124 BACTERIOLOGY 



Bouillon. As has been stated, the fundamental con- 

 stituent of culture media is beef-tea or bouillon. The 

 directions of Koch for the preparation of this medium 

 have undergone many modifications to meet special cases; 

 but for general use the formula now employed is as follows: 

 500 grams of finely chopped lean beef free from fat and 

 tendons, are to be soaked in 1 liter 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 temperature. 

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

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

 To this are to be added 10 grams (1 per cent.) of dried 

 peptone and 5 grams (0.5 per cent.) of common salt 

 (NaCl). It is then to be rendered neutral or very slightly 

 alkaline by one of the foregoing methods. The mixture is 

 then placed in an agateware or porcelain-lined saucepan 

 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 by steam. 



Not infrequently the filtered bouillon, neutralized and 

 sterilized, will be seen to contain a fine, flocculent precipi- 

 tate. This may be due either to excess of alkalinity or to 

 incomplete precipitation of the albumin. The former may 

 be corrected with dilute acetic or hydrochloric acid, and the 

 bouillon again boiled, filtered, and sterilized; or, if due 

 to the latter cause, subsequent boiling and filtration usually 

 result in ridding the bouillon of the precipitate. 



Another modification now generally employed is in the 

 substitution of meat extracts for chopped meat in making 

 the bouillon. Almost any of the meat extracts of com- 

 merce answer the purpose, though we usually employ 



