PREPARATION OF ORDINARY CULTURE MEDIA 31 



To ensure that the temperature all around shall be the same, 

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

 special gas burner at the side to heat it. This is the form 

 originally used, but serum sterilisers are now constructed in 

 which the test-tubes are placed in the sloped position, and in 

 which inspissation (vide p. 40) can 

 afterwards be performed at a higher 

 temperature. 



THE PREPARATION OF ORDINARY 

 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 FlG 5. steriliser for blood 

 the pathogenic bacteria isolated. These serum, 



consist of proteids or carbohydrates 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 particular 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 gelatin, 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 gelatin is fluid, and there- 

 fore another gelatinous substance called agar, which does not 

 melt below 98 C., was substituted. Bouillon made from 

 meat extract, gelatin, and agar media, and the modifications 

 of these, constitute the chief materials in which bacteria are 

 grown. 



