CHAPTER VI. 



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 preparation 

 of this medium has undergone many modifications to 

 meet special cases, but for general use his original for- 

 mula 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 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 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 on a water-bath, or over 

 a free flame, and Ivept 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 modifi'cations 

 of this method are of sufficient value to justify mention. 

 Most important is the neutralization. Ordinarily, this 



