CHAPTEE V. 



Preparation of media — Bouillon, gelatin, agar-agar, potato, blood- 

 serum, etc. 



As has been stated, the fundamental constituent of 

 culture-media is beef-tea, or bouillon. 



Bouillon. — The directions of Koch for the prepara- 

 tion of this medium have undergone many modifications 

 to meet special cases ; but for general use his original 

 formula is still retained. It is as follows : 500 grammes 

 of finely chopped lean beef, free from fat and tendons, 

 are to be soaked in one litre of water for twenty-four 

 hours, during which time the mixture is to remain in 

 an ice-chest or to be otherwise kept at a low tempera- 

 ture. At the end of twenty-four hours it is to be strained 

 through a coarse towel and pressed until a litre of fluid is 

 obtained. To this are to be added 10 grammes (1.0 per 

 cent.) of dried peptone and -6 grammes (0.5 per cent.) of 

 common salt (NaCl). It is then to be rendered exactly 

 neutral or very slightly alkaline to litmus with a few 

 drops of saturated sodium carbonate solution. The flask 

 containing the mixture is then to be placed either in a 

 steam sterilizer or in a water-bath, or over a free flame, 

 and kept at the boiling-point until all the albumin is 

 coagulated and the fluid portion is of a clear, pale 

 straw color. It is then filtered tlirough a folded paper 

 filter and sterilized in the steam sterilizer by the frac- 

 tional method. Certain modifications of this method 

 are of sufficient value to justify mention. Most im- 



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