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. -Other 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 tem- 

 perature at which this nutrient gelatin is fluid, and therefore 

 another gelatinous substance called agar, which does not 

 melt below 98° O, 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. 



Fig. 5.- 



Steriliser for blood 

 serum. 



