CHAPTER IV. 

 CULTURE-MEDIA. 



Culture-media are substances in which bacteria are artifi- 

 cially cultivated. The number of such substances is very 

 large, different materials being suited to different purposes and 

 to different kinds of bacteria. The most important ones are 

 nutrient bouillon or beef-tea, nutrient gelatin, and nutrient 

 agar-agar. The two last have a jelly-like consistency, owing 

 to the addition of a gelatinizing substance, but otherwise are of 

 the same composition as the bouillon. 



In all cases the media must be either free from bacteria 

 originally or they must be rendered free from bacteria in order 

 that the organisms under cultivation may be studied in pure 

 cultures. This is effected, as a rule, by steaming in the steam 

 sterilizer or in the autoclave. For special purposes filtration 

 through porcelain filters is resorted to. 



Preparation of Media. 



NUTRIENT BOUILLON. 



Beef-extract (such as Liebig's) 3 grams. 



Peptone, pure (Witte's)* . 10 grams. 



Sodium chloride (common salt) . 5 grams. 



Water . . i Uter. 



The solid ingredients are dissolved in water, and the mixture 

 is boiled for a few minutes. It is made neutral or very faintly 

 alkaline by the addition of a solution of sodium hydroxide, 

 drop by drop, the reaction being tested at intervals with 

 litmus-paper. As soon as the proper reaction is reached, it is 



♦Commercial "peptones" are mixtures of albumose and a small amount of 

 peptone. 



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