-shaped, 

 some spherical, some twisted rods (Fig. 86). By shifting the mirror 

 of the microscope, it is possjble to see them better. Some of these 

 organisms (bacteria) swim rapidly by means of cilia so small that 

 they cannot be seen with the ordinary microscope. Watch the 

 swimming of some of the largest bacteria visible. Remember that 

 the apparent rate of speed has been magnified as much as have 



