CULTURE-MEDIA AND THEIR PREPARATION 93 



LlQUEFIABLE SOLID MEDIA 



Nutrient Gelatin. This is prepared by the addition of 10-15 

 per cent, of gelatin to bouillon as prepared above. The gelatin 

 should be the best " gold label." Care must be used in heating 

 the solution while dissolving the gelatin or the latter will stick 

 to the bottom of the vessel and burn. It is best to use an asbestos 

 pad, a double boiler, or a rice-cooker. The gelatin is itself acid, 

 so that it is necessary to adjust the reaction after it has dissolved. 

 The medium is then cooled to 60, and the white of an egg thor- 

 oughly mixed with it. It is again heated to the boiling-point 

 without stirring. The coagulation of the egg removes suspended 

 dust-particles and makes nitration easier. The nutrient gelatin 

 is tubed and sterilized in the autoclave at 120 for ten minutes. 

 It should be cooled at once after removal from the sterilizer. 

 Care must be exercised not to heat the medium too long or it 

 may fail to solidify when cooled. 



Other Gelatin Media. Any of the liquid media already dis- 

 cussed, with the exception of the milk and serum broth, may be 

 made solid by the addition of 10 to 15 per cent, gelatin. Among 

 the more commonly used are dextrose, lactose, and glycerin 

 gelatin. 



Nutrient Agai'. This is prepared by the addition of 1.5 per 

 cent, of shredded or powdered agar-agar to bouillon. Agar-agar 

 is a carbohydrate-like material, probably related to the vegetable 

 gums, which is prepared from certain of the seaweeds of the 

 Pacific and Indian Oceans. The mixture must be boiled vigorously 

 for half an hour to insure thorough solution of the agar. This 

 medium does not burn as readily as does gelatin, and long- 

 continued heating does not interfere with its solidification when 

 cooled. The nutrient agar may be sterilized in the autoclave 

 for fifteen minutes at 120. 



Other Agar Media. Agar may be used as the solidifying agent 

 for any of the liquid media described. It has the advantage 

 over gelatin that it may be kept at blood heat, while gelatin under 

 such conditions would liquefy. 



