198 Culture Media and Bacteria Cultivation 



culture of the colon bacillus to the meat-infusion and per- 

 mitting fermentation to continue overnight before finishing 

 the bouillon and adding the known quantity of whatever, 

 sugar is desired. About i per cent, of dextrose, lactose, 

 saccharose or galactose is all that is required. More may be 

 injurious. If the bouillon be made from meat extract, fer- 

 mentation may not be necessary. 



The sugar bouillons should not be sterilized in the auto- 

 clave, as the high temperatures chemically alter the sugars. 



GELATIN. 



The culture medium known as gelatin has the decided ad- 

 vantage over bouillon that it is not only an excellent food 

 for bacteria, and, like the bouillon, transparent, but also is 

 solid at the room temperature. Nor is this all. It is a 

 transparent solid that can be made liquid or solid at will. 

 It is prepared as follows: 



To 1000 c.c. of meat-infusion or to 1000 c.c. 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 ingredients are dissolved. The solution reacts 

 strongly acid and must be corrected by titration, as already 

 described. It must then be returned to the fire and boiled 

 for about an hour. As gelatin is apt to burn when boiled 

 over the direct flame, double boilers have been suggested, 

 but unless the outer kettle is filled with brine or saturated 

 calcium chlorid solution, they are very slow, and when 

 proper care is exercised there is really no great danger of 

 the gelatin burning. It must be stirred occasionally, and 

 the flame should be so distributed by wire gauze or by placing 

 a sheet of asbestos between it and the kettle as not to act 

 upon a single point. At the end of the hour the albumins 

 of the meat-infusion will be coagulated and the gelatin 

 thoroughly dissolved. Gunther 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 vaporiza- 

 tion during the process of boiling should be replaced. It 

 is well to cool the liquid to about 60 C., add the water mixed 

 with the white of an egg to clear the liquid, boil again for 

 half an hour, and filter. 



If the filter paper be of good quality, properly folded 



