CHAPTEE V. 



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



As has been stated, the fundamental part of our cul- 

 ture media is beef tea, or bouillon. 



Bouillon. — The formula of Koch for the prepara- 

 tion of this medium has undergone many modifications 

 to meet special cases, but for general use his original 

 formula is still retained. It is as follows : Five hundred 

 grammes of finely-chopped lean beef, free from fat and 

 tendons, is to be soaked in one litre of water for 

 twenty-four hours. During this time it is to remain in 

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

 ture. It is then to be strained through a coarse towel and 

 pressed until a litre of fluid is obtained. To this is to be 

 added ten grammes (1.0 per cent.) of dried peptone and 

 five grammes (0.5 per cent.) of common salt (NaCl). It 

 is then to be rendered exactly neutral or very slightly 

 alkaline, with a few drops of saturated soda solution. 

 The flask containing the mixture is then to be placed 

 either in the 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 through a 

 folded paper filter, and sterilized in the steam sterilizer 

 by the fractional method. Certain of the modifications 

 of this method are of sufficient value to justify mention. 

 Most important is the neutralization. Ordinarily, this 

 is accomplished with the saturated soda solution, and 



