190 PATHOGENIC BACTERIA. 



saccharose, etc. Smith 1 points out that the bouillon 

 as usually prepared is apt to contain considerable muscle 

 sugar; this should be destroyed before the new sugar is 

 added, else confusion of results must be expected. To 

 exclude the muscle sugars and secure dextrose-free 

 bouillon he inoculates the beef-infusion in the evening 

 with the colon bacillus and stands it in the incubator. 

 Next morning, the growth of the colon bacillus having 

 destroyed the sugars, the bouillon is prepared from the 

 sugar-free meat-infusion as already described, and the 

 requisite percentage of sugar added. 



The sugar bouillons should not be sterilized in the 

 autoclave, as the high temperatures alter the sugars. 



Geiatin. — The culture-medium known as gelatin has de- 

 cided advantages over the bouillon, not only because it is 

 an excellent food for bacteria, and, like the bouillon, trans- 

 parent, but because it is also solid. Nor is this all : it is 

 a transparent solid which can be made liquid or solid at 

 will. It is prepared as follows: To 1000 c.cm. of meat- 

 infusion or to 1000 c.cm. of water containing 2 grams of 

 beef-extract in solution, 10 grams of peptone, 5 grams of 

 salt, and 100 grams of gelatin ("Gold label " is the best 

 commercial article) are added, and heated until the ingre- 

 dients are all dissolved. It is then titrated or alkalinized 

 to litmus by adding sodium hydroxid solution as described. 

 Double boilers are very slow, and if proper care is exer- 

 cised there is little danger of the gelatin burning. It 

 must be stirred occasionally, and the flame should be so 

 distributed by wire gauze as not to act upon a single point 

 of the bottom of the kettle. The preparation is now re- 

 turned to the fire and boiled for about an hour. At the 

 end of the hour the albumins of the meat-infusion will 

 be coagulated and the gelatin thoroughly dissolved. 

 Giinther has shown that the gelatin congeals better if 

 allowed to dissolve slowly in warm water before boiling. 

 As much water as has been lost by vaporization during 

 the process of boiling should be replaced. It is well to 



1 Journal of Experimental Medicine, ii., No. 5, p. 546. 



