PREP A RA TION OF ME A T EXTRA CT. 4 1 



mate as closely as possible to that on which the bacterium 

 grows naturally. In growing pathogenic bacteria, the 

 medium therefore should resemble the juices of the body. 

 The serum of the blood satisfies this condition and is 

 often used, but its application is limited by the difficulties 

 in its preparation and preservation. Other media have 

 been found which can support the life of all the pathogenic 

 bacteria isolated. These consist of proteids or carbo- 

 hydrates in a fluid, semi-solid or solid form, in a transparent 

 or opaque condition. The advantage of having a variety 

 of media lies in the fact that growth characters on parti- 

 cular media, non- growth on some and growth on others, 

 etc., constitute specific differences which are valuable in the 

 identification of bacteria. The most commonly used media 

 have as their basis a watery extract of meat. Most bacteria 

 in growing in such an extract cause only a grey turbidity. 

 A great advance resulted when Koch, by adding to it 

 gelatine, provided a transparent solid medium in which 

 growth characteristics of particular bacteria become evident. 

 Many organisms, however, grow best at a temperature at 

 which this nutrient gelatine melts, and therefore another 

 gelatinous substance called agar, which does not melt below 

 98 C., was substituted. Bouillon made from meat extract, 

 gelatine, and agar media, and the modifications of these, 

 constitute the chief materials in which bacteria are grown. 



Preparation of Meat Extract. 



Ox flesh or horse flesh (preferably the latter for most 

 purposes on account of its cheapness and freedom from 

 fat) is taken. It ought to be from an animal recently 

 killed, and should therefore be markedly acid to litmus 

 paper. It must be freed from fat, and finely minced. For 

 each pound of mince add 1000 c.c. distilled water, and 

 mix thoroughly in a shallow dish. Skim off any fat present, 

 removing the last traces by stroking the surface of the fluid 

 with pieces of filter paper. Set aside in a cool place for 

 twenty-four hours. Place a clean linen cloth over the mouth 



