PREPARATION OF ORDINARY CULTURE MEDIA 31 



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 iu 



the 



position, and in which 



inspissatiori (vide p. 40) can after- 

 wards 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 grow r s 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 F]G . 5 ._ steriliser for blood 

 preparation and preservation. Other serum. 



media have been found which can 



support the life of all the pathogenic bacteria isolated. These 

 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 

 therefore another gelatinous substance called agar, which does 

 not melt below 98 C., was substituted. Bouillon made from 

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

 of these, constitute the chief materials in which bacteria are 

 grown. 



