PREPARATION OF CULTURE MEDIA. 41 



lid also is hollow and filled with water, and there is a 

 special gas burner at the side to heat it. 



THE PREPARATION OF CULTURE MEDIA. 



The general principle to be observed in the artificial 

 culture of bacteria is that the medium used should approxi- 

 mate as closely as possible to that on which the bacterium 

 grows naturally. In the case of 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. 



The flesh of the ox, calf, or horse is usually employed. 

 Horse-flesh has the advantage of being cheaper and con- 

 taining less fat than the others ; though generally quite 



