BACTERIA 



The work that may be done with bacteria is 

 limitless but, to advance very far, we shall need 

 facilities for obtaining what are known as " pure 

 cultures." Let us make the term clear. Suppose 

 we take milk, water, butter, anything in fact upon 

 which bacteria will grow and examine them care- 

 fully. If we have the requisite knowledge and 

 recognise what we see, we shall find not one kind 

 [of bacterium but a number of different bacteria. 

 Now by certain manipulations, which need not be 

 described here, all the different kinds of bacteria 

 may be sorted out, so that we have colonies con- 

 sisting of one kind of bacterium only and such a 

 colony is known as a "pure culture." 



In practice, bacteriologists do not use the rough 

 and ready methods that we used in dealing with 

 the Hay Bacillus. They prepare pure cultures and 

 cultivate the bacteria on various substances, dif- 

 fering markedly from those on which they originally 

 lived. For example, a jelly-like substance, mainly 

 composed of beef broth and gelatine is one of the 

 favourite substances on which to grow bacteria, 

 milk is also used in some cases and also slices of 

 potato. All this may seem to have little to do with 

 the microscope, but indeed the bacteriologist relies 

 as much on the behaviour of his pure cultures, 

 growing on gelatine, etc., as on their appearance 

 under the microscope. Some bacteria will not grow 

 on the surface of the gelatine but only in the body of 

 the substance, where air cannot reach them; others 

 cause the gelatine to become liquid; others give 



165 



