194 Cultivation of Micro-organisms 



Sugar bouillon is bouillon containing in solution known percent- 

 ages of such sugars as glucose, lactose, saccharose, etc. As Smith* 

 has pointed out, if the quantity of sugar in the bouillon is to be ac- 

 curately known, it is necessary to first destroy the muscle sugars in 

 the meat-infusion. This can be done by adding a culture of the 

 colon bacillus to the meat-infusion and permitting fermentation to 

 continue overnight, then finishing the bouillon and adding the 

 known quantity of whatever sugar is desired. If the bouillon be 

 made from meat extract, fermentation may not be necessary. One 

 per cent, of dextrose, lactose, saccharose or galactose is the standard 

 addition. 



The sugar bouillons should be steriHzed in the Arnold apparatus 

 not in the autoclave, as the high temperatures chemically altfer the , 

 sugars. 



GELATIN 



The culture-medium known as a gelatin is bouillon to which lo 

 per cent, gf gelatin is added. It has the decided advantage over 

 bouillon that it is not only an excellent food for bacteria, and, like 

 the bouillon, transparent, Imt also is solid at the room temperature. 

 Nor is this all: it is a transparent solid that can be made hquid or 

 sohd at will. Lefifmann and La Wall have examined commercial 

 gelatins and found that many of them contain sulphur dioxid in 

 quantities as great as 835 parts per milUon. As the varying quan-, 

 tity of this impurity may modify the growth of the culture, pure 

 gelatin should be demanded, and all gelatin should be washed for 

 some hours in cold running water after being weighed and before 

 being added to the bouillon. It is prepared as fbllows: 



To 1000 cc. of meat-infusion or to 1000 cc. 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 titra- 

 tion, 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 solu- 

 'tion, they are very slow, and when proper care is exercised there is 

 really no great danger of the gelatin burning. It must be stirred 

 oc'casionally, 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. 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 



* "Jour, of Exp. Med.," u, No S- p. 546- 



