Artificial Cultivation of Bacteria. 19 



everything coming in contact with the culture in any 

 way is wholly free from living bacteria, i. e., sterile. 



Many substances frequently contain so many bacteria 

 that it is impossible to add a small enough amount to 

 the gelatine directly, and not have the cultures so 

 thickly dotted with colonies that it would be impossible 

 to count them.' To overcome this difficulty a small but 

 definite amount of the substance is added to a definite 

 amount of sterile water, and intimately mixed. A 

 quantity of the mixture is then added to the melted 

 gelatin. In this manner it is possible to obtain 1-1000 

 of a drop of water or milk, a procedure that will be 

 seen to be necessary when it is known how many qf the 

 bacteria there may be in a drop of these liquids. 



Pure cultures of bacteria. Since each colony has re- 

 sulted from the growth of a single cell, it follows that 

 all of the cells of the colony are of the same kind. If 

 a small bit of the mass of growth is transferred from 

 the original colony to a tube of fresh food, the resulting 

 growth is known as a "pure culture." If some other 

 form of bacteria should accidentally fall into the tube 

 from the air when the tube is opened, the culture no 

 longer contains a single kind but a mixture and is now 

 called an impure or mixed culture. 



The shape and appearance of the colonies of the dif- 

 ferent kinds of bacteria differ as do their growth on 

 various media. These differences often aid the experi- 

 enced bacteriologist in determining the kind of bacteria 

 in the materials examined. 



In order to separate a pure culture from a mixture of 

 many kinds or in order -to determine the kinds of bac- 

 teria present in any substance, essentially the same 

 method is used as described above. 



