36 The Story of the Bacteria 



such as beef -tea, milk, blood-serum, etc., we 

 can arrive at a series of characteristics in 

 the mode of growth of various bacteria by 

 which, together with their form when seen 

 under the microscope, we can distinguish 

 them one from the other, just as the naturalist 

 distinguishes from each other nearly related 

 animals and plants. 



! It is obviously of the greatest importance, 

 as we have seen above, that we should be able 

 to separate different species of bacteria from 

 one another in the living condition, so that we 

 may have growths or colonies which shall con- 

 tain one species alone without admixture with 

 any other. These are called "pure cultures.'* 

 This is by no means an easy task, as will be 

 appreciated when we consider how exceedingly 

 minute the organisms are, and how much dan- 

 ger there is that the bacteria floating every- 

 where invisibly in the air may become mixed 

 with those forms which we are studying. By 

 a very simple device elaborated by Dr. Koch, 

 of Berlin, we are, nevertheless, able at any 

 time to separate one species from another with 

 the utmost certainty, or from a mixture of 



