CHAPTEE V. 



Preparation of media— Bouillon, gelatin, agar-agar, potato, blood-serum, etc. 



As has been stated, the fundamental constituent of 

 our culture media is beef tea, or bouillon. 



BouiLLO>f. — 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 : five hun- 

 dred 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 this time the mixture 

 is to remain in the ice-chest or to be otherwise kept at a 

 low temperature. It is then to be strained through a 

 coarse towel and pressed until a litre of fluid is obtained. 

 To this are to be added ten grammes (1.0 per cent.) of 

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

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

 neutral or very slightly alkaline with a few drops of 

 saturated sodium carbonate solution. The flask con- 

 taining the mixture is then to be placed either in the 

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

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

 coagulated and the fluid portion is of a clear, pale 

 straw color. It is then filtered through a folded paper 

 filter, and sterilized in the steam sterilizer by the frac- 

 tional method. Certain of the modifications of this 

 method are of sufficient value to justify mention. Most 



