BACTERIA. 63 



correcting the chemical reaction, clarifying and sterilizing. To this broth various 

 substances are added for special purposes; gelatin and agar, in order to solidify the 

 media, and various sugars and other chemical substances for the purpose of determin- 

 ing the physiological characteristics of various bacteria. One of the difficulties with 

 the present methods of the artificial cultivation of bacteria is the inconstancy of the 

 composition of the media, due to the fact that the extract of beef, the peptone, and other 

 ingredients, cannot be obtained chemically pure. If it should prove possible to use 

 synthetic substances, such as the polypeptids, it would mark a great step in advance, 

 but it is probably quite impossible to devise a single medium upon which all bacteria 

 will grow. Some bacteria, such as those which produce nitrification, refuse to grow 

 on ordinary media containing organic material. The cultivation of bacteria in pure 

 culture is dependent upon isolation, and the method of isolation suggested by 

 Robert Koch in 1880, and known as the plate culture method, has given eminent 

 satisfaction. This method is dependent upon the use of a liquefiable solid medium, 

 such as gelatin or agar. 



